What is the structural/chemical difference leading to different shapes in facial/head/pubic hair?

What is the structural/chemical difference leading to different shapes in facial/head/pubic hair?

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Different hairs have different shapes, thicknesses, properties.

What exactly is different between these types of hairs? a previous post implies that the root of the differing length (pun intended) is the rate at which the skin sheds the hair. However, this doesn't explain the different degrees of curliness, thicknesses, and textures.

hair shape texture is really easy. the biggest effect is the shape, size, and angle of the hair follicle. Hair with different cross sections have different properties. If hair is stiffer in one plane than other it will tend to bend along the weaker plane. Larger follicles will of course produce thicker, thus stiffer hair and the angle of the follicle can influence the hairs cross sections as mush as the follicles cross sectional shape can. Note the chart below is simplified triangular and kidney shaped cross sections also exist.

Similar to the differences between curly and straight hair, different hairs have different hair follicles.

Sex hormones at the onset of puberty rebuild the local vellus hair into terminal hair, which tends to be longer, thicker, and darker. The anagen/telogen/catagen cycle is different for these follicles and as a result they get longer.

What is the structural/chemical difference leading to different shapes in facial/head/pubic hair? - Biology

The same with any hair on any specific part, really. Arm hair is a different length, color, grows at a different rate, etc. Also, pubic hair faces different levels of sweat and oil, so that makes a different texture.

boydieshere ( 166 />) “Great Answer” ( 0 />)

Androgen hormones are the cause-this hormone becomes present in pubescent-age m/f and thereafter, modified existing hair shafts and sebaceous glands which produce thicker follicles especially around the genital areas and armpits.

Some believe the thicker hair was once for protection or for drawing heat away from these anatomical parts (like a radiator of sorts), also radiators for other pheremones secreted by the individual in various stages of sexual (or other) states of arousal but there is no substantial proof to verify this.

sndfreQ ( 11729 />) “Great Answer” ( 1 />)

And… pubic hair has a flattened shape. Imagine your normal hair is really big and you slice it, the top view of the hair is almost exactly round. If you would do the same to your pubic hair it is shaped like a flattened oval, therefore forcing it to curl more and change it’s structure.

Jax ( 356 />) “Great Answer” ( 1 />)

It’s also suspected that one of multiple purposes of pubic hair is to act as a sort of surrogate lubrication for sex. If it were just skin on skin (no addition lube, we’re talking the wild here), that would cause alot of friction leading to rashes and abrasion.

I also imagine another important purposes would be to help shade against potentially detrimental sunburns. Ouch. After all, the only reason we exist (like every other organism) is to procreate, and if our genitals are burnt, we’re useless.

Spargett ( 5382 />) “Great Answer” ( 1 />)

Spargett, I GA’d you for “if our genitals are burnt, we’re useless.” So wise.

susanc ( 16134 />) “Great Answer” ( 0 />)

Best Overall: Nair Hair Remover Cream for Face

Sometimes, classic beauty brands are a go-to for a simple reason: They just plain work. “Nair is still the tried and true depilatory cream. This product will soften the hair to either remove it at the surface or soften it enough to remove it from the root,” says Condon. “It’s extremely important to follow the directions closely so you don’t burn or irritate your skin,” she adds. (Related: The Best Hair Removal Creams for Silky, Smooth Results)

Condon also recommends avoiding this product if you take prescription acne medications, including Retin-A, because the combo could make your skin hyper-sensitive and potentially lead to irritation. Over 2,000 Amazon shoppers have given Nair’s face cream a five-star rating, with many reviews raving about how well it works. One said it “works like a charm” while another wrote: “I have always had a hard time getting all my hair off my lip it’s some fine and some thick hair. The first time I used this it took IT ALL OFF!! Didn’t leave any redness or irritation which I always get with other products, even the Nair for the body will make me red. I am so relieved to finally find a decent hair removal for the face! Buy this you will not regret it.” Bonus: You can get it for just $4 on Amazon right now, making it one of the cheapest hair removal options. 

What causes hair shaft defects?

Hair shaft defects may be due to external injury or genetic abnormality.

External injury to the hair shaft.

Repeated physical injury is the commonest cause of increased hair fragility . This may be due to:

  • Excessive grooming
  • Traction from braiding or tight ponytail
  • The heat from a hairdryer

A chemical injury may be caused by:

Traction alopecia

Genetic hair shaft abnormalities

Genetic hair shaft abnormalities may be broadly divided into two types: those with increased hair fragility and those without increased hair fragility.


Cellulitis is a painful, erythematous infection of the dermis and subcutaneous tissues that is characterized by warmth, edema, and advancing borders (Table 1) . Cellulitis commonly occurs near breaks in the skin, such as surgical wounds, trauma, tinea infections ( Figure 1 ) , or ulcerations, but occasionally presents in skin that appears normal. Patients may have a fever and an elevated white blood cell count. Cellulitis can occur on any part of the body. Among the patients in the cohort above, the most common sites of cellulitis were the legs and digits, followed by the face, feet, hands, torso, neck, and buttocks (data taken from primary physician diagnosis codes from January 1, 1999 to December 1, 1999 for health plan members of Intermountain Health Care, Salt Lake City).

Descriptions of Bacterial Skin Infections

A network of furuncles connected by sinus tracts

Painful, erythematous infection of deep skin with poorly demarcated borders

Fiery red, painful infection of superficial skin with sharply demarcated borders

Papular or pustular inflammation of hair follicles

Painful, firm or fluctuant abscess originating from a hair follicle

Large vessicles and/or honey-crusted sores

Descriptions of Bacterial Skin Infections

A network of furuncles connected by sinus tracts

Painful, erythematous infection of deep skin with poorly demarcated borders

Fiery red, painful infection of superficial skin with sharply demarcated borders

Papular or pustular inflammation of hair follicles

Painful, firm or fluctuant abscess originating from a hair follicle

Large vessicles and/or honey-crusted sores

Cellulitis secondary to tinea infection.

Cellulitis secondary to tinea infection.

In otherwise healthy adults, isolation of an etiologic agent is difficult and unrewarding. If the patient has diabetes, an immunocompromising disease, or persistent inflammation, blood cultures or aspiration (some physicians inject sterile nonpreserved saline before aspiration) of the area of maximal inflammation may be useful.2 – 4 For infection in patients without diabetes, empiric treatment with a penicillinase-resistant penicillin, first-generation cephalosporin, amoxicillin-clavulanate (Augmentin), macrolide, or fluoroquinolone (adults only) is appropriate.5 Limited disease can be treated orally, but more extensive disease requires parenteral therapy. Marking the margins of erythema with ink is helpful in following the progression or regression of cellulitis ( Figure 2 ) . Outpatient therapy with injected ceftriaxone (Rocephin) provides 24 hours of parenteral coverage and may be an option for some patients. The patient should be seen the following day to reassess disease progression.

Inked margins of cellulitis.

Inked margins of cellulitis.

Most cases of superficial cellulitis improve within one day, but patients who exhibit thickening of the dermis usually take several days of parenteral antibiotics before significant improvement occurs. Antibiotics should be maintained for at least three days after the resolution of acute inflammation.5 Adjunctive therapy includes the following: cool compresses appropriate analgesics for pain tetanus immunization and immobilization and elevation of the affected extremity.6

A parenteral second- or third-generation cephalosporin (with or without an aminoglycoside) should be considered in patients who have diabetes, immunocompromised patients, those with unresponsive infections, or in young children.5 The patient may also require a plain radiograph of the area or surgical debridement to evaluate for gas gangrene, osteomyelitis, or necrotizing fasciitis.6

Recurrent episodes of cellulitis or undergoing surgery, such as mastectomy with lymph node dissection, can compromise venous or lymphatic circulation and cause dermal fibrosis, lymphedema, epidermal thickening, and repeated episodes of cellulitis. These patients may benefit from prophylaxis with erythromycin, penicillin, or clindamycin (Cleocin).5 , 7

Periorbital cellulitis is caused by the same organisms that cause other forms of cellulitis and is treated with warm soaks, oral antibiotics, and close follow-up.8 Children with periorbital or orbital cellulitis often have underlying sinusitis.9 If the child is febrile and appears toxic, blood cultures should be performed and lumbar puncture considered. Haemophilus influenzae type b (Hib) in young children was a significant concern until the widespread use of the Hib vaccine and coverage with a parenteral third-generation cephalosporin was used routinely. Recently, some researchers have recommended no longer routinely covering for H. influenzae .8 – 10

Orbital cellulitis occurs when the infection passes the orbital septum and is manifested by proptosis, orbital pain, restricted eye movement, visual disturbances, and concomitant sinusitis. Complications include abscess formation, persistent blindness, limited eye movement, diplopia, and, rarely, meningitis.11 This ocular emergency requires intravenous antibiotics, otorhinolaryngology, and ophthalmologic consultation.12

Perianal cellulitis is caused by group A beta-hemolytic streptococcal infection and occurs most often in children. A study13 of children with perianal cellulitis found a mean age of onset of 4.25 years. Ninety percent of patients presented with dermatitis, 78 percent with itching, 52 percent with rectal pain, and 35 percent with blood-streaked stools. Despite 10 days of oral antibiotics (primarily penicillin or erythromycin), the recurrence rate was high at 39 percent. If there is recurrence, the presence of an abscess should be considered, with needle aspiration of the site for bacteriology being more accurate than a skin swab.14

Hair Plucking & Tweezing: Side Effects and Risks

It’s a natural impulse for many women: See a stray hair, pluck it away. But before reaching for the tweezers, it’s important to consider the hair removal risks being taken. For most women, the side effects of tweezing are minor and worth suffering through to save money. However, there are some serious plucking risks that women need to be aware of before diving into that group of wandering chin hairs.

Epilation, or plucking, is the process of removing a hair from its roots below the surface of the skin. Epilation wounds the hair follicle, and over time it can result in finer and thinner hair. Most women are elated with this result, and since tweezing is one the easiest and most economical ways of removing unwanted facial hair, they are more inclined to head for the tweezers. But plucking has its risks, and they can be painful.

Tweezing Side Effects: #1 – Pain and Skin Irritation

Let’s face it: tweezing hurts! Yanking hair from the roots one by one is no easy task. For women who have never, or rarely, tweezed, the pain can be too much. Even experienced pluckers feel the sting. Apart from pain, tweezing can lead to irritation and redness of the skin. For most people, a swipe of makeup can conceal the redness associated with tweezing, but for others wearing makeup isn't a viable option. While these side effects of tweezing sound minimal, there are more serious hair plucking risks.

Tweezing Side Effects: #2 – Ingrown Hairs

If hair is thick, tweezing it can result in the formation of ingrown hairs, where hair curls back or grows sideways under the skin and creates a small lesion. This plucking side effect can lead to redness, swelling, itching or infection. Luckily, this event happens most often when hair is broken off short, like with shaving, but tweezing can have similar consequences when a hair is not properly or thoroughly removed. In order to prevent ingrown hairs from tweezing, try to pluck the hair from the root to reduce the chance of breakage.

Tweezing Side Effects: #3 – Folliculitis

The most important part of tweezing is technique. If done improperly, tweezing can result in blood vessels breaking around the follicles. This can lead to folliculitis, when hair follicles become inflamed and infected&mdashnot pretty! The condition appears as small, pimple-like white dots on the skin, causing itching and pain. Folliculitis can easily be mistaken as an embarrassing breakout. Mild folliculitis will usually clear up on its own in a few days, but it can still be uncomfortable and embarrassing. More severe infections can cause permanent hair loss and scarring, and should be treated by a dermatologist.

Tweezing Side Effects: #4 – Hyperpigmentation

A particularly unpleasant side effect of tweezing is hyperpigmentation, the darkening of skin in patches around the face. This is caused when melanin forms deposits in the skin from prolonged inflammation due to plucking. This is common in certain skin types and may not affect everyone who tweezes.

Preventing Hair Plucking Risks

In order to avoid some of these side effects, it’s essential to take the necessary precautions. Make sure tweezers are sterile by washing them with warm water and soap after each plucking session. Applying medicated lotions as well as ice after tweezing can help reduce redness, swelling and irritation. While plucking, women should also be sure they are pulling hair in the direction of growth to avoid breaking the hair midway and causing ingrown hairs.

For quick facial hair removal, epilation is still a quick, economical option. But there are other alternatives for women who would like to avoid the side effects of plucking. Waxing is a popular method to shape eyebrows, and skin remains smooth for a longer period of time (though skin irritation and pain are still common). Threading is a technique involving a line of hairs being removed with a single thread of cotton. It is similar to tweezing, with the exception that more hair is removed at one time. For a more advanced, permanent hair removal treatment, there is also electrolysis and laser hair removal, both of which can eliminate the need for tweezing and shaving once and for all.

To explore the advantages of a professional hair removal treatment, including advanced facial laser hair removal, you can contact us today and schedule a free consultation with a trusted cosmetic clinic near you. After all, as much as we all love plucking those rogue hairs, a life without tweezers is certainly a pleasant thought!

Physical Development during Adolescence

Puberty Begins

Puberty is the period of rapid growth and sexual development that begins in adolescence and starts at some point between ages 8 and 14. While the sequence of physical changes in puberty is predictable, the onset and pace of puberty vary widely. Every person’s individual timetable for puberty is different and is primarily influenced by heredity however environmental factors—such as diet and exercise—also exert some influence.

Adolescence has evolved historically, with evidence indicating that this stage is lengthening as individuals start puberty earlier and transition to adulthood later than in the past. Puberty today begins, on average, at age 10–11 years for girls and 11–12 years for boys. This average age of onset has decreased gradually over time since the 19th century by 3–4 months per decade, which has been attributed to a range of factors including better nutrition, obesity, increased father absence, and other environmental factors (Steinberg, 2013). [1] Completion of formal education, financial independence from parents, marriage, and parenthood have all been markers of the end of adolescence and beginning of adulthood, and all of these transitions happen, on average, later now than in the past. In fact, the prolonging of adolescence has prompted the introduction of a new developmental period called emerging adulthood that captures these developmental changes out of adolescence and into adulthood, occurring from approximately ages 18 to 29 (Arnett, 2000). [2] We’ll learn more about this phase in the next module on early adulthood.

Figure 1. Major physical changes in males during puberty.

Hormonal Changes

Puberty involves distinctive physiological changes in an individual’s height, weight, body composition, and circulatory and respiratory systems, and during this time, both the adrenal glands and sex glands mature. These changes are largely influenced by hormonal activity. Many hormones contribute to the beginning of puberty, but most notably a major rush of estrogen for girls and testosterone for boys. Hormones play an organizational role (priming the body to behave in a certain way once puberty begins) and an activational role (triggering certain behavioral and physical changes). During puberty, the adolescent’s hormonal balance shifts strongly towards an adult state the process is triggered by the pituitary gland, which secretes a surge of hormonal agents into the blood stream and initiates a chain reaction.

Puberty occurs over two distinct phases, and the first phase, adrenarche, begins at 6 to 8 years of age and involves increased production of adrenal androgens that contribute to a number of pubertal changes—such as skeletal growth. The second phase of puberty, gonadarche, begins several years later and involves increased production of hormones governing physical and sexual maturation.

Sexual Maturation

Figure 2. Major physical changes in females during puberty.

During puberty, primary and secondary sex characteristics develop and mature. Primary sex characteristics are organs specifically needed for reproduction—the uterus and ovaries in females and testes in males. Secondary sex characteristics are physical signs of sexual maturation that do not directly involve sex organs, such as development of breasts and hips in girls, and development of facial hair and a deepened voice in boys. Both sexes experience development of pubic and underarm hair, as well as increased development of sweat glands.

The male and female gonads are activated by the surge of the hormones discussed earlier, which puts them into a state of rapid growth and development. The testes primarily release testosterone and the ovaries release estrogen the production of these hormones increases gradually until sexual maturation is met.

For girls, observable changes begin with nipple growth and pubic hair. Then the body increases in height while fat forms particularly on the breasts and hips. The first menstrual period (menarche) is followed by more growth, which is usually completed by four years after the first menstrual period began. Girls experience menarche usually around 12–13 years old. For boys, the usual sequence is growth of the testes, initial pubic-hair growth, growth of the penis, first ejaculation of seminal fluid (spermarche), appearance of facial hair, a peak growth spurt, deepening of the voice, and final pubic-hair growth. (Herman-Giddens et al, 2012). [3] Boys experience spermarche, the first ejaculation, around 13–14 years old.

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Physical Growth: The Growth Spurt

During puberty, both sexes experience a rapid increase in height and weight (referred to as a growth spurt) over about 2-3 years resulting from the simultaneous release of growth hormones, thyroid hormones, and androgens. Males experience their growth spurt about two years later than females. For girls the growth spurt begins between 8 and 13 years old (average 10-11), with adult height reached between 10 and 16 years old. Boys begin their growth spurt slightly later, usually between 10 and 16 years old (average 12-13), and reach their adult height between 13 and 17 years old. Both nature (i.e., genes) and nurture (e.g., nutrition, medications, and medical conditions) can influence both height and weight.

Before puberty, there are nearly no differences between males and females in the distribution of fat and muscle. During puberty, males grow muscle much faster than females, and females experience a higher increase in body fat and bones become harder and more brittle. An adolescent’s heart and lungs increase in both size and capacity during puberty these changes contribute to increased strength and tolerance for exercise.

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Watch this video to see a summary of the main biological changes that occur during puberty.

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Reactions Toward Puberty and Physical Development

The accelerated growth in different body parts happens at different times, but for all adolescents it has a fairly regular sequence. The first places to grow are the extremities (head, hands, and feet), followed by the arms and legs, and later the torso and shoulders. This non-uniform growth is one reason why an adolescent body may seem out of proportion. Additionally, because rates of physical development vary widely among teenagers, puberty can be a source of pride or embarrassment.

Most adolescents want nothing more than to fit in and not be distinguished from their peers in any way, shape or form (Mendle, 2015). [4] So when a child develops earlier or later than his or her peers, there can be long-lasting effects on mental health. Simply put, beginning puberty earlier than peers presents great challenges, particularly for girls. The picture for early-developing boys isn’t as clear, but evidence suggests that they, too, eventually might suffer ill effects from maturing ahead of their peers. The biggest challenges for boys, however, seem to be more related to late development.

As mentioned in the Khan Academy video about physical development, early maturing boys tend to be stronger, taller, and more athletic than their later maturing peers. They are usually more popular, confident, and independent, but they are also at a greater risk for substance abuse and early sexual activity (Flannery, Rowe, & Gulley, 1993 Kaltiala-Heino, Rimpela, Rissanen, & Rantanen, 2001). Additionally, more recent research found that while early-maturing boys initially had lower levels of depression than later-maturing boys, over time they showed signs of increased anxiety, negative self-image and interpersonal stress. (Rudolph, Troop-Gordon, Lambert, & Natsuaki, 2014). [5]

Early maturing girls may be teased or overtly admired, which can cause them to feel self-conscious about their developing bodies. These girls are at increased risk of a range of psychosocial problems including depression, substance use and early sexual behavior (Graber, 2013). [6] These girls are also at a higher risk for eating disorders, which we will discuss in more detail later in this module (Ge, Conger, & Elder, 2001 Graber, Lewinsohn, Seeley, & Brooks-Gunn, 1997 Striegel-Moore & Cachelin, 1999).

Late blooming boys and girls (i.e., they develop more slowly than their peers) may feel self-conscious about their lack of physical development. Negative feelings are particularly a problem for late maturing boys, who are at a higher risk for depression and conflict with parents (Graber et al., 1997) and more likely to be bullied (Pollack & Shuster, 2000).

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Forensic Hair Comparison: Background Information for Interpretation

Hair evidence is one of the most common types of evidence encountered in criminal investigations. During the course of the normal hair-growth cycle, hairs are readily lost from individuals, and these hairs may be transferred during the course of a criminal activity.

Edmond Locard was the first forensic scientist to formally articulate the foundation for the transfer event (Locard 1930). Now known colloquially as the Locard Exchange Principle, it states that any time there is contact between two surfaces, an exchange of materials will occur. One of the materials that can be readily collected, identified, and compared is hair evidence.

The forensic analysis of hair evidence can be extremely valuable in the examination of physical evidence by (1) demonstrating that there may have been an association between a suspect and a crime scene or a suspect and a victim or (2) demonstrating that no evidence exists for an association between a suspect and a crime scene or a suspect and a victim. Although the science of microscopic hair examination can never result in an identification, that is, conclude that a hair came from one individual to the exclusion of all others, the vast amount of both macroscopic and microscopic information available from hair analysis can provide a strong basis for an association and certainly provides strong exculpatory evidence. The final aim of any forensic examination must be to provide statements based on objective scientific observation that will be of value in a court of law or to any interested party involved in an investigation.

The purpose of this document is to review the bases for microscopic hair analysis and comparison. Hair examinations involve the analysis and comparison of the morphological characteristics present in hair. Based on these morphological characteristics, the first determination that can be made is whether the hair came from a human or an animal (for the purposes of this document, any reference made to animal means nonhuman animal). Within each of these two groups, additional information regarding the potential donor can be obtained using these same microscopic characteristics. Finally, a comparison can be conducted between a hair of unknown origin and a known sample of hairs from an appropriate known sample

Scientific Basis for Microscopic Hair Examinations

All organisms differ widely in many dimensions, including morphological appearance, physiology, and genetic makeup. Some groups of organisms clearly are more similar to some groups than to others. For instance, a monarch butterfly is more similar to a tiger swallowtail butterfly than either is to a ladybird beetle. Biologists seek to identify these differences and use them to organize and classify the world around them. They use these differences to generate classification schemes that can be used for many purposes, from examining how traits evolve to solving crimes.

These classification schemes have their roots in the field of taxonomy. Taxonomy is the practice of classifying biodiversity, and it has a long and venerable history. In 1758, Carl Linnaeus proposed a system that has dominated classification for centuries. He proposed a system of binomial nomenclature to describe living organisms (as cited in Wikipedia 2009). However, the term taxonomy is now applied in a wider, more general sense and refers to a classification of things, as well as to the principles underlying such a classification. Almost anything—animate objects, inanimate objects, places, concepts—may be classified according to some scheme.

Evaluation of shared and distinguishing characteristics is essentially the process used in forensic hair examinations. The microscopic characteristics allow for hair to be categorized into smaller groups, such as human or animal, racial group, body area, color, phase of growth, etc. This is considered the identification phase, for example, classifying the hair as being human, exhibiting Caucasian characteristics, coming from the head, being brown in color, and possessing a telogen root.

The next phase of the examination process is conducting a microscopic comparison. This involves evaluating the microscopic characteristics present in the hair samples, evaluating the points for comparison, and determining whether or not a questioned hair can or cannot be excluded as originating from the source of a known sample.

Hair comparisons are a combination of a pattern-recognition process and a step-by-step analysis of a questioned hair and a known sample. An example of the pattern-recognition process is the manner in which we identify a friend in a crowd of people. It is an instant recognition, based on our experience with that person. It is not conducted in a logical, step-by-step process, evaluating first the height, hair color, skin color, eye color, and other characteristics. It is an almost instantaneous evaluation of all of these characteristics together. The identification of our friend does not carry any less weight based on the mechanism we used to identify him or her.

The same process is used for hairs, but in a more methodical manner. In a microscopic hair comparison, the examiner is determining whether or not similar patterns of microscopic characteristics exist at each point of comparison along the hair shaft. This pattern-recognition process then continues in a step-by-step fashion along the length of the hair.

To be considered an association, the microscopic characteristics of a questioned hair also must be exhibited in the known sample. Forensic hair examination involves the analysis of objective characteristics and a subjective interpretation of the relative weight of these characteristics. The subjective component of hair examination almost dictates that two different examiners will place slightly different weight on individual characteristics or may describe these characteristics using slightly different words.

However, if two examiners have been trained properly, possess adequate experience, and use proper procedures, they should reach the same conclusion. Accordingly, the amount of experience gained by examining a large number of hairs and conducting a large number of hair comparisons is critical.

The scientific method involves generating a hypothesis and testing it to determine if it is false. In order for the hypothesis to be valid, it must be able to be supported repeatedly via reproducible experiments. This process distinguishes science from other professional endeavors. By establishing a reliable, repeatable set of procedures and criteria by which the results are evaluated, an objective scientific methodology can be achieved. This, coupled with a properly trained, qualified examiner operating within a rigorous quality assurance/quality control program, provides credible and reliable results.

Hair identification is not employed solely by forensic scientists. Hair identification is an important tool used by wildlife biologists, archeologists, anthropologists, and textile conservators. Many researchers have investigated the morphological characteristics of hair, devised keys, and reviewed the science of animal-hair identification (Appleyard 1960 Day 1966 Mathiak 1938 Mayer 1952 Moore 1974 Oyer 1939 Stains 1958, 1962 Wildman 1954, 1961 Williams 1938). These works have aided in ecological studies, food-habit studies, and law enforcement investigations by providing descriptions, keys, and photographs of the microscopic characteristics of animal hairs.

Brown (1942) attempted to develop a technique for identifying hairs and wools from various types of materials recovered from archeological works. Hausman (1930) used hair examination in his laboratory to perform archeological work, examine stomach remains, identify fur, and conduct legal proceedings.

Animal-hair studies also have been conducted within the field of forensic science. Peabody et al. (1983) determined that the medullary fraction could be used to reliably distinguish between dogs and cats. Hicks (1977) and Deedrick and Koch (2004a) described the microscopic characteristics that can be used to discriminate between animal hairs that are most likely to be encountered in forensic casework.

It is important to note that although microscopic analysis and comparison of animal hairs can be conducted, the significance ascribed to an animal-hair association often is less than that of a human-hair association. Accordingly, when an animal-hair association is reported, this decreased significance must be highlighted. For example, in a report for a dog-hair association, the FBI Laboratory would use a statement similar to the following:

It should be noted that dog hairs do not possess enough individual microscopic characteristics to associate a questioned hair to a particular dog to the exclusion of other dogs of a similar breed.

Despite the difference in the significance of animal-hair comparisons to human-hair comparisons, it does not detract from its potential usefulness in a forensic investigation. The presence of a dog hair on an item from the victim that can be microscopically associated with the known hair sample from the suspect’s dog may be very important.

Like the analysis of animal hair, the analysis of human hair is not conducted solely by forensic scientists. Hairs are analyzed by the cosmetics industry in the area of hair-care products and by the medical field in many areas such as nutritional status and toxic-element levels, as well as for certain dermatological diseases. However, the microscopic characterization and comparison of human hairs are largely the domain of forensic scientists.

The first reported use of forensic human-hair comparison was by Rudolf Virchow in 1861 (as cited in Bisbing 1982). He reported the following:

The greatest majority of the hairs of the victim represent a thorough and complete accord with the hairs found on the defendant that there exists no technical ground opposite to looking at the hairs found on the defendant as being the hairs of the victim. . . . However, the hairs found on the defendant do not possess any so pronounced peculiarities or individualities that no one with certainty has the right to assert that they must have originated from the head of the victim (as cited in Bisbing 1982).

Paul Kirk conducted some of the first studies on the potential forensic application of microscopic comparison of hair in the United States (Gamble and Kirk 1941 Greenwell et al. 1941 Kirk 1940). In addition to his publications on the microscopic characteristics of human hairs, he conducted hair-comparison studies using his criminology students. All of the students were required to compare a single hair to 20 known samples, where all of the known samples were of a similar color and from individuals of a similar age. He reported that no student who completed the routine examination failed to report the association correctly (Kirk 1940). He further stated that although this falls short of individualization, the following five factors must be considered:

(a) that twenty suspects in a single crime is rather an exceptional number (b) that the eliminative value of a failure to identify the hair as that of any of the suspects is great (c) that time imposes on students a restriction on the total number of hairs that can be examined, and it is impossible as yet to say whether they could equally well pick, e.g., from 100 standards (d) that in any random group of suspects there would be greater normal variations than are present in the selected group of similar hairs used for this exercise and (e) that the students in question have never before examined hairs and are in no sense experts in this examination (Kirk 1940).

Based on his work, Kirk expected that it would be likely to make a determination of individuality using human-hair comparison. Obviously, this has not yet occurred, nor will it likely ever occur. However, his work laid much of the foundation for the methods of microscopic human-hair comparison that remain in use today.

Microscopic Characteristics—Hair Identification

Many characteristics must be considered in microscopic hair identification (Bisbing 1982 Deedrick and Koch 2004b Hicks 1977 Kirk 1974 Lee and DeForest 1984 Moore 1974 Robertson 1999 Saferstein 1995 Seta 1988). Three distinct anatomical regions are associated with hair: the cuticle, the cortex, and the medulla. Using a wooden pencil as an analogy, we can think of the cuticle as the paint on the outside of the pencil, the cortex as the wooden portion of the pencil, and the medulla as the graphite.

The cuticle is the outermost layer of the hair. It protects the hair from environmental insults. The cuticle is composed of flattened, scale-like cells, which overlap one another much like the scales on a fish or shingles on a roof. These scales slope outward from their attachment point on the cortex, and their free ends point toward the tip of the hair. The free ends interlock with the cells of the inner root sheath and hold the hair in the follicle. In human hairs, the scales form an imbricate pattern, that is, they have no repeating pattern. This characteristic serves to distinguish human hairs from animal hairs many animal hairs have a very regular, repeating pattern to their scales.

A number of microscopic characteristics associated with the cuticle are used in a hair comparison. The thickness of the cuticle, the variation in the thickness, the presence of pigment, and the color are all useful characteristics. In addition, the nature of the outer cuticular margin may be smooth, looped, ragged, or damaged. When damage or artificial treatment to the hair is extreme, the cuticle may be removed, thereby causing damage to the next innermost region of the hair, the cortex.

The cortex is the main body of the hair and contains many of the characteristics used in the microscopic comparison process. The cortex is composed of elongated and spindle-shaped cells. The cortex contains the structures that primarily give hair its color, the pigment granules. There are two chemical forms of pigment in human hairs: eumelanin and phaeomelanin. The pigment eumelanin manifests in the colors of brown and black, and phaeomelanin in the colors of yellow and red, with each pigment having a slightly different size and shape. From a forensic standpoint, the organization, density, size, and distribution of these pigment granules are the most informative features of the cortex. They vary tremendously between racial groups, between individuals, and, to a much lesser extent, even within an individual.

In addition to pigment granules, small air spaces called cortical fusi are found in the cuticle. These air spaces form during the keratinization process of the hair. They are readily observed with compound microscopy and are typically found near the root end of the hair.

The final structures associated with the cortex are the ovoid bodies. These are large, well-defined, oval-shaped structures that may be found dispersed throughout the hair. According to Robertson (1999), ovoid bodies are well-defined, highly dense clumps of undispersed pigment. Their presence is not rare in human hair, but neither are they commonly seen.

In addition to the cortical structures just discussed, a number of characteristics are associated with the cortical cells themselves. Their texture, size, damage, and shape are all useful characteristics in the comparison process.

Another region of the hair is the innermost layer of cells called the medulla. This layer of cells may be continuous, discontinuous, fragmentary, or absent.  In animal hairs, the medullary structure often is used to identify the family and sometimes the species of animal. In comparison to animal hairs, human hairs have no regular structure or pattern. The medullary cells may appear opaque or translucent or may vary even within a single hair. The opaque regions contain trapped air, and the translucent regions are caused by the mounting medium’s having displaced the air. The diameter of the medulla is also a useful characteristic in the identification and comparison process.

In addition to the three anatomical regions of the hair, many other characteristics are useful in the microscopic comparison process. These characteristics relate to the growth stage, environmental influences, and disease influences on the hair.

The nature of the root may contribute information regarding the species of origin of the hair, and in addition, the growth stage of the hair when it was separated from the body. In animal hairs, the root morphology may assist in the identification of the group of animals from which the hair came. For example, the hairs of dogs, cattle, horses, and members of the deer family have very distinct root morphologies from one another and from humans. In human hairs, the nature of the root depends on the growth stage of the hair. Many authors have offered classification schemes for the description of hair roots (Bisbing 1982 Harding and Rogers 1984 Lee and DeForest 1984 McCrone 1982 Shaffer 1982 Strauss 1983).

Hair grows from the dermal papillae, which lie in the base of the hair follicle. As new material is added to the hair, the “older” portion of the hair is slowly pushed out of the follicle until the hair naturally sheds from the body. At any given time, between 80 and 95 percent of the hairs on the human body are in an actively growing, or anagen, phase (Orentreich 1969 Pinkus 1981 Zviak and Dawber 1986).

The presence of an anagen root implies that some amount of force was required to remove the hair from the body (Ludwig 1969). The hair is still actively growing and is therefore still attached to the follicle. No statement can typically be made as to how much force was required however, one would not expect to see this stage of root on hairs that fell from the body as a normal part of daily activities.

The second growth phase of hair is a transitional stage, called the catagen stage. During this short phase, the bulbous root of the hair begins to develop. At any given time, approximately 2 percent of hairs are in this growth phase (Pinkus 1981). No consensus exists with regard to the microscopic characteristics that are specific to this growth phase.

The third and final growth phase of hair is the dormant stage, called the telogen stage. These hairs are characterized by decreased pigment near the root, lack of medullation near the root, and increased cortical fusi near the root (Petraco 1988). In telogen hairs, the bulbous root is fully formed and is no longer attached to the dermal papillae. The hair is anchored in the follicle because of the interlocking cuticular scales of the hair and the inner root sheath of the hair follicle. Approximately 10 to 20 percent of the hairs are in this growth phase (Orentreich 1969 Pinkus 1981 Zviak and Dawber 1986).

In addition to the characteristics resulting from the growth phase of the hair, other characteristics are associated with the root. At times, when hairs are forcibly removed, follicular material may be attached. This material may be suitable for nuclear DNA analysis, if warranted. In forensic cases, hairs from dead bodies are sometimes examined. The decomposition process can impart specific characteristics on hairs (Linch and Prahlow 2001 Petraco et al. 1988), which can be used in the comparison process.

Hair color depends on the pigment granules present in the hair and on the other physical properties that affect how light is transmitted through the hair. Hair color is a useful feature in the hair-comparison process. Within an individual, hair color will show a degree of variation. In fact, variation may be observed within a single hair because of differences in exposure to the environment. However, the degree of variation within an individual is less than the variation among individuals (Robertson 1999).

Many authors have offered classification schemes for hair color (Bisbing 1982 Gaudette and Keeping 1974 Harding and Rogers 1984 Lee and DeForest 1984 McCrone 1982 Robertson 1982 Strauss 1983 Trotter 1939). Regardless of the classification scheme used, the hue (color shade), the value (light versus dark), and the intensity (saturation) must be considered during the comparison process (Hicks 1977).

The tip, or the distal end, of the hair also can vary greatly in morphology. The tip of a newly formed hair will taper naturally to a point. As the hair is subjected to grooming, abrasion, cutting, and possibly artificial treatment, microscopic characteristics are imparted to the tip of the hair. As they have with hair roots and hair color, numerous authors have offered classification schemes for the nature of hair tips (Bisbing 1982 Gaudette 1976 Gaudette and Keeping 1974 Harding and Rogers 1984 Lee and DeForest 1984 Robertson and Aitken 1986 Shaffer 1982). Regardless of the classification scheme used, the nature of the tip must be considered during the comparison process.

The length of the hair should be considered, keeping in mind that hairs may have been cut in the time between the deposition of the hair at the crime scene and the collection of the known sample. Conversely, the known hair sample may have grown if a significant length of time has elapsed between the deposition of the hair and the collection of the known sample. These additional factors must be considered during the comparison process.

The diameter of the hair is another feature that can be used in the comparison process. The overall shaft diameter may range from very fine (40󈞞 micrometers) to very coarse (110� micrometers). The diameter of the hair plays a significant role in the classification of racial group and determination of the body area from which the hair may have arisen.

The presence of artificial treatment may give the hair a characteristic color. Bleaching will remove pigment from the hair and give a Caucasian hair a characteristic yellow color. Dyes will add color to the hair and often result in a hair color that is outside the normal range of color expected in human hairs. Repeated artificial treatments will result in distinct regions of varying color.

When an artificial treatment is applied to the hair, it often results in a line of demarcation, that is, a notable change in color along the length of the hair. This is due to the treatment’s interaction with the hair shaft down to the skin layer. As the hair continues to grow, the newly formed hair is not subjected to the same treatment and retains its natural color, resulting in the line of demarcation between the treated and untreated areas. Artificial treatment may also result in changes to the color of the cuticle, either within the cells of the cuticle or on the exterior of the cuticle.

Any damage present in the hair also should be noted. Cutting a hair with scissors typically results in a sheared or squared-cut appearance. This can be contrasted with hairs that are cut with a razor, which typically have an angular-cut appearance. Hairs that have been crushed, broken, burned, or chewed by insects all have very distinctive characteristics. These characteristics provide value to the comparison process, such as finding crushed or damaged hairs on a tool used to strike the head of the victim.

Diseases and other hair abnormalities may be informative to the hair-comparison process. A number of diseases may cause specific microscopic characteristics to appear in the hair. Seta et al. (1988) summarized the diseases and abnormalities that can result in these characteristics. Because these disease conditions are very rare, considerable weight is given to the presence of these characteristics.

Hairs are remarkably robust, retaining their comparable microscopic characteristics for a very long time, making them very suitable for forensic analysis. Hairs recovered from Ice Age sites, between 10,000 and 18,000 years old, were still able to be identified as human hairs. In fact, one hair still had its follicle attached (Bonnichsen and Schneider 1995). In its reference collection, the FBI Laboratory has hair samples collected from mummies identified to be more than 2000 years old (Oien unpublished data).

Transfer and Persistence of Hairs

The primary mechanism for the transfer of trace evidence is described by the Locard Exchange Principle (Locard 1930). Although there will always be a transfer of trace evidence, in some instances, the material exchanged may be too small to detect or may be rapidly lost. Numerous authors have addressed the transfer and persistence of fibers in forensic cases, including Kidd and Robertson 1982 Pounds and Smalldon 1975a, 1975b, 1975c and Robertson et al. 1982.

These authors investigated the mechanisms involved in the transfer of textile fibers and the persistence of the fibers after the transfer occurred. Although these studies primarily involved textile fibers, wool fibers were used in these studies therefore, the results of these studies also apply to human hair. These authors found that the number of fibers transferred depended on the amount of pressure involved in the contact and the duration of the contact.

With regard to persistence, these authors found that the nature of the recipient garment, the size of the transferred fiber, and the movement of the recipient garment had a dramatic effect. If the garment containing transferred fibers was worn, most fibers were lost rather quickly (within a few hours). If the garment containing transferred fibers was held in a fume hood, the rate at which fibers were lost was much lower.

Gaudette and Tessarolo (1987) stated that many of the variables affecting fiber transfer and persistence were also important in hair transfer and persistence. In order to document some of these variables, they conducted several experiments on hair transfer. They identified two mechanisms of hair transfer: primary and secondary transfer.

Primary transfer can be either direct (from person A’s scalp to another location) or indirect (from person A’s scalp to person A’s environment and then to another location). Secondary transfer is an indirect transfer (from person A’s environment to person B’s environment to person C’s environment). The authors demonstrated that secondary transfer of human scalp hair can and does occur in casework situations and that persistence of the transferred hair is similar to that previously found for fibers by Pounds and Smalldon (1975a, 1975b, 1975c). Robertson and Somerset (1987) conducted a similar study on persistence and found comparable results that is, most transferred hairs are lost with normal wear after about three hours.

Quill (1985) recovered 81 hairs from his clothing over a 31-day period. Of the hairs that were suitable for microscopic comparison, all had been transferred from family members. Quill concluded that for a foreign hair to be present on clothing, close personal contact is required. Simons (1986) found that although most hairs are removed from clothing during the laundering process, some hairs do remain on clothing and hair transfers can occur as a result of the laundering process.

Peabody et al. (1985) investigated the shedding of hairs into various types of headgear. They found that the number of hairs shed varies with the type of headgear worn and with the individual. They also noted the importance of collecting head-hair combings, because the nature of the hairs shed in their study were more similar to the naturally shed hairs encountered in combings than to the hairs encountered in plucked, known head-hair samples.

Based on these studies, it can be concluded that it is reasonable to find hair evidence in forensic cases. Hair is ubiquitous in the environment and, therefore, may be transferred during a crime. However, it is imperative for proper and timely collection of evidentiary materials, including known hair samples, to occur if hair examinations are going to be valid, reliable, and meaningful.

Hair-Collection Process

In order for hair evidence to be meaningful, the hair must not only be transferred, it must persist and be recovered as evidence. There are two distinct locations where evidence recovery typically occurs: at the crime scene and at the laboratory. Because of the potential loss of hair evidence, it is crucial that evidentiary items be collected as soon as possible properly packaged to prevent loss, contamination, or deleterious change and transported to the laboratory expeditiously. Any additional handling or wear increases the likelihood that the evidence will be lost.

Once the evidentiary item is transported to the laboratory, it must be handled in much the same manner. If the evidentiary item is going to be subjected to a variety of forensic disciplines, it is imperative that the trace evidence (including the hairs) be recovered before these other disciplines analyze the evidence, to protect against loss, contamination, or deleterious change. A number of techniques may be employed to collect and preserve the debris from the evidentiary item, including but not limited to taping, scraping, picking, and vacuuming.

Once the debris has been collected and preserved, the next step involves low-magnification microscopic analysis of the debris. Using a stereomicroscope, the debris is examined, and hairs are removed from the debris and mounted on glass microscope slides. This allows the hairs to be examined using a high-magnification compound microscope. Depending upon the number of hairs encountered in the debris sample, all of the hairs can be mounted on glass microscope slides, or a representative sample can be mounted.

Two methods may be used in determining which hairs are mounted when a representative sample is employed. In the first method, a sample of each of the distinct types of hairs observed with the stereomicroscope can be mounted, that is, some of each color, length, diameter, and texture.

The second method involves using a targeted search. This may be used in cases where known hair samples are submitted along with the evidentiary items. The known samples may be examined and mounted first. Once proper precautions have been taken to prevent contamination (cleaning the work area, cleaning the tools, changing gloves), the debris from the evidentiary items is then examined. Hairs macroscopically similar to those in the previously mounted known hair samples can be identified and preserved on glass microscope slides.

After the hairs have been preserved on glass microscope slides, they can then be examined with a high-magnification compound microscope. Using magnification ranges from 50x to 400x, the microscopic characteristics can be observed. Based on the analysis of these microscopic characteristics, a number of possible determinations can be made.

Race and Body-Area Identification

A human hair can be classified into one of three racial groups: Caucasian, Negroid, or Mongoloid. A classification of Caucasian typically means of European descent. Negroid typically means of Sub-Saharan African descent. Mongoloid typically means of Asian or Native American descent. It must be understood that designation of these racial groups is based upon an evaluation of the microscopic characteristics present in the hair. The microscopic designation of racial group may or may not coincide with how a person self-identifies his or her racial group.

If a hair or a hair sample cannot be easily associated with a particular racial designation, these hairs may be described as either exhibiting mixed racial characteristics or as not classifiable to one of the three groups. Even if a hair or a hair sample cannot be classified as to race, it still may be of value for meaningful microscopic comparison purposes. The inability to classify a hair into only one of these three groups serves as an additional characteristic that can be used in the comparison process.

A human hair also can be classified as to the region of the body from which it came. Using the same features listed previously, this designation can be made with considerable accuracy. Typically, the body-area determinations that can be made are head hairs (from the scalp), pubic hairs, facial hairs (beard and mustache), limb hairs (arm/leg), chest hairs, axillary hairs (armpit), and eyebrow/eyelash hairs. However, hairs may be encountered that cannot be categorized into one of these groups. These may consist of “transitional” hairs, that is, those hairs growing between two body regions, hair fragments that are not large enough to be identified, or hairs from other body areas.

Procedure for Microscopic Hair Comparisons

Once the race and body-area determinations have been made, the suitability for comparison is determined. Hairs that have been characterized as head hairs or pubic hairs are generally considered suitable for comparison with a known head-hair or pubic-hair sample. Hairs from other body areas generally are not considered suitable for comparison because these other body-area hairs generally do not contain sufficient variation in their microscopic characteristics to reliably distinguish between hairs from different individuals. In limited circumstances and with limited significance ascribed to an association, a microscopic comparison may be conducted between these other body-area hairs and an appropriate known sample. However, the limitations of these comparisons must be understood by the hair examiner and conveyed in a report.

Once a hair has been determined to be suitable for microscopic comparison, it is compared with an appropriate known hair sample. Head hairs must be compared with known head-hair samples, and pubic hairs must be compared to known pubic-hair samples.

The comparison process involves the side-by-side analysis of a questioned hair and known hair samples using a comparison microscope. This allows for a direct comparison of the microscopic characteristics of the questioned hair within the same relative area of the known sample, at the same time and in the same field of view. This comparison must occur over the entire length of the hair.

In 1982, an ad-hoc Committee on Forensic Hair Comparisons was formed with Barry Gaudette as the chairman. This committee represented 10 U.S. states, Canada, and Great Britain laboratories and included representatives from law enforcement laboratories, private laboratories, academic institutions, and the National Bureau of Standards. This group of highly qualified hair examiners met to advance forensic hair comparison as a science (as cited in Federal Bureau of Investigation [FBI] 1985).

After two meetings, the committee published its recommendations in the following seven areas:

  1. Terminology definition and standardization.
  1. Establishment of a protocol for microscopical human-hair comparison.
  1. Investigation and standardization of macroscopic and microscopic hair comparison characteristics.
  1. Conclusion, report writing, and court testimony in forensic hair comparison.
  1. Training of hair examiners.
  1. Quality assurance in forensic hair comparison.
  1. Nonmicroscopical methods of forensic hair comparison. (as cited in FBI 1985)

These meetings were followed in 1985 by an International Symposium on Forensic Hair Comparisons.

The Committee on Forensic Hair Comparison recommended using a comparison microscope, stating that the hair examiner must use a high-quality comparison microscope at different magnifications to conduct a thorough and careful examination of both the gross and microscopic characteristics exhibited by properly prepared hairs (as cited in FBI 1985). Robertson (1999) also expressed the opinion that the microscopic comparison of hairs cannot be conducted without a comparison microscope.

The Scientific Working Group on Materials Analysis (SWGMAT), which evolved from the original Committee on Forensic Hair Comparison, has one of its subgroups dedicated to forensic hair comparisons. SWGMAT states that the use of a high-quality transmitted light microscope is necessary to examine and identify the microscopic characteristics of hairs (SWGMAT 2005).

A number of authors have published examination procedures for the forensic examination of hair, including the Committee on Forensic Hair Comparison (as cited in FBI 1985), Robertson (1999), Shaffer (1982), Strauss (1983), and SWGMAT (2005). According to the SWGMAT guidelines (2005), in order to conclude that a questioned hair and a known sample are consistent with sharing a common origin (association), it must be determined that there are no significant differences between the two. In other words, for a conclusion of an association to be made, it must be determined that the characteristics exhibited by the questioned sample are represented in the known sample.

The starting point in a forensic hair examination must be an attempt to look for differences, not similarities, between a questioned hair and a known sample (Robertson 1999). Even within an individual, the expectation must be that a known sample will exhibit a range of microscopic characteristics. Because hairs are a biological product and there are both genotypic and phenotypic influences on the arrangement of their microscopic characteristics, no two hairs, even from the same person, can look exactly alike. Therefore, even when an association is made, it does not mean that the questioned hair and a single hair from the known sample will be identical in all features along the entire length of the hairs.

The assessment of what is or is not a meaningful or significant difference lies at the core of the training and experience of the forensic hair examiner. These examinations should be conducted only by a properly trained hair examiner employing a side-by-side comparison of a questioned hair and a known sample.

Three general conclusions can be reached as a result of microscopic hair analysis: exclusion, no conclusion, or association. Within the categories of exclusion and association, there are two subcategories (see Gaudette 1985). When a questioned hair (a hair of unknown origin) is compared with a known hair sample (a sample of hairs removed from a particular body area of a person) and differences in the observed microscopic characteristics are found, the hair examiner can conclude that the questioned hair is not consistent with originating from the source or donor of the known hair sample.

Is it possible to definitively exclude an evidence sample as originating from a particular donor based on microscopic hair comparisons? Stating categorically that a questioned hair definitely did not come from the donor of a known sample implies that the probability of an incorrect exclusion is zero (or so small to be effectively zero). Some possible causes of an incorrect exclusion are examiner error, too few reference hairs comprising the known sample, the known sample is not representative of the body region, incomplete hairs in the known sample, a large amount of time between the deposition of the questioned hair and the collection of the known sample, and the possibility that the questioned sample is an atypical hair. If an examiner is properly trained and follows appropriate procedures, then the probability of an incorrect exclusion based on these possible scenarios is minimal.

Regardless, a hair examiner is limited when providing an interpretation that the hair definitely did not come from a particular person (as an examiner would be if he or she were to state that the hair definitely came from a particular person—an almost impossible conclusion given the limitations of the evidence and/or technology). Although a false exclusion is not as serious as a false inclusion, there are still consequences associated with a false exclusion.

For example, an examiner can state that the questioned hair (a short, colorless [gray] hair) is not consistent with originating from the suspect, but the number of gray hairs on the suspect’s scalp was so few that they were not represented in the known sample. A categorical exclusion in this case might direct an investigation in an improper course.

There are few circumstances in which an absolute exclusion can be rendered, such as when the questioned hair and the known hair samples exhibit different racial characteristics. In this scenario, the examiner may be able to conclude that “the questioned hair is microscopically dissimilar to hairs in the known hair sample and therefore could not have come from the donor of the known hair sample.”

The scenario of the gray-hair exclusion cited above, however, would not meet any of these potential exclusion circumstances. The examiner would have to conclude that “the questioned hair is microscopically dissimilar to the hairs in the known hair sample, and the questioned hair is not consistent with originating from the source of the known sample.” Although the characteristics of the questioned hair may not encompass the range of characteristics exhibited by the known hair sample, a low percentage of gray hairs may not be seen simply because of sampling error. The great majority of microscopic hair exclusions have similar limitations.

When a significant difference is observed between a questioned hair and a known hair sample, then it must be concluded that the questioned hair and the known hair sample are not consistent with sharing a common origin. More precisely, the hair is not consistent with originating from the donor of the known sample as represented by the hairs present in the known sample.

Another category of conclusion that can be reached by a microscopic-hair examiner is that “no conclusion” can be reached as to whether or not the questioned hair is consistent with originating from the donor of the known sample. This conclusion is reserved for circumstances when the questioned hair exhibits similarities to the hairs in the known sample but also exhibits some slight microscopic differences however, these differences are not sufficient to conclude that the hair is not consistent with originating from the donor of the known sample. Some possible causes for these slight differences are (1) a significant amount of time has occurred between the deposition of the questioned hair and the collection of the known sample (typically more than one year) (2) the questioned hair is significantly longer (or shorter) than the hairs in the known head-hair sample (for example, the donor of the known sample may have cut his or her hair after deposition of the questioned hair) (3) the questioned hair has not been artificially treated, but the known sample has been (4) the questioned hair is not a full-length hair that is, a portion(s) of the hair is/are missing (5) the known sample contains too few hairs for an adequate comparison and (6) the questioned hair comes from a different donor.

The typical wording for a “no conclusion” is: “The questioned hair exhibits similarities and slight microscopic differences, and therefore, no conclusion can be reached as to whether or not the questioned hair is consistent with originating from the donor of the known hair sample.”  When this conclusion is reached, these hairs may be submitted for mitochondrial DNA (mtDNA) analysis.

The final category of conclusion that can be reached in a microscopic hair comparison is that of an association. In contrast to the two subcategories of an exclusion result, there is only one conclusion for association, that the questioned hair exhibits the same microscopic characteristics as the hairs in the known hair sample and therefore cannot be excluded from originating from the source of the known sample. It is possible to contrive a scenario where one might logically report a strong positive association, that is, state that the questioned hair originated from the donor of the known hair sample.

For example, if one knew that the person with the longest hair in the world had hair that was 26 feet long and if a 26-foot-long hair was recovered from a crime scene that exhibited all of the same microscopic characteristics as the hairs in the known hair sample, it would be possible to state that the hair definitely came from that person. However, the likelihood of such an event occurring is so rare that it is not meaningful.

When an association is made, the conclusion would be stated as follows: “The questioned hair exhibits the same microscopic characteristics as the hairs in the known hair sample, and accordingly, the questioned hair is consistent with originating from the same source as the known sample.” This means that all of the microscopic characteristics expressed by the questioned hair are represented within the range of characteristics exhibited by the known hair sample. In other words, no significant differences can be found.

The final step of a hair examination should be a verification or confirmation procedure. This step involves having a second qualified examiner conduct an independent microscopic comparison of the hair that has been associated (SWGMAT 2005). The second examiner must conduct a thorough, complete analysis and must be free to reach his or her own conclusion on each hair. Only upon the second examiner’s reaching agreement should a microscopic hair association be reported. In accordance with FBI Laboratory standard operating procedures, when this conclusion is reached, these hairs are submitted for mtDNA analysis.

Not all conclusions of a hair association can be weighted equally. Consider the following two examples. First, a questioned hair is recovered from a hat that was found on the side of the road and believed to be used in a bank robbery. Microscopic examination reveals that the hair is a blond, Caucasian head hair. Comparison with known hair samples from suspects in a bank robbery results in the questioned hair’s being associated with one of the suspects and excluded from the other two suspects.

Second, a questioned hair is recovered from the backseat of a vehicle used to transport the victim of a kidnapping. Microscopic examination reveals that the hair is a long, dark brown, Caucasian head hair that exhibits characteristics of being artificially treated three separate times over the length of the hair. Comparison with known hair samples from the victim and persons that were known or suspected to be in the car results in the questioned hair’s being associated with the victim and excluded from the persons known or suspected to have been in the car.

In these two scenarios, more relative weight can be ascribed to the hair association in the second scenario, because additional characteristics are present for comparison, namely the artificial treatment (dark brown color) and the length of the hair. Because there is no known mechanism for a hair examiner to quantitatively assess the additional weight of these special characteristics, it is incumbent upon the hair examiner to explain this to the trier of fact. Fact finders, from their own experiences, can assess the significance of the results conveyed by the hair examiner.

Most important, when conducting hair examinations, properly trained and qualified examiners would not and should not opine that a hair could be attributed to an individual to the exclusion of all others. This basic tenet of the science has been espoused since the inception of the discipline (as first described by Rudolf Virchow in 1861 cited in Bisbing 1982). The hair examiner must convey, both in the written report and in courtroom testimony, the limitations of the science and, especially, an interpretation of an association.

In order to ensure that proper weight is put on the results and interpretation of a microscopic hair comparison, FBI Laboratory reports for the past 40 years have stated that hair comparisons are not a means of individualization in reports of examination containing microscopic hair comparisons. In addition, FBI hair examiners are trained to include, at a minimum, the same information during their testimony on cases involving a hair association.

Not only must fact finders be aware of the conclusion reached by the examiner, they also must be aware of the limitations of the science so they can properly appreciate the significance of a result. Providing an explanation of the results will best overcome possible confusion from similar but differently phrased conclusions such as “is consistent with” or “could have come from.” Regardless of the phraseology used, the fact finder must be given supporting information beyond the statement of an association.

Some critics emphasize the fact that microscopic-hair examiners are unable to statistically quantify the significance of an association (see, for example, Robertson 1999). The development of a statistical model would involve frequency data across the entire population for all microscopic characteristics present in hair. Although this is an attractive idea, the difficulties associated with generating such a database have been, to date, practically insurmountable. In order to generate frequency data for hair characteristics, microscopic-hair examiners might be required to use a “checklist” or “archetype” approach rather than the pattern-recognition process normally used.

However, two hairs that may be “alike” based on a checklist may very well be microscopically different (see Gaudette and Keeping 1974 Strauss 1983). In addition, different examiners are likely to describe hairs in different ways (see Gaudette and Keeping 1974 Podolak and Blythe 1985). Finally, the same examiner may vary his or her description of the same hair on different days (see Wickenheiser and Hepworth 1990).

These examples do not reflect a flaw in the science of microscopic hair comparisons or an error by a microscopic-hair examiner rather, they serve to highlight the limitations of generating a useful database. The database approach confines the examiner to documenting the status of single characteristics at a specific location in a single focal plane as opposed to a holistic approach. That characteristic may change slightly at a different focal plane even at the exact same location and may change dramatically at a different location in the hair. In a single hair, there are hundreds or even thousands of possible different fields of view.

It has been pointed out previously (see Wickenheiser and Hepworth 1990) that this classification method would force the examiner to choose, subjectively, from the myriad possible fields of view, the one that best represents that characteristic. This subjective choice then must be repeated for all of the remaining microscopic characteristics present in hair. Because of the inherent variation in the microscopic characteristics of hair, the use of such an approach likely would result in a situation where two hairs that “match” according to the checklist in reality look nothing alike.

Given that useful statistical data are not generated regarding the relative frequency of an evidentiary hair, one must accept that the answer to the question, what proportion of the population would have characteristics that are the same as the evidentiary hair? is we do not know. Similarly, the answer to the question, what is the probability of a coincidental match between the questioned hair and the known sample? is we do not know. Rather, the fundamental question that can be addressed is, what is the value of the evidence in establishing the association? (see Gaudette 1986). Numerous empirical studies exist detailing the ability of microscopic hair comparisons to reach the correct conclusion these empirical studies can provide some guidance on the significance of an association.

Studies Supporting Microscopic Hair Comparison

Numerous studies have been conducted that support the science of microscopic hair comparisons. Strauss (1983) conducted a study using 100 individuals comprising 54 Caucasian, 19 Negroid, and 27 Mongoloid. From each of the 100 individuals, 7 hairs were chosen to represent the widest variation possible. These were mounted on glass microscope slides and were designated as the known samples. One hair was also chosen from each of the 100 samples, mounted on glass microscope slides, and designated as questioned hair samples. All 800 hairs (700 known hairs and 100 questioned hairs) were individually characterized using a checklist and punch cards.

A series of seven experiments was conducted. A neutral party selected a total of 10 single questioned hairs to be compared with 10 known samples. Comparison microscopy resulted in 100 percent accuracy in associating the correct questioned hair with its known source, showing that they could reliably associate a questioned hair with a known sample. In addition, the study showed that the examiners correctly identified each of the 100 individuals in the questioned hair pool to the correct known hair group, that is, 54 Caucasian, 19 Negroid, and 27 Mongoloid.

Gaudette and Keeping (1974) obtained head-hair samples from 100 individuals. Within the group, 92 were Caucasian, 6 were Mongoloid, and 2 were Negroid hairs. From each of these samples, 6 to 11 macroscopically dissimilar hairs were selected to represent the range of microscopic characteristics present in the known sample. These hairs were then mounted individually on glass microscope slides. The hairs were characterized, and the microscopic characteristics were categorized using punch cards. Each hole in the punch card was associated with a specific microscopic characteristic. The cards from each individual were combined with all of the others and were sorted based on similar holes in the punch cards. The hairs for each of these similar cards were then compared microscopically. Using this system, a total of 861 hairs from 100 different individuals were examined and compared, for a total of 370,230 comparisons. From all of these comparisons, only 9 pairs of hairs were found to be indistinguishable.

In a similar study, Gaudette (1976) obtained 30 pulled pubic hairs from 60 different individuals. All of these were Caucasian hairs. From these, 6 to 11 dissimilar hairs were selected randomly to represent the range of characteristics present in the 30 hairs. As in the previous study, the characteristics were coded on punch cards, and the cards were combined and sorted. With the 454 hairs, the total number of comparisons made was 102,831. A total of 16 pairs of hairs were found to be indistinguishable.

From each of these studies, the authors attempted to obtain probability estimates for head-hair and pubic-hair comparisons. The probability estimates proposed by Gaudette and Keeping (1974) and Gaudette (1976) for the frequency of head and pubic hairs cannot apply to the population at large. The probabilities they derived refer to the process of distinguishing between two hairs that the examiner knew originated from different people. In addition, the authors found that different examiners obtained different results in the single-hair comparison study. This would mean that even if the data were correct, the probability estimate would have to be generated by each new person using the technique. This is not comparable to the normal casework scenario for a microscopic-hair examiner, where a questioned hair is compared to known hair samples (Barnett and Ogle 1982). In a later paper, Gaudette (1978) stated that “the significance of this research is not in the actual probability numbers found but in experimental proof of the proposition that macroscopic and microscopic hair comparison is a useful technique and that hair evidence is good evidence” (Gaudette 1978).

In the same study (1978), Gaudette provided each of three examiner trainees with one separate known head-hair sample, consisting of 80 scalp hairs. Each of the trainees was then given 100 questioned hairs from different individuals, one of which was the one represented by the known standard. Without being told how many individuals the unknowns were from or how many, if any, of the hairs were supposed to be similar to the standard, the trainees were instructed to compare the questioned hairs with the known standards and report their results. Two of the trainees correctly identified one and only one hair with the known standard.

The third trainee initially concluded that there were four hairs similar to the standard. However, upon further examination and consultation with other examiners, he was able to eliminate one of the four. However, he still concluded that the three remaining hairs could not be eliminated: the correct one and two others. All of the hairs that remained were of the common, featureless type.

Another experienced examiner evaluated the three remaining hairs and concluded that the correct hair could not be eliminated and, in addition, that one of the two others could not be eliminated. Yet another examiner looked at the remaining three hairs and agreed that the correct hair could not be eliminated and, in addition, that the other of the remaining two could not be eliminated.

Another experiment was conducted, again using 100 representative scalp hairs from 100 individuals. From these, one sample was selected. From this sample, a single hair was then selected at random. Thus, there was one questioned hair that was compared with 100 known standards. This experiment was then repeated. On both occasions, it was found that the unknown hair could be associated with one and only one standard—the correct one.

In a third experiment, the unknown hair was chosen specifically to be a common, featureless hair. This hair was found to be similar to two standards: the correct one and one additional one.

Based on these series of experiments, Gaudette found that when an experienced hair examiner conducts a hair comparison using all of the available microscopic characteristics, these comparisons were reliable and repeatable. He also offered that special training in hair comparison of at least a year in length is required to enable a person to develop the required level of discrimination.

Wickenheiser and Hepworth (1990) obtained head-hair samples from 97 different individuals, including a number of closely related individuals from several generations. Between 5 and 13 dissimilar hairs from each sample were chosen to represent the range of characteristics in the known sample. These were numbered randomly by an independent party. In addition, 53 additional hairs randomly chosen from the original 97 known samples were also numbered randomly. In total, 930 hairs were selected and placed on glass microscope slides. All of these hairs were examined to determine how many matching pairs were present. Each hair was compared to all of the other 929 hairs, for a total of 431,985 hair comparisons.

Two different examiners developed comparison checklists and used a computer program to sort the hairs based on these checklists. As a result of the computer sorting based on gross macroscopic and microscopic characteristics, the first examiner conducted 749 one-to-one microscopic comparisons, and the second examiner conducted 2006 comparisons. The first examiner found seven pairs of hairs that were microscopically indistinguishable, and the second examiner found six pairs. In every case where a one-to-one association was found, the hairs were truly from the same source. No incorrect associations were made by either examiner. Based on their findings, the authors determined that if a one-to-one match is the requirement in a microscopic hair comparison, then the incidence for an error is very low.

Bisbing and Wolner (1984) conducted a series of studies using known head-hair samples recovered from 17 sets of twins and 1 set of identical triplets. Of the twins included in the study, 9 were fraternal twins, 6 were identical twins, and 2 were of unknown zygosity. All of the twins were Caucasian, and 11 of the 18 sets were blond. In addition, all of the samples were cut. The authors commented that the predominance of blond hair and the absence of hair roots made these comparisons unusually difficult. In fact, many of the samples were considered by the authors to be common, featureless hairs.

From each of the individuals in the study, two known hair samples were mounted on glass microscope slides and were assigned a random number. This resulted in a total of 74 known samples. The authors then conducted comparisons of each twin sample with all other samples. By visual and microscopic examination, both authors were able to correctly distinguish all of the known samples and were able to accurately associate the duplicate samples with each other. The specimens were never incorrectly associated, even with the known hair sample of the twin.

In order to more closely resemble true forensic casework, a second study was conducted. This study involved removing 2 or 3 hairs from 7 randomly selected unmounted samples, which were then mounted on glass microscope slides. For each of these 7 “questioned” samples, between 5 and 10 known samples were randomly selected from the 74 mounted known samples for microscopic comparison. There were 52 comparisons made by each of the two examiners, for a total of 104 comparisons. Because of the random sampling, none of the true known samples for the questioned hairs was present in any of the comparison scenarios. The two examiners correctly excluded 96 of the known samples as being possible donors of the questioned hairs. Eight of the questioned hair samples were incorrectly associated to the known samples (5 by one examiner and 3 by the second examiner). In one of these cases, a sample of the fraternal twin’s hair was present in the known pool and was correctly eliminated. In the other simulated cases, the questioned hairs were incorrectly associated with control samples that were neither the true source nor the twin of the true source.

It is interesting to note that 7 of the 8 incorrectly associated hairs were classified by the authors as being blond, common, featureless hairs. These results serve to reinforce that human hair cannot be associated with one person to the exclusion of all others. In addition, this study served to show that caution is necessary when comparing common, featureless hairs. Finally, the authors stated that the verification process might measurably reduce the possibility of Type II errors.

Suzanski (1988) conducted a blind study involving comparison of 15 questioned hairs with known hair samples obtained from 25 purebred German shepherd dogs. He made no false inclusions and correctly assigned 6 of the 15 questioned hairs to their known sample of origin. In a later study (1989), Suzanski compared 25 questioned hair samples of approximately 10 hairs each with known samples from 100 mixed-breed and purebred dogs. He was able to assign all 25 of the questioned hair samples to the known samples, with no incorrect associations.

From these studies, we can conclude that microscopic hair comparisons are reliable and are indeed a valid scientific method. If a properly trained hair examiner uses a valid procedure, the examiner can achieve the correct result. It is important to note that hairs are not a means of personal identification, and this information must be conveyed both in a written report and during testimony. It is acknowledged that the microscopic characteristics exhibited by a questioned hair can be encompassed by the range of characteristics exhibited by more than one person. However, if an examiner associates a questioned hair with a known sample that is known to be from a different person, it does not imply an error or a mistake on the part of the microscopic-hair examiner. Rather, it highlights the limitations of the science.

For the past 100 years, microscopic hair comparisons have been the only method available to determine if an association exists between two people or between a person and an object based on hairs recovered from evidentiary items. These comparisons have been routinely conducted in forensic laboratories and accepted both in the scientific community and in the legal community for the past 75 years. Because of the limits of the science of microscopic hair comparison, the strongest conclusion that a microscopic-hair examiner can ever make is that a hair “is consistent with” or “could have come from” the donor of the known sample. However, the studies cited above do show that the method is reliable and repeatable. In addition, these studies demonstrate both the exclusionary and a degree of inclusionary power of microscopic hair comparison.

A wealth of information can be gained from the microscopic analysis and comparison of hairs—information that may be crucial to a case, such as the ability to exclude persons who are not the source of an evidence hair. Fortunately, another tool is available to augment microscopic hair analysis. With the advent of DNA testing, both nuclear and mitochondrial, an additional, independent examination is available to compare the questioned hair and the donor of the known hair sample.

DNA Analysis of Hairs

The tools of molecular biology have enabled forensic scientists to characterize biological evidence at the DNA level, both nuclear and mtDNA. Any material, including hair, that contains nucleated cells can potentially be exploited using nuclear DNA typing. Typically, these hairs must contain sheath material for nuclear DNA typing to be successful. When this material is found, nuclear DNA testing is the best, most discriminating technique available for the comparison of a questioned hair with a known sample. No other technique in the field of hair comparison can result in the potential individualization of the questioned hair to a known source. When a questioned hair can be microscopically associated with a known sample and sufficient root material is present, this hair should be subjected to nuclear DNA analysis.

Kolowski et al. (2004) conducted a study to evaluate the accuracy of short tandem repeat (STR) DNA analysis and microscopic comparison. Pubic-hair samples consisting of at least 50 hairs were collected from 27 volunteers employed in their laboratory. Laboratory volunteers were used because each employee’s DNA was present in a laboratory database. From these samples, a neutral third party created five sets of four hairs each to act as questioned hair samples. Of each of these samples, two of the hairs had to be in the anagen, or actively growing, phase. For the known hair samples, five hairs were selected and mounted on glass microscope slides. The root end of each of the two anagen hairs from each sample was cut, the nuclear DNA was extracted, and this extract was analyzed using STR typing technology. The remaining portion of these two hairs as well as the two remaining hairs from the questioned hair samples were then compared microscopically with five of the hairs selected from the known pubic-hair samples according to SWGMAT guidelines (2005).

Short tandem repeat typing of the unknown hair samples correctly identified the source of three of the questioned hair samples. Of the two remaining hair samples, one could not be DNA-typed using either of the questioned hairs in the sample, and therefore, no conclusion could be reached. On the final questioned hair sample, no DNA profile was obtained from the first hair, and only a partial profile could be obtained from the second hair. Because it was only a partial profile, the profile could be attributed to two people present in the laboratory staff database. Overall, STR analysis was able to identify only three out of five unknown hairs with their known source. The microscopic hair analysis was able to associate four of the five questioned hair samples with their source. One of the five questioned hair samples was incorrectly associated with the incorrect source by microscopic analysis.

The authors noted that the questioned hairs in this set were very small and would not normally have been analyzed in normal casework. In addition to the small size of the hairs in the questioned hair sample incorrectly associated with a known, it is also important to note that the authors did not mention whether or not a microscopic verification was conducted by an independent examiner. The addition of the verification step may very well have detected the incorrectly associated questioned hair. Also, the authors used only five hairs from each known sample for the microscopic comparison process. This also may have contributed to the incorrectly associated questioned hair sample. SWGMAT guidelines suggest collecting 25 pubic hairs in a pubic-hair known sample and 50 hairs in a known head-hair sample.

Although nuclear DNA analysis is a powerful individualizing tool, it does not lend itself well to screening through a tremendous volume of evidence to identify hairs of interest. Microscopic analysis is better suited to distinguishing between a hair or a textile fiber, an animal hair or a human hair, a head hair or a pubic hair, a Caucasian hair or a Mongoloid hair, and a forcibly removed hair or a naturally shed hair. Microscopic examination also may contribute contextual information regarding the hair in question that may be critically important to the case.

For example, in a case where a husband is accused of bludgeoning his wife with a hammer, finding a hair on the hammer that can be microscopically associated with the victim would likely be of limited significance. However, if the hair also exhibits microscopic characteristics of being crushed or damaged, the significance of the association increases greatly. Similarly, finding hairs that can be microscopically associated with a victim in the trunk of her spouse’s car would not be unusual unless these hairs also exhibited signs of decomposition, showing that they were deposited sometime after the victim’s death.

One of the limiting factors with performing STR analysis on hairs recovered in casework rests in the fact that most of the hairs encountered are shed naturally and contain little nuclear DNA-containing tissue. Naturally shed hairs typically do not provide sufficient amounts of nuclear DNA for analysis. In contrast, the vast majority of these hairs have ample amounts of mtDNA present. Because a cell can contain more than 5000 copies of mtDNA (Bogenhagen and Clayton 1974), this analysis is currently the best approach to the genetic typing of these hairs (DiZinno et al. 1999). The guidelines for this analysis with respect to routine forensic casework were first outlined in Wilson et al. (1993).

Unlike nuclear DNA, mtDNA is maternally inherited (Case and Wallace 1981 Giles et al. 1980 Hutchinson et al. 1974) and is not unique to an individual. Therefore, mtDNA, much like the microscopic analysis of hair, cannot be used to positively identify an individual, but it can be used to exclude a potentially large portion of the population as a possible donor of the hair.

Houck and Budowle (2002) reviewed 170 human hairs submitted to the FBI Laboratory and subjected to both microscopic and mtDNA analysis between 1996 and 2000. The results of the microscopic human-hair comparison were placed in four different categories: a positive association (i.e., cannot exclude) a negative association (i.e., can exclude) an inconclusive result (i.e., cannot render an opinion) or no examination (i.e., insufficient or unsuitable sample to attempt an examination). The results of the mtDNA analysis were placed in four similar categories: concordant (or inclusion), inconclusive, exclusion, or insufficient DNA to reach a conclusion.

Of the 80 hairs that were associated using microscopic hair comparisons, 69 were consistent with mtDNA analysis, 1 was inconclusive, 9 were excluded, and 1 had insufficient mtDNA present. Excluding the inconclusive and insufficient mtDNA results yields agreement between the microscopic and mtDNA hair analysis more than 88 percent of the time. Four of the 9 hairs excluded by mtDNA analysis were categorized microscopically as being blond in color. Blond hairs, as discussed previously, have fewer microscopic characteristics available for use in the microscopic comparison process.

These nine mtDNA exclusions cannot be used as an error rate for the microscopic comparison method rather, they demonstrate the limitation of the resolving power of microscopic comparison of the hairs examined in this study. There were no known mistakes made in the microscopic hair or mtDNA analyses. Although the mtDNA analysis did exclude the donor of the known sample as the source of the questioned hair, it does not mean that the microscopic characteristics of the questioned hair were different than those of the known sample. It does not imply a mistake on the part of the science or the microscopic-hair examiner. As explained previously, microscopic hair comparisons are not the basis for personal identification, and some people will share similar hair morphological characteristics.

Imagine the following scenario: Two brothers are both suspects in a case, and one is the true donor of a hair found at the crime scene. Both of the brothers will have the same mtDNA sequence, and therefore, mtDNA analysis would not be able to exclude a questioned hair as having come from either brother. However, microscopic analysis may be able to exclude one brother and not exclude the other brother as the donor of the hair. This does not imply an error on the part of the mtDNA analysis or the mtDNA examiner but rather identifies the limitation of the technology. Both of the techniques have value when their application is understood.

In Houck and Budowle (2002), there were no apparent differences in the exclusions obtained by either method. Of the 19 hairs excluded by microscopic comparison, 17 were confirmed, and the remaining 2 were either inconclusive or insufficient by mtDNA analysis. Of the 71 hairs that were either inconclusive or where no microscopic examination was conducted (that is, when the hairs were categorized as not suitable for comparison with a known sample), mtDNA analysis was able to give a definitive result of either association or exclusion on 66 of the hairs. Therefore, when an important evidentiary hair recovered from the crime scene cannot be microscopically compared, mtDNA analysis should be sought when possible.

In summary, a combination of microscopic examination of hairs followed by DNA examination (either nuclear or mtDNA) will yield the best possible information on the evidentiary questioned hairs. Because of the destructive nature of both DNA techniques, the microscopic analysis must be conducted first, followed by the DNA examination, because that portion of the hair used in the DNA analysis will be destroyed, making it unavailable for the microscopic examination.

The primary challenge of microscopic hair analysis is in the biological variation of microscopic characteristics that exists within the hairs from a single individual. This is further compounded by the fact that variation exists even along the shaft of a single hair (Gaudette 1978). It is generally accepted within the forensic science community that hairs are not a means of positive identification, but they can provide substantial information because of the variation in hair among individuals.

Also, if a questioned hair and a known sample are significantly different, they can be positively excluded as having come from a common source (such as differences in racial identification). Frequency data for the myriad observable microscopic characteristics are not available (Ogle 1998 Robertson 1982 Robertson and Aitken 1986), and a method for properly combining these features has not yet been produced. Therefore the significance of their occurrence within a single sample cannot be expressed and compared numerically (Kind and Owen 1976). It must be done qualitatively or semiqualitatively.

When a questioned hair is associated with a known sample, there is no accepted number of characteristics that must be similar between the two known samples. Rather, it is generally accepted that all of the characteristics exhibited by the evidentiary hair must be represented by the hairs in the known hair sample in order to support a conclusion of association. Jones (1956) stated: “The probability of identity may grow with every point of resemblance and with the number of hairs available for comparison, but that probability should never be stated as a certainty.”

The limitation of the science is that there is always the possibility of a coincidental match. The possibility of this event should not be construed as an error. Simply put, some people can share the same microscopic characteristics.

One should not construe that a probability statement equates to reliability. Simply because the statistical probability of hair evidence cannot be calculated does not make the comparison unreliable. To ensure proper weighting of an association, it is essential that the limitations of microscopic hair comparisons be understood by the examiner and conveyed to all interested parties.

Houck and Budowle (2002) stressed that microscopic hair comparisons are not a “screening test” and mtDNA analysis is not a “confirmatory test.” The combination of the two methods (microscopic hair comparison coupled with either nuclear DNA or mtDNA analysis) provides the judicial system with significantly more powerful information than either method does alone. Yet each technique on its own is useful.

At many crime scenes, hairs may be the most common trace material recovered. In fact, hairs may be the only forensic evidence available. Often, the condition of these hairs may be an important factor in determining their “history,” such as the presence of damaged hairs on a murder weapon or in the known sample from the victim. To ignore them limits the proper analysis of the crime scene and may leave very important questions unanswered.

Robertson (1999) stated that the greatest danger is to have hair examinations conducted by generalists who lack proper training and competence. This author agrees. Some of the greatest disservices to the field of microscopic hair comparison have been contributed by those lacking the necessary training, experience, and ability to properly explain the process, limitations, meaning, and significance of a microscopic hair association.

Crocker (1991) stated that “the greatest challenge faced by forensic hair examiners is to be able to leave the witness box with a feeling of assurance that members of a jury, or a judge acting alone, have the same appreciation as the examiner does of the proper level of significance to be given to the hair evidence.” It is the responsibility of the examiner to clearly state, both in the report and in any testimony, the limitations of the science of forensic hair examinations as well as the significance or weight of an association or exclusion.

Because of the importance of experience in the field of microscopic hair analysis and comparison, it is critical that personnel undertaking these examinations spend a considerable amount of their time conducting these examinations. One cannot expect an examiner to conduct these examinations part-time, remain technically competent, and be able to provide reliable results. Robertson (1999) made an excellent point when he stated:

It is also time for laboratory systems and scientists that are either unable or unwilling to embrace hair examinations with the necessary investment in time and commitment to withdraw altogether from the field. Half-hearted commitment can only continue to do damage to the credibility of hair examination. (Robertson 1999)

Guidelines have been established by SWGMAT for the forensic examination of hair. This consensus document produced by representatives of the microscopic-hair examination community describes:

  • Procedures for microscopic examination of hair.
  • Recommended instrumentation.
  • Integration of the microscopic examination and DNA analysis of hair.
  • Documentation and report writing.
  • Interpretation and testimony.
  • Training.
  • Quality assurance and proficiency testing. (SWGMAT 2005)

 It is imperative that any examiner practicing the science of microscopic hair comparison adhere to a guideline such as this. In addition, there is no substitute for a well-documented, rigorous training program in the microscopic examination and comparison of hair. Given these conditions, forensic hair comparison can provide reliable, scientifically valid, and important information in a criminal investigation.

This is publication number 09-03 of the Laboratory Division of the Federal Bureau of Investigation. Names of commercial manufacturers are provided for identification only, and inclusion does not imply endorsement by the FBI.

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The notochord: structure and functions

The notochord is an embryonic midline structure common to all members of the phylum Chordata, providing both mechanical and signaling cues to the developing embryo. In vertebrates, the notochord arises from the dorsal organizer and it is critical for proper vertebrate development. This evolutionary conserved structure located at the developing midline defines the primitive axis of embryos and represents the structural element essential for locomotion. Besides its primary structural function, the notochord is also a source of developmental signals that patterns surrounding tissues. Among the signals secreted by the notochord, Hedgehog proteins play key roles during embryogenesis. The Hedgehog signaling pathway is a central regulator of embryonic development, controlling the patterning and proliferation of a wide variety of organs. In this review, we summarize the current knowledge on notochord structure and functions, with a particular emphasis on the key developmental events that take place in vertebrates. Moreover, we discuss some genetic studies highlighting the phenotypic consequences of impaired notochord development, which enabled to understand the molecular basis of different human congenital defects and diseases.

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Neurobiology and Neurodevelopment of Pedophilia

Introduction and conceptual framework

Research regarding the etiology of pedophilia suggests the view of a complex and multifactorial phenomenon in which the influences of genetics (Blanchard et al., 2007), stressful life events, specific learning processes (Jespersen et al., 2009a), as well as perturbations in the structural integrity of ‘pedophilic’ brains may generate this specific phenotype of a sexual preference (Schiffer et al., 2007 Schiltz et al., 2007 Cantor et al., 2008). Initial theories relied mainly upon psychological mechanisms to account for a pedophilic preference, including classical and operant conditioning, as the behavioral mechanism through which the �used-abuser’ theory by Freund et al. (1990) and Freund and Kuban (1994) could be explained as well as attachment style in childhood as a marker for dysfunctional cognitive sexual schemas in adulthood (Beech and Mitchell, 2005).

The first theories to account for sexual behavior disorder associated with pedophilia suggested masturbatory conditioning [e.g., Laws and Marshall (1990)] or childhood sexual abuse (Freund et al., 1990 Fedoroff and Pinkus, 1996) as causal explanations. However, as Seto purports, due to lack of stringent methodology that includes proper control groups, small experimental or treatment effect sizes, and lacking knowledge of effect duration, these theories are not well supported. Beyond this, the majority of victims are female, whereas the majority of offenders are male, and if conditioning were the only logical theory to explain the etiology of pedophilia, it stands to reason that there would be more female pedophiles than are clinically seen (Seto, 2008 Jespersen et al., 2009b). However, a study by Klucken et al. (2009) showed that men are more easily conditioned through exposure to sexual stimuli than are women, casting significant doubt on the conditioning theory as it applies to female pedophiles. Currently, there is a strong push to understand the brain’s role in sexual preference development, particularly as it relates to pedophilia.

As discussed in a previous review by Seto (2008), there are three major neurobiological theories, which have come to be connected to pedophilia but all have the same shortcoming that they rely on data based on cases of pedophiles who have other psychological disorder diagnoses, are incarcerated or otherwise legally sanctioned, or are not sufficiently diagnostically classified (i.e., not differentiating between the exclusive or the non-exclusive type, etc.).

The first is the 𠇏rontal lobe” theory that refers to orbitofrontal and left and right dorsolateral prefrontal cortex differences that are often seen in pedophilic men (Graber et al., 1982 Flor-Henry et al., 1991 Burns and Swerdlow, 2003 Schiffer et al., 2007, 2008a,b). As the orbitofrontal cortex is responsible for behavior control (Bechara et al., 2000 O𠆝oherty et al., 2003), especially inhibiting sexual behavior, volume differences or dysfunction in this area may explain the sexual behavior disorder associated with pedophilia, although not pedophilic sexual preference.

The second major theory is the “temporal lobe” theory, referring to reports of hypersexuality accompanying pedophilia. Studies have shown that disturbances of the temporal lobes can result in an increase in pedophilic behaviors or an increase in the breadth of deviant sexual interests (Hucker et al., 1986 Langevin et al., 1988). These disturbances include temporal lesions and hippocampal sclerosis (Mendez et al., 2000). Ponseti noticed further differential temporal lobe activations in pedophilic men that highlight a hypersexuality-specific activation profile, further supporting the role of the temporal lobe in the expression of hypersexuality that is often seen with sexual behavior disorders (Schiltz et al., 2007 Ponseti et al., 2012). However, this theory also does not fully explain the etiology of the preference.

The third major neurobiological theory holds that differences in the sex dimorphic brain structures affected by the masculinization of the male brain would more strongly affect pedophilia development. Furthermore, the volumes of these structures would be influenced, but the hypothesis failed to state in what direction these changes occur, i.e., either increased or decreased volumes as a result of testosterone exposure. In the frontal and temporal lobes, these differences would be limited to those sexually dimorphic structures, rather than a generalized difference in region volume, but research has not supported the hypothesis (Cantor et al., 2008).

Furthermore, there is an additional theory that combines the frontal and temporal lobe theories. It states that the frontal and temporal lobes affect pedophilic sexual preference expression and its associated behaviors differently, with the frontal lobe (orbitofrontal and dorsolateral prefrontal cortices) accounting for committing the sexual offenses against children and the temporal lobe (amygdala and hippocampus) accounting for the sexual preoccupation with children often seen in pedophilic men (Seto, 2008, 2009 Poeppl et al., 2013).

Currently, pedophilia is often viewed as an interaction among neurodevelopmental factors based on genes and the (in utero-) environment as previously discussed (Becerra Garc໚, 2009). This theory holds that pedophilic sexual preference is a neurodevelopmental disorder corroborated by increased rates of non-right-handedness, shorter stature, lower intelligence, head injury, prenatal androgen levels, and the associated neuronal structural and functional differences that are present since childhood and/or adolescence. The exact directions of these relationships to pedophilic sexual preference, committing child sexual offenses, or consuming child pornography are still to be disentangled. There is currently no causal evidence yet to support a role in pedophilic sexual preference development.

Neurodevelopmental correlates of pedophilia

The prevailing perspective among biologists was that sex differences are linked solely to the exposure to testosterone in utero [see Phoenix et al. (1959) and Ehrhardt and Meyer-Bahlburg (1979)]. The masculinization of an initially undifferentiated human female brain is caused by testosterone’s induction of organizational effects during a limited period of time, as extrapolated from animal research. Sexual differentiation and development of subsequent sexual preference are likely an interplay between the impact of sex chromosomes on gene expression and sex hormones (Bao and Swaab, 2010). In pedophilia, research investigating biological differences is underway and studies have already highlighted structural and functional differences. The following is a discussion of findings that are classified as neuropsychological however, the onset of these differences is in utero, childhood, and adolescence, thus suggesting that these findings are actually a part of human development and contribute to pedophilic preference onset rather than acting as consequences thereof.

As a group consisting of primarily incarcerated individuals, pedophilic men show a doubled rate of head injuries before age 13, though after 14 years of age the difference is no longer significant, highlighting possible causative effects in multiple areas of cognitive functioning. While prenatal perturbations influence cognitive functioning and disorder development, so can head injuries resulting in unconsciousness in childhood, especially before age 13 years (Blanchard et al., 2002, 2003). This is a result of cortical development plasticity during childhood, when synaptic myelination and pruning are at their peak (Zhong et al., 2013). Of 725 originally tested, 685 pedophilic men participated in a study investigating the role of head injuries with associated loss of consciousness in pedophilia development. Pedophilic participants reported a significantly higher number of head injuries that resulted in a loss of consciousness prior to age 13 than did non-pedophilic child sexual offender participants. These results also positively correlated with a diagnosis of attention deficit-hyperactivity disorder and left-handedness among pedophilic participants.

More importantly, the more child victims each pedophile had correlated positively with each additional head injury before age 13, but not those sustained later in adolescence or adulthood (Blanchard et al., 2003). However, no studies have yet been conducted investigating head injuries in non-incarcerated pedophilic men with histories of CSA, or those with no such histories. Also lacking are studies on the prevalence of head injuries in children in general, as well as for the number of children with head injuries who subsequently go on to commit sexual offenses against children in adulthood.

The organizational�tivational hypothesis was initially developed by Phoenix and his colleagues in the 1950s in consequence to observations that a temporary rise in prenatal and early post-natal testosterone shapes development by masculinizing and defeminizing neural networks in males, whereas the absence thereof results in the development of female-typical neural phenotype (Phoenix et al., 1959 Schulz et al., 2009). According to the organizational�tivational hypothesis, pre- and perinatal as well as pubertal/adolescent androgens are able to shape cortical circuits (organization), whereas in adults androgens can only modulate the activity of these circuits (activation). The process of sexual differentiation occurs between weeks 12 and 18 of prenatal life and during the first 2 months after birth, periods during which testosterone has organizational effects on the brain. During this time, not only behavior is programed, depending on the level of exposure to testosterone, but also handedness, total digit length, and second to fourth finger length ratios (George, 1930 Rahman, 2005). These neuroendocrinological developmental differences are then activated during puberty and their relationship to pedophilia development will be discussed further in the coming paragraphs.

In understanding the relationship between testosterone, the brain, sexual behavior, and the rise of sexual deviancy, one must first understand how testosterone influences the brain. In vertebrates, androgen receptors (ARs) can be found in several brain regions, including the lateral septum, posteromedial bed nucleus of the stria terminalis (BNSTpm), medial preoptic nucleus of the hypothalamus, ventral premammillary nucleus, ventromedial nucleus of the hypothalamus, and the medial amygdaloid nucleus, otherwise found in the temporo-occipital, superior-parietal, and orbitofrontal cortices (Wood and Newman, 1999 Jordan et al., 2011a).

Research has shown a relationship between prenatal androgen exposure and hand preference in pedophilic men with a history of sexual offending against children. These men show a trend for increased rates of sinistrality – more efficient use of the left side/hand and is preferred – whereas hebephilic men show increased rates of ambiguous-handedness (Fazio et al., 2014) as compared to teleiophiles, and this has been discussed as an indicator of developmental perturbations resulting from a lack of prenatal testosterone exposure (Cantor et al., 2004). Homosexuality has also been associated with a higher prevalence of left-handedness (Cantor, 2012), and it would be of interest to see whether the higher prevalence of left-handedness seen among pedophilic men is attributable to pedophilia specifically or to a higher rate of homosexuality within this population as compared to teleiophilic men. More specifically, approximately 11% of the general non-offender population is non-right-handed, whereas pedophilic men with histories of sexually offending against children are approximately 15% non-righthanded, this difference being significant (Bogaert, 2001 Cantor et al., 2004, 2005 Blanchard et al., 2007 Rahman and Symeonides, 2008). Future studies should control for sexual orientation (homosexuality vs. heterosexuality) when examining handedness in pedophilia.

These effects also influence the second to fourth finger length ratio (D2:D4) (Voracek et al., 2007), a marker altered also in other psychiatric disorders including alcohol dependence (Lenz et al., 2012). The D2:D4 ratio is smaller in men than in women and is used as an indirect marker of prenatal testosterone exposure (Beaton et al., 2011). Additional differences in sexual orientation exist, such that the D2:D4 ratio is smaller in homosexual women compared to heterosexual women, as well as homosexual men compared to heterosexual men (Williams et al., 2000 Rahman and Wilson, 2003 Rahman, 2005 Manning et al., 2007). Although prenatal testosterone exposure affects both hand preference and D2:D4 ratio, the data here are equivocal and no firm conclusions have been drawn regarding the absolute relationship between hand preference and D2:D4. However, exposure to prenatal testosterone does not affect the D2:D4 ratio between 9 weeks gestation and birth, in contrast to hand preference, where differences are noted here and possibly after puberty (Lenz et al., 2012). How this applies to pedophilia is currently under investigation.

The following markers of neurodevelopmental abnormality have also been implicated in the neurodevelopmental processes contributing to pedophilia: sibling sex composition, maternal and paternal age at birth, and the fluctuating asymmetry of finger lengths and wrist widths. Pedophiles have a greater number of older brothers (Lalumière et al., 1998 Côté et al., 2002). Greater paternal age at birth was related to an increased chance of homosexuality, whereas greater maternal age increased risk for pedophilia, specifically (rather than generalized paraphilia) (Rahman and Symeonides, 2008).

Considering the effects of neurodevelopmental perturbations and executive functioning on pedophilia development, it seems worthwhile to consider the effect of intelligence. Research results have been contradictory: for example, generalized sexual delinquency is related to lower intelligence, whereas among groups of non-sexual offenders, pedophiles, and non-pedophiles, neither education level nor intelligence differed significantly. However, when pedophilic participants were separated by use of child pornography, those who had no history of child pornography use showed a decreased IQ and lower mean education level as compared to those who did (Briken et al., 2006 Blanchard et al., 2007 Schiffer and Vonlaufen, 2011). The main caveat to this research is that child pornography is considered a reliable indicator of pedophilic sexual interest, therefore confounding any results found with education or intelligence level because those pedophiles with child sexual offense histories are also more likely to have used child pornography (Seto, 2010). Research is currently focusing on the role of intelligence among pedophilic men who have only consumed child pornography and those who have committed CSA offenses, particularly differentiating those who have been incarcerated from those who have not (Babchishin et al., 2011 Seto et al., 2011, 2012).

As these results indicate, pedophiles do seem to differ from HC on neurodevelopmental measures. However, these results are varied and few strong conclusions can be drawn, including increased rates of left-handedness and increased rates in head injuries before age 13. The next section will discuss the relationship of neurological and neurobiological differences to the development of pedophilia, as both are the focus of current research determining the neural correlates of pedophilia. Please refer to Table ​ Table2 2 for a summarization of neuroimaging findings in pedophilia.

Table 2

Findings from previous neuroimaging studies in pedophilia.

Author (year)MethodStructural/functionalPPT groups (n)Paradigm/softwareCorrectionThreshold/SigFindings
Schiffer et al. (2007)MRIFrontostriatal and cerebellum structureHeterosexual (9) and homosexual pedophiles (9)
Heterosexual (12) and homosexual (12) controls
VBM-whole brain/SPM 2FDR (whole brain)/FWE corrected within ROIsp <𠂐.05GM volume reductions in pedophiles: PHc L/R, IFG L/R, OFC L/R, Ins L/R, Cer L/R Cin L/R, Posterior Cin L, STG L/R, MiTG R, Pcu L/R, Put L/R (Amy L/R in unpublished re-analysis)
Schiltz et al. (2007)MRIAmygdala structurePedophilic (15) Community controls (15)VBM/manual morphometry/SPM2 ROIs/MRIcroFWE/corrected for multiple comparisons within ROIsp <𠂐.05GM reductions in pedophiles: Amy R, Hyp L/R, SI L/R, Septal Region R, Bed Nucleus
Striae Terminalis L/R
Enlargement of Temporal Horn R
Poeppl et al. (2013)MRIPrefrontal cortex and amygdala structureHeterosexual (2) and homosexual (7) pedophiles
Heterosexual (11) controls
VBM 8 toolbox/SPM 8FWE corrected within ROIsp <𠂐.05GM volume decreases in pedophiles: only in Amy R pedosexual interest and sexual recidivism associated with GM volume decreases in insular cortex and DLPFC L, preference for younger children associated with GM decreases in the OFC and Ang L/R
Cantor et al. (2008)MRIWhite matter structurePedophiles (44) Teleiophilic sexual offenders (21)
Non-sexual Offender (53)
VBM whole brain/SPM 2
Parcelation with ANIMAL
FDRp <𠂐.05Reduced WM volumes in pedophiles in Superior Fronto-Occipital Fasciculus L, Arcuate Fasciculus R
No differences in GM
Cantor and Blanchard (2012)MRIWhite matter structurePedophiles (19)
Hebephiles (49)
Teleiophiles (47)
VBM Whole brain/SPM 2Not specifiedp <𠂐.05Reduced WM volumes in Temporal Lobe L/R and Parietal Lobe L/R in pedophiles/hebephiles compared to teleiophiles
Cohen et al. (2002)PETFrontal and temporal functionHeterosexual pedophiles (7)
Community controls (7)
Auditory stimulus/software not specifiedBonferronip <𠂐.05No differences seen in glucose metabolism after an erotic auditory paradigm lower metabolism in ITC and in Superior VFG during neutral auditory condition in pedophiles compared to controls no survival after correction
Dressing et al. (2001)fMRIOrbitofrontal functionHomosexual pedophiles (1)
Controls (2)
Visual stimuli block design/brain voyagerNot specifiedNot specifiedStronger recruitment in pedophiles in response to erotic pedohomosexual stimuli: ACC, Brain Stem R, PFC R, Basal Ganglia R, OFC R
Walter et al. (2007)fMRIHypothalamus and lateral prefrontal cortex functionPedophiles (13)
Controls (14)
Visual stimuli/SPM2Uncorrectedp <𠂐.005Decreased activations in pedophiles to sexual >𠂞motional arousal contrast: DLPFC R (Precentral), DLPFC R (MFG/SFG), DLPFC L (SFG), Occipital Cortex L
Schiffer et al. (2008a)fMRIFrontal and temporal functionHomosexual pedophiles (11)
Homosexual matched controls (10)
Visual stimuli/SPM2Whole brain analysis uncorrected/false discovery ratep <𠂐.001/p <𠂐.05Stronger Activations in pedophiles compared to controls in contrast nude children/adults >𠂝ressed children/adults: Fus L/R, HC L/R, Tha R
Schiffer et al. (2008b)fMRIAmygdala functionHeterosexual pedophiles (8)
Heterosexual matched controls (12)
Visual sexual stimuli/SPM2Whole brain analysis uncorrected/FDRp <𠂐.001/p <𠂐.05Activations seen in pedophiles compared to controls in contrast nude children/adults >𠂝ressed children/adults: MFG R, ACC L/R
Sartorius et al. (2008)fMRIAmygdala functionHomosexual pedophiles (10)
Heterosexual controls (10)
Visual stimuli/SPM2Uncorrectedp <𠂐.005Activation in pedophiles to children (Boys/girls) < neutral geometric stimuli contrasts in Amy R
Poeppl et al. (2011)fMRICortical and subcortical functionHeterosexual (2) and homosexual (7) pedophiles
Heterosexual non-sexual offender controls (11)
Visual sexual stimuli/SPM5Whole brain analysis uncorrected/FWE/FDRp <𠂐.001/p <𠂐.05Activations in pedophiles compared to controls in contrast nude children > scrambled images of children: MFG R, Ins L/R, MTG R, IPL L, Pos R, MCC R, PCC R, HC R, Tha L, Cer R
Ponseti et al. (2012)fMRIPattern classification functionHeterosexual (11) and homosexual (13) pedophiles
Heterosexual (18) and homosexual (14) controls
Visual stimuli pattern classification/SPM8Uncorrectedp <𠂐.001/p <𠂐.001Deactivations in homosexual pedophiles compared to controls in boys < men contrast: Cer L/R, Lin L/R, Anterior Tha L, HC R, Occ L, Fus L, ITG R, Ang R
Deactivations in heterosexual pedophiles compared to controls in girls < women contrast: NC L/R, SPG L/R, ITG L/R, Fus L/R, Cin L, Occ L, Amy L, Ins L, IFG R, Tha L, Cer R
Habermeyer et al. (2013a)fMRIFunctionHeterosexual pedophiles (8)
Heterosexual controls (8)
Erotic sexual stimuli/brain voyager 2.3.0Uncorrected/cluster-level threshold correctionp <𠂐.005/p <𠂐.05Activations in pedophiles in sex ×𠂚ge × group voxel-wise ANOVA analysis in MiFG R
Kärgel et al. (2015)rsfMRIFunctionPedophiles +𠂜SA (12)
Pedophiles −𠂜SA (14)
Healthy Controls (14)
SPM8 and rsfMRI toolkit RESTUncorrected at voxel level Family wise error corrected at cluster levelp <𠂐.005/p <𠂐.05DMN: (P-CSA > P +𠂜SA) Diminished connectivity to left MSF, left OFC. No differences in opposite contrast (P +𠂜SA > P-CSA). (HC > P +𠂜SA): VM PFC, OFC. No differences in P +𠂜SA > HC contrast
Limbic Network: (P-CSA > P +𠂜SA) diminished connectivity between L Amy and VM PFC, ACC, OFC, anterior PFC. No differences in P +𠂜SA > P-CSA. In HC > P +𠂜SA contrast: increased connectivity between L Amy and L anterior/inferior PFC, L Lin. No differences in P +𠂜SA > HC contrast
Poeppl et al. (2015)rsfMRIFunctionHeterosexual (2) and homosexual (7) pedophiles
Heterosexual (11) controls
Meta-analytic connectivity modeling (MACM) and ALEFEW at cluster levelp <𠂐.05Seed area: R Amy connected to HC, R ventral striatum, R Tha, L Amy, L Cla, L hyp, L Put, L HC, L Mid, L Tha for psychosexual arousal
L DLPFC: L Ant Ins, DMPFC, L Per, L SPL, L VLPFC for cognition and perception, spec. working memory
L Ins: L PaO, L Ant Ins, L Pos, L STG, L Put, R PaO, R STG, R DLPFC/Ant Ins, R Put, R pMC, L Tha, R Tha, L Ext for perception and cognition

ACC, anterior cingulate cortex Amy, amygdala, Ang, angular gyrus, Cau, caudate, CC, corpus callosum Cer, cerebellum Cin, cingulate gyrus Cla, claustrum DLPFC, dorsolateral prefrontal cortex Ext, extrastriate cortex FPPFC, frontopolar prefrontal cortex (Brodmann area 10) Fus, fusiform gyrus HC, hippocampus Hyp, hypothalamus IFG, inferior frontal gyrus Ins, insula IPL, inferior parietal lobule ITC, inferior temporal cortex ITG, inferior temporal gyrus L/R, left/right Lin, lingual gyrus MCC, middle cingulate cortex MFG, medial frontal gyrus MSF, medial superior frontal Mid, midbrain MiFG, middle frontal gyrus MOG, middle occipital gyrus MTG, middle temporal gyrus NC, nucleus caudatus Occ, occipital lobe OFC, orbitofrontal cortex PaO, parietal operculum Par, paracentral lobule PCC, posterior cingulate cortex Pcu, precuneus Per, peristriate cortex PHc, parahippocampal gyrus Pos, post central gyrus Pre, precentral gyrus PSS, posterior cingulate cortex Put, putamen SFG, superior frontal gyrus SI, substantia innominata SPG, superior parietal gyrus SPL, superior parietal lobule SOG, superior occipital gyrus STG, superior temporal gyrus Tha, thalamus VFG, ventral frontal gyrus.

Structural brain alterations in pedophilia

For the purposes of this review, we focused on providing an overview of recent neuroimaging work in pedophilia research starting in 2007, with case studies from 2000 to 2003. This was done for space and readability reasons such that another recently published review provides an excellent in-depth discussion of neuroimaging in pedophilia (Mohnke et al., 2014). That review summarizes the state of the art of neuroimaging in pedophilia as being in its infancy, with a general consensus that findings are scattered and need to be replicated. Most results from neuroimaging studies in pedophilia have found neurostructural or neurofunctional correlates of CSA, not pedophilia per se. The amygdala remains a region of high interest, but Mohnke et al. (2014) suggest stricter methodology to replicate these findings. Our discussion parallels and expands upon the aforementioned review.

A famous case study that highlighted a neurological disease that caused impulsive sexual behavior and could have been an expression of an underlying pedophilic orientation was a right orbitofrontal tumor in a 40-year-old man (Burns and Swerdlow, 2003). Prior to the discovery of his tumor, the patient had overtly claimed no sexual interest in children, but after the tumor progressed, he made sexual advances to his prepubescent stepdaughter and began a pornography collection, including child pornography, resulting from impulse control loss associated with orbitofrontal cortex dysfunction. Although his behavior was non-exclusive and his preference was not explicitly tested, the most striking fact about his symptoms is that all pedophilia-like symptoms disappeared after resection of the tumor. Even more, after the tumor recidivated, the pedophilia-like symptoms remerged and disappeared again after the second resection, thus showing a clear causal link between behavior and brain function. However, the clear majority of orbitofrontal tumors do not result in pedophilic behavior, meaning this case study should be interpreted cautiously.

A further publication with two case studies highlights the role of the temporal cortex in regulating sexual behavior (Mendez et al., 2000). In the first case, a 60-year-old man developed fronto-temporal dementia and presented with increased sexual drive the molestation of extrafamilial children. The second case was a 67-year-old man who developed hippocampal sclerosis that similarly increased his sexual desire. He attempted to molest extrafamilial children. Both patients sexually abused their own young children, suggesting a latent predisposition to pedophilic behaviors existed in these patients prior to disease onset. Both patients showed hypometabolism of the right temporal lobe as measured with FDG-PET. After treatment with antidepressants (paroxetine for the former patient and sertraline for the latter), a decrease in pedophilic behaviors and desires was reported (Mendez et al., 2000). These findings support that dysfunction in the prefrontal cortex may prompt a latent predisposition to sexual attraction to children through disinhibition, whereas a dysfunction in the temporal cortex might elicit this response through sexual preoccupation (Jordan et al., 2011b). This does not explain the etiology of pedophilia as a sexual preference but as an acquired hypersexual behavioral disorder, and furthermore one that rarely presents in the realm of fronto-temporal dementia and hippocampal sclerosis. Clear here is the expression of pedophilic behaviors resulting from the neurological diagnoses, but not why these behaviors were pedophilic rather than hypersexual in nature. For further discussion of dementia and its relation to hypersexual/pedophilic disorders, please refer to Mohnke et al. (2014).

Only a handful of studies of MRI-based structural differences in pedophilia have been published so far. By means of voxel-based morphometry (VBM), several alterations of gray matter (GM) and white matter (WM) were found. In 18 incarcerated exclusive heterosexual and homosexual pedophilic men with histories of sexual offending against prepubertal children, a significantly lower GM volume in the bilateral orbitofrontal cortex, bilateral insula, bilateral ventral striatum (putamen), precuneus, left posterior cingulate, as well as superior and right middle temporal, parahippocampal gyrus, and in the cingulate compared to 24 teleiophiles was found. These findings were corrected for multiple comparisons using the false discovery rate within the whole brain (Schiffer et al., 2007). However, only the left parahippocampal gyrus would have remained significant had a Bonferroni correction for the 15 additional ROI analyses been applied. The authors proposed a theoretical frontal-executive dysfunction and suggested that – similarly to obsessive-compulsive spectrum disorders – these findings may form a neurophysiological circuit contributing to the pathophysiology of pedophilia.

In another study with 15 pedophilic forensic inpatients in comparison to a healthy teleiophile group, GM reductions were found in four pre-defined ROIs comprised of right amygdalae in right septal region, the bed nucleus striae terminalis (BNST), hypothalamus, and the substantia innominate bilaterally (Schiltz et al., 2007). Later on, amygdalar volume reduction was confirmed by a post hoc manual volumetric analysis, unpublished until now (Schiltz, personal communication). These results could be related to a developmental hypoplasia and underscores the influence of right amygdalar lateralization on regulation of sexual behavior, supporting the temporal lobe hypothesis of pedophilia.

One study was published showing that, in comparison to non-sexual offender controls (n =�), convicted pedophilic offenders (n =𠂙) show only GM volume decreases in the centromedial nuclei group of the right amygdala which extended into the laterobasal nuclei group and the cornu ammonis of the hippocampus, although this finding did not survive correction for the large number of predefined ROIs (Poeppl et al., 2013). Interestingly, pedosexual interest, including the strength of such interest, and sexual recidivism were associated with GM volume decreases in the left insular and dorsolateral prefrontal cortices, while preference for younger children was associated with GM decreases in the orbitofrontal cortex and bilateral angular gyri (Poeppl et al., 2013).

What the studies of Schiffer et al. (2007) and Schiltz et al. (2007) have in common is a comparison between a group of sentenced sex offenders recruited from forensic institutions with healthy teleiophiles without criminal histories, leading to potential confounds in the results with factors other than pedophilia, such as criminality or stress of imprisonment. However, an advantage of the study by Schiffer et al. (2007) is that they included only pedophiles of the exclusive type, allowing for interpretations including sexual preference.

By comparing 44 pedophilic men with histories of sexually offending against children or child pornography consumption, with 53 men with histories of non-sexual offenses, differences were found in the WM only, highlighting a bilateral connection route traveling the superior fronto-occipital fasciculus, as well as a right-sided alteration in the arcuate fasciculus. No differences in GM were observed (Cantor et al., 2008). These findings were upheld in a follow-up confirmatory reanalysis (Cantor and Blanchard, 2012) and interpreted as insufficient connectivity in pedophilic individuals, rather than simply GM reductions in disparate (sub-) cortical regions (Cantor and Blanchard, 2012).

Studies to date contain shortcomings either due to the sample sizes, to the configuration of the control group, or because the methodology of VBM was used in a restricted way by focusing on a priori regions of interest. The take home message of the present structural imaging MRI studies of pedophilia is that while there have been different results from different studies, one finding has been replicated across studies: reduced right amygdala volumes in pedophiles compared to teleiophilic controls (Mendez et al., 2000 Schiffer et al., 2007 Schiltz et al., 2007 Poeppl et al., 2013). This finding supports the temporal lobe theory of pedophilia referred to in Section “Introduction and Conceptual Framework.” Diffusion-tensor imaging is a method of WM imaging that holds promise to validate and expand previous VBM results.

Functional brain alterations in pedophilia

Only a few functional imaging studies have been conducted to investigate possible differences during the processing of sexual stimuli in the brains of pedophiles. With only one exception, they were visual sexual stimulation studies, thereby inducing a strong visual bias while making this modality the dominant model of perceptual processing alterations in paraphilias, although sensory systems offer potential other routes to sexual responsiveness. However, with the background of recent evidence explaining how hetero- or homosexual teleiophilic brains process visual sexual information and regulate the psychosexual and physiosexual components of sexual arousal [please refer to Safron et al. (2007), Georgiadis and Kringelbach (2012), Stoléru et al. (2012), and Poeppl et al. (2014)] for a deeper discussion), it is a reasonable step toward the understanding of pedophilia to study whether there are functional differences in the brain network associated with sexually arousing visual pictures of children.

Research has highlighted alterations in pedophiles through positron emission tomography (PET) and functional MRI. For example, in a PET study of pedophilia, a decreased regional cerebral metabolic rate for glucose was found in the right inferior temporal cortex and superior frontal gyrus, without Bonferroni correction. This rate decreased in the pedophilic group after presentation of girl and women cues, whereas it increased in the teleiophilic group (Cohen et al., 2002). The authors interpreted this as a consistent brain abnormality underlying decreased glucose metabolism in the temporal and frontal cortices implicated in cortical regulation of sexual arousal. The small sample size of seven participants in each group limits the generalizability and confidence with which the results can be interpreted.

In fMRI research, the first study that included a single homosexual pedophile found increased activity of the anterior cingulate gyrus, right prefrontal cortex, and basal ganglia in response to pictures of minimally clothed boys, regions that comprise the attentional brain network with the right orbitofrontal cortex (Dressing et al., 2001).

Decreased activations were seen in the hypothalamus, dorsal midbrain, dorsolateral prefrontal cortex, and right lateral parietal, right ventrolateral, and right occipital cortices, as well as in the left insula in 13 hetero- and homosexual forensic pedophiles when viewing sexual stimuli as compared to emotional stimuli as compared to teleiophiles (Walter et al., 2007). Based on these findings, it was suggested that the missing sexual interest toward adults could be explained by impairment in subcortical regions associated with the autonomic component of sexual arousal, i.e., lack of activation seen in hypothalamus and dorsal midbrain in pedophilia. Additionally, using a regression analysis approach, the activation in the left DLPFC was inversely correlated with the score on the child abuse subscale of the multiphasic sexual inventory (MSI), indicating also possible alterations of cognitive processing of sexual stimuli in these subjects (Schiffer et al., 2008a,b).

Homosexual and heterosexual incarcerated pedophiles were examined with fMRI to determine whether there were differences associated with age and child gender preference. Among homosexual pedophiles with a history of sexual offenses against children (n =�) in comparison with homosexual (n =�) controls, the substantia nigra, caudate nucleus, the occipitotemporal and prefrontal cortices, thalamus, globus pallidus, and the striatum were activated in response to male child sexual stimuli, whereas these were not among the matched homosexual teleiophiles (Schiffer et al., 2008a). This was interpreted as an increased effort in evaluating respective stimuli in pedophilic compared to control participants. In another investigation, heterosexual pedophiles (n =𠂘), when compared to heterosexual teleiophiles (n =�), after presentation with female child sexual stimuli displayed significant activations in the amygdala, hippocampus, substantia nigra, caudate nucleus, the medial dorsal thalamic nucleus, and the inferior temporal gyrus, suggesting a similar response pattern to sexually preferred stimuli as seen in healthy heterosexual males (Schiffer et al., 2008b). Pedophilic males showed a signal increase only in the right dorsolateral prefrontal cortex in response to the preferred sexual stimuli (no activation was seen in the control group to sexual stimuli of adult women). An interesting finding was that whereas the healthy male teleiophiles activated the orbitofrontal cortex in response to both sexually explicit adult female and female child imagery, this activation was not seen among male pedophiles. All together, the authors suggest that orbitofrontal deactivation, as shown in pedophilic participants, represents a dysfunction of the neural network necessary for the appropriate cognitive component of sexual arousal processing.

There were also attempts to investigate the perception and emotional processing of visual sexual stimuli. For example, the right amygdala showed greater activation in homosexual pedophiles when they were presented with male child sexual stimuli compared to heterosexual male teleiophiles who observed female adult sexual pictures, although the participants were not matched for sexual orientation, thus potentially obscuring true ‘pedophilic’ activations (Sartorius et al., 2008). The authors interpreted this increased amygdala activation to stimuli depicting children that were observed in pedophiles as a possible fearful emotional reaction combined with sexual arousal, supported by the lack of an appropriate amygdala activation to adult female stimuli (Sartorius et al., 2008).

Poeppl et al. (2011) used a block design in their study to investigate sexual interest in pedophiles (nine pedophiles with a history of contact offenses and 11 non-sexual offender controls) that consisted of male and female nude Tanner scale imagery, including Tanner scales I, III, and V, corresponding to prepubescent, pubescent, and adult images. Results of whole brain analyses showed significantly greater activation in the middle temporal lobe, hippocampus, posterior cingulate cortex, thalamus, medial frontal lobe, and culmen of the cerebellum in pedophiles to the Tanner I > neutral contrast. When compared to control teleiophiles in the Tanner V > neutral contrast, pedophiles showed a significant deactivation in the right insula. Furthermore, in the between group contrast of interest (pedophiles > Tanner I, teleiophiles > Tanner V), there were significantly greater activation signals seen in the postcentral gyrus, right middle temporal gyrus, anterior midcingulate cortex, and the amygdalae bilaterally (Poeppl et al., 2011). The authors interpreted these findings as an easier sexual arousability in pedophilic as compared to non-paraphilic participants when stimulated with purposefully non-erotic material (Poeppl et al., 2011).

In a similar study, Habermeyer et al. (2013a) investigated eight pedophiles (three with a history of contact offenses, five with a history of child pornography consumption) and eight heterosexual teleiophilic controls in an event-related design consisting of erotic pictures of boys, girls, men, and women. In an ROI analysis including the middle frontal gyrus, only the pedophilic participants showed activation in the girl contrast, whereas controls showed deactivation (Habermeyer et al., 2013a). A further finding showed that during the immediate processing of erotic stimuli, both groups showed significant activations in the dorsomedial prefrontal cortex, a finding the authors attributed to the crucial role this region occupies in the critical evaluation of and attention to sexual stimuli (Habermeyer et al., 2013a).

Two recent studies investigated functional connectivity in pedophilia and have supported decreased connectivity associated with CSA, but not with pedophilia. Specifically, Kärgel et al. (2015) examined functional connectivity at rest (RSFC) in 26 pedophilic men stratified according to offense status (14 P+CSA, 12 P𠄼SA) and 14 HC within (1) the default mode network and (2) the limbic network. Pedophiles who engaged in CSA depicted diminished RSFC in both networks compared with HC and P𠄼SA with diminished RSFC between the left amygdala and orbitofrontal as well as anterior prefrontal regions. These findings highlight a diminished resting state functional connectivity in offending pedophiles as compared to controls, suggesting a relationship to CSA more than to pedophilia. Using complex multimodal integration of brain structure and function analyses, Poeppl et al. (2015) found that the functional role of brain regions that are altered in pedophilia were linked to non-sexual emotional as well as neurocognitive and executive functions, which were previously reported to be impaired in pedophiles. They suggested that structural brain alterations affect neural networks for sexual processing by way of disrupted functional connectivity and that structural alterations also account for common affective and neurocognitive impairments in pedophilia.

Further, new methods have been investigating differences that go beyond regional activations. Pattern classification is a new method of analyzing neural activation patterns. The idea of pattern classification is to use activation patterns in different brain regions in a multivariate approach rather than relying on region by region comparisons (Linden, 2012). It can be used for classifying groups. For example, in the field of sexology pattern classification has been applied successfully to classify heterosexual and homosexual male teleiophiles (Ponseti et al., 2009).

Research found that the activations seen in heterosexual and homosexual pedophiles to child stimuli are nearly indistinguishable from those in heterosexual and homosexual healthy males to adult stimuli (Ponseti et al., 2012) this supports the assumption that pedophilia is primarily a sexual age preference similarly to teleiophilia. The activation pattern among heterosexual and homosexual pedophiles and healthy male teleiophiles includes the caudate nucleus, cingulate cortex, insula, fusiform gyrus, temporal cortex, occipital cortex, thalamus, amygdala, and cerebellum. Despite the similarity in activation patterns between pedophilic and teleiophilic men, the novel pattern classification technique has been successfully applied based on the presentation of preferred sexual stimuli and resulted in a mean accuracy of 95%, with 100% specificity and 88% sensitivity (Ponseti et al., 2012 Mohnke et al., 2014), thereby showing a promising new approach for classifying subjects. Please refer to Figure ​ Figure3 3 for a visual explanation of pattern classification according to Ponseti et al. (2012). These studies included fully admitting pedophilic participants only therefore, further research should verify its use with partially- or non-admitting pedophiles. The promise of functional predictors is, however, also supported by a similar study which, in contrast to Ponseti et al. (2012), used a highly hypothesis-driven approach of several impaired functions.