3. Human Hair Types

The most noticeable hair on humans is the hair on the head, which shows far more variations in texture and color and can grow much longer than the hair on other mammals. It is commonly theorized that the long head hair characteristic of humans has evolved as a byproduct of secondary natural selection, serving as a kind of adornment after other types of somatic hair in humans disappeared. Another theory is that this characteristic is the result of runaway sexual selection, in which longer hair would serve as an indication of an individual’s health. The absence of head hair in humans is called alopecia, commonly known as baldness. 

Other types of hair on the human body can vary greatly in their occurrence and appearance according to the age, sex, health, or race of the individual. Beginning in childhood and regardless of sex or race, fine vellus hair covers much of the body. The only places on the body not covered by vellus hair are the lips, nipples, palms, soles of the feet, naval (along with any other scar tissue), and some external genital areas. The thickness, color, and density of these hairs vary from one person to another. 

During puberty, increasing levels of male hormones called androgens (primarily testosterone and its derivatives) trigger hair follicles to transform the fine vellus hair into terminal hair on many parts of the body for both sexes. The terminal hair in these areas is generally characterized by an increase in both thickness and the rate of growth and is often called androgenic hair. As testosterone levels in the body increase, the appearance of androgenic hair changes in response to androgen sensitivity. Most sensitive are the pubic areas, which are generally the first to respond to changing androgen levels through an increase in hair thickness. 

Both men and women exhibit terminal hair growth in the underarm and pubic areas as androgen levels increase during puberty. However, in general only men grow androgenic hair in other areas, in particular on the face, chest, abdomen, legs, and arms. Females usually maintain the finer vellus hair in these areas, both during puberty and throughout adulthood. Increased androgenic hair on women in these areas is referred to as hirsutism, while excessive and abnormal hair growth on these and other areas (for both sexes) is called hypertrichosis. 

Hair color and texture are inherited characteristics. Hair color, or pigmentation, can lessen with illness or stress and eventually cause the hair shaft to whiten. The normal whitening of the hair that occurs with increasing age, however, is largely determined by heredity. Individual hair shape and texture varies greatly with race. Native Americans, Eskimos, and Mongoloids exhibit very straight hair that is almost perfectly round in cross section. Thick, kinky hair that is very flat in cross section is characteristic of Negro populations, and wavy or curly hair that is oval in cross section is commonly exhibited among Caucasians.