Head hair is much longer than hair elsewhere on the body. This lengthening has involved several evolutionary changes: faster rate of growth, longer growing phase, increased density, and greater resistance to physical damage (Khumalo, 2005; Loussouarn et al., 2005). The multiplicity of these changes is consistent with sexual selection: the selective pressure seems to have acted on an overall visual effect rather than on one incidental factor. In some non-human primates, head hair has lengthened for apparently similar reasons, perhaps because visual attention tends to focus, as in humans, on the face and its surrounding frame (Darwin, 1936 , p. 906).
Head hair has lengthened only in those human populations that have lived in the temperate and Arctic zones, including some that have back-migrated to the tropical zone, e.g., Austronesians in Southeast Asia and Oceania, Amerindians in the tropical New World. Darwin noted "the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground" (Darwin, 1936 , p. 906).
This point seems to be lost on those who think that hominids acquired long head hair at a very early date. Advocates of the 'Aquatic Ape Hypothesis', for example, believe that head hair lengthened during a putative aquatic phase of human evolution when an infant would have to hang on to its mother's hair while in the water "and if the hair floated around her for a yard or so on the surface [the infant] wouldn't have to make so accurate a beeline in swimming towards her" (Morgan, 1972, p. 36). Yet such lengthening could not have occurred until modern humans had begun spreading out of Africa—some 50,000 years ago at the earliest. This same point is also lost on those who argue that long head hair improved mating success among ancestral humans because it signified health and hence mate quality (Hinsz et al., 2001; Mesko and Bereczkei, 2004). Fine. But does health matter less in the Tropics?
In the literature, this lengthening of head hair outside the tropical zone is often ascribed to relaxation of natural selection: short frizzy hair helps dissipate body heat and thus loses its adaptive value in colder climates. But why would a colder climate cause head hair—and only head hair—to lengthen to such an extent and over such a short span of evolutionary time? If we turn to the sexual selection model, we see that non-tropical climates do alter selective pressures, but indirectly so—by altering human demography. When early modern humans left the Tropics, they entered an environment that reduced the supply of mateable men, thus making them a limited resource. This new environment intensified sexual selection of women and would have favored physical traits that retain visual attention, particularly from men.
Why would sexual selection favor longer-haired women? We know that most societies consider a greater amount of head hair to be an appropriate female characteristic (Synnott, 1987). Even in sub-Saharan Africa, where people are naturally shorthaired, women have traditionally lengthened their head hair with vegetable fiber, sinew, or hair from relatives, apparently to enhance their beauty (Bernolles, 1966; Sieber and Herreman, 2000). Head hair is identified with femininity partly because men begin to go bald as early as their 20s, but also because the scalp hairs of women seem to have a higher mean diameter and hence more volume, even in the naturally shorthaired New Guineans (Walsh and Chapman, 1966). It may be that men perceived a greater amount of head hair as more feminine from an early date, and this perceptual bias could have influenced male mate-choice wherever female-female rivalry for mates had become sufficiently intense.
Bernolles, J. (1966). Permanence de la parure et du masque africains. Paris : G.P. Maisonneuve et Larose.
Darwin, C. (1936) . The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.
Hinsz, V.B., Matz, D.C., and Patience, R.A. (2001). Does women's hair signal reproductive potential? Journal of Experimental Social Psychology, 37, 166-172.
Khumalo, N.P. (2005). African hair morphology: macrostructure to ultrastructure. International Journal of Dermatology, 44(Suppl. 1), 10-12.
Loussouarn, G., El Rawadi, C., and Genain, G. (2005). Diversity of hair growth profiles. International Journal of Dermatology, 44(Suppl. 1), 6-9.
Mesko, N., and Bereczkei, T. (2004). Hairstyle as an adaptive means of displaying phenotypic quality. Human Nature, 15, 251-270.
Morgan, E. (1972). The Descent of Woman. London: Souvenir Press.
Sieber, R., and Herreman, F. (2000). Hair in African art and culture. African Arts, 33, 54-69.
Synnott, A. (1987). Shame and glory: a sociology of hair. The British Journal of Sociology, 38, 381-413.
Walsh, R.J., and Chapman, R.E. (1966). A study of the quantitative measurement of human head hair fibres. Man, new series, 1, 226-232.