Human skin-color sexual dimorphism: a test of the sexual selection hypothesis.

Women have lighter skin than men do across a wide range of populations, even on the unexposed skin of the upper inner arm, possibly because of sexual selection by men for lighter-skinned women. If this hypothesis is true, human skin color should become more sexually dimorphic with increasing distance from the equator, since sexual selection for lighter skin in women would be less constrained by natural selection for darker skin in both sexes. Yet when Madrigal and Kelly (2006) analyzed skin reflectance data from 53 different samples, they found that the most dimorphic human populations were actually those of medium skin color at medium latitudes. Their finding does not necessarily falsify the sexual selection hypothesis. It may be that this sexual dimorphism cannot fully express itself in light-skinned populations at higher latitudes. Skin reflectance becomes sexually dimorphic during adolescence, when girls progressively lighten in color, and this lightening may be less effectively expressed if melanin production is already low (Frost, 2005; 57). It is perhaps significant that this sex difference seems to disappear or even reverse itself when skin reflectance is close to the physiological maximum, notably in Dutch and Belgian subjects (Leguebe, 1961; van Rijn-Tournel, 1966; Rigters-Aris, 1973). The authors of these studies attribute the absence of dimorphism to their female subjects being more likely to wear sleeveless shirts. Madrigal and Kelly (2006) cautiously attribute it to the popularity of tanning among modern European women. Yet the skin color of Spanish subjects becomes sexually dimorphic during adolescence even though young women in post-Franco Spain wear sleeveless shirts and sun themselves (Mesa, 1983). If we eliminate samples near the maximum of skin reflectance, can we still tease out the effects, if any, of sexual selection? Doubtful. For one thing, the ‘‘ceiling effect’’ likely begins to manifest itself well before the limit of maximum skin reflectance. For another, there is probably too much noise in the data for such a fine-grained analysis. Skin color is less sexually dimorphic at some ages, particularly childhood (when it is absent) and adolescence (when it is still emerging), and most studies are poorly controlled for age. Madrigal and Kelly (2006) tried to minimize this problem by limiting their meta-analysis to adult men and women: ‘‘By excluding samples which worked with children and even teenagers, we feel assured that our sample will not suffer from age-related heterogeneity. ’’ Many of the papers they cite, however, apply the term ‘‘adults’’ or ‘‘men and women’’ to anyone past the age of puberty. For example, when Weiner et al. (1964) studied Black Bushmen at Bagani kraal, the ‘‘adults’’ were defined as those whose third molars had erupted. Leguebe (1961) studied ‘‘women’’ who ranged in age from 17 to 23. Banerjee (1984) examined Punjabi ‘‘mothers’’ and ‘‘fathers’’ without specifying their ages, yet many Punjabi women begin to have children in their teenage years. Even after adolescence, age-related noise continues to muddy the data. Human skin may change more slowly during adulthood, but it does change. In particular, there is some evidence that skin color becomes less sexually dimorphic after 40, with some papers finding that women become nonsignificantly darker than men (Chamla and Demoulin, 1978; Mori and Tokuhashi, 1956; Rigters-Aris, 1973). This is a problem for any meta-analysis because relatively few skin reflectance studies exclude adults over 40 years of age. The most serious problem with Madrigal and Kelly’s meta-analysis, however, is not a methodological one. When they modeled the selection pressures on human skin color, they assumed that the only one to vary with latitude was natural selection for darker skin in both sexes. Yet sexual selection for lighter skin in women may also have varied with latitude, especially during the long period of time when all humans were hunter-gatherers. The intensity of sexual selection is determined by the operational sex ratio (i.e., the ratio of men to women among adults available for mating) and this ratio becomes more female-biased in hunter-gatherers the further away they are from the equator: first, polygyny becomes costlier for men because women cannot gather food in winter and require more food provisioning; second, more men die at a younger age because they cover longer distances while hunting. As a result, in proportion to distance from the equator, fewer men are available for mating, women compete much more for mates, and sexual selection of women intensifies. Thus, sexual selection for lighter skin in women may have varied with latitude just as natural selection for darker skin varies with latitude. The two selection pressures are confounded. Madrigal and Kelly (2006) allude to this problem when they deplore the assumption that everywhere, and to the same extent, men can pick and choose among a broad range of possible mates. How, then, can the sexual selection hypothesis be falsified? Does it make predictions that may be proven true or false? In my opinion, there are two testable predictions: