The human mind seems to use facial color to determine whether a person is male or female. A man has a relatively dark facial color that contrasts poorly with his lip and eye color. Conversely, a woman has a relatively light facial color that contrasts sharply with her lip and eye color (Russell, 2003; Russell, 2009; Russell, in press).
This kind of sex-recognition algorithm has been a channel for sexual selection in many species. When selecting a mate, an animal tends to choose the ones most easily recognizable as the opposite sex. Over many generations, such selection will cause the relevant sex-specific cues to be accentuated (Manning, 1972, pp. 47-49).
The degree of accentuation will depend on the intensity of sexual selection and on whether males have been selecting females or females selecting males. Among ancestral humans, sexual selection seems to have varied in both intensity and direction along a north-south gradient (Frost, 2006; Frost, 2008). In the tropical zone, women gathered food year-round, so a second wife would cost little in terms of food provisioning. With more men becoming polygynous, fewer women were left unmated. The pressure of sexual selection was thus on men, with women being the ones who could pick and choose mates.
This situation reversed outside the tropical zone. First, polygyny was costlier because women could not gather food in winter. Second, male mortality exceeded female mortality because men had to hunt over longer distances. Together, these two trends resulted in too few men competing for too many women. This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances. The pressure of sexual selection was thus on women, with men being the ones who could pick and choose mates.
Sexual selection and lightening of skin color
If light skin is perceived as a sign of femininity, sexual selection of women should tend to lighten female skin. This kind of selection became possible once ancestral humans had left the tropical zone. On the one hand, there was less natural selection for dark skin as a barrier to intense UV radiation. On the other, as explained above, there was stronger sexual selection of women because they outnumbered men on the mate market. Women should thus be increasingly lighter-skinned than men with increasing distance from the tropical zone, this sex difference being greatest among those humans that once inhabited the large expanses of continental steppe-tundra in northern and eastern Europe. Since most skin pigmentation genes are not sex-linked, selection for lighter-skinned women would also lighten mean skin color (i.e., both males and females). Thus, mean skin color should likewise lighten along the same north-south gradient.
How do these predictions stack up against reality? They accurately describe geographic variation in mean skin color (Frost, 2008). But they poorly describe geographic variation in female depigmentation relative to male skin color. In fact, female skin reflectance exceeds male skin reflectance the most among humans at medium latitudes with medium skin color (Frost, 2007; Madrigal & Kelly, 2006). This may be a ‘ceiling effect’. Northern and eastern Europeans are close to the physiological limit of skin depigmentation. Their women cannot be much whiter than the mean skin color because they have, so to speak, very little headroom left—the mean skin color is already scrunched up against the ceiling of maximum skin whiteness.
Sexual selection and increase in facial color contrast
There seems to be similar geographic variation in the contrast between facial color and lip/eye color. This contrast is weakest among tropical humans. It is strongest, however, not among northern/eastern Europeans but among East Asians (Russell, in press). This is largely because East Asians have dark eyes and relatively light facial skin. The contrast effect is even stronger if we factor in their jet-black hair, which further sets off the lightness of the female face. Nonetheless, facial color contrast is no more sexually dimorphic among East Asians than it is among Europeans (Russell, in press).
Why would Europeans score lower than East Asians on facial color contrast? It may be that sexual selection for dark eyes and dark hair relaxed among ancestral Europeans once their facial skin had lightened to the point of becoming pinkish-white. At that point, the color contrast was more than sufficient. This, in turn, may have allowed rare color preference to generate sexual selection for diverse hair and eye colors. This process may have then acquired a dynamic of its own that competed with the older dynamic of facial color contrast. Alternately, rare color preference may have always been a weak selection pressure that manifests itself only under conditions of intense sexual selection (Frost, 2006; Frost, 2008).
In sum, if we examine geographic variation in skin color and in facial color contrast, the pattern is largely consistent with increasingly intense sexual selection of women along a north-south gradient. This selection would have been minimal among tropical humans and maximal among arctic humans, particularly those that once lived on continental steppe-tundra—where polygyny was constrained the most and where male mortality exceeded female mortality the most. There are, however, deviations from the expected pattern that may be due to ceiling effects and release of sexual selection for rare hair and eye colors.
Among ancestral Europeans, this process of sexual selection seems to have been a multi-stage process. It likely began c. 30,000 BP with the first penetration by modern humans of the steppe-tundra belt (southwestern France). This initial stage would correspond to certain physical changes that are common to Europeans and East Asians. Stage I ended with the onset of the glacial maximum (c. 20,000 BP), which blocked East-West gene flow by merging the Fenno-Scandian and Ural icecaps and by forming large glacial lakes along the Ob (Rogers, 1986; Crawford et al, 1997). Stages II and III would correspond to later physical changes that are specific to Europeans.
Stage I – head hair lengthens, face shape feminizes, skin lightens (30,000–20,000 BP ?)
Stage II – skin lightens to pinkish-white (20,000–15,000 BP ?)
Stage III – hair and eye color diversifies (15,000–10,000 BP ?)
Crawford, M.H., Williams, J.T., & Duggirala, R. (1997). Genetic structure of the indigenous populations of Siberia. American Journal of Physical Anthropology, 104, 177-192.
Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.
Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.
Frost, P. (2006). European hair and eye color – A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.
Madrigal, L., & W. Kelly. (2006). Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 132, 470-482.
Manning, A. (1972). An Introduction to Animal Behaviour, 2nd edition, London: Edward Arnold.
Rogers, R.A. (1986). Language, human subspeciation, and Ice Age barriers in Northern Siberia. Canadian Journal of Anthropology, 5, 11?22.
Russell, R. (in press). Why cosmetics work. In Adams, R., Ambady, N., Nakayama, K., & Shimojo, S. (Eds.) The Science of Social Vision. New York: Oxford University Press.
Russell, R. (2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219.
Russell, R. (2003). Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.