Tuesday, April 8, 2008

Questions on skin color, beauty, and sexual selection

A student recently sent me the following questions:

-How does skin color influence beauty from a biological standpoint?

Women are lighter-skinned than men in all human populations. This sex difference seems to be wider in populations that are medium in color and narrower in populations that are very dark or very light in color. It appears at puberty and seems to be related to other changes that the skin undergoes at that time of life.

There are three theories as to why this sex difference exists:

1) Women have become lighter in skin color for the same reason that they have smoother skin, higher-pitched voices, and more 'baby-like' faces. These are all infant characteristics. In other primates, adults become less aggressive and more willing to provide care when they see an infant's physical features. So women may have evolved a lighter skin color and a more child-like physique as a means to reduce male aggression and to increase male care-taking behavior.

2) Men subconsciously use lighter skin as a visual cue for recognizing sexually mature women. Several controlled studies have shown that people can distinguish a man's face from a woman's by complexion alone, even when the picture is blurred and offers no other details. The gender cue seems to be the contrast between facial color and eye/lip color. According to this hypothesis, the sex difference in skin color began accidentally as a side-effect of the influence of sex hormones on skin pigmentation. Women then became even more light-skinned because men tended to select those women who looked unambiguously feminine.

3) Women evolved a lighter skin as a means to increase their production of vitamin D and hence meet their greater need for calcium during pregnancy and lactation.

These theories are not incompatible with each other. Once this sex difference had become established, for whatever reason (infantile mimicry, physiological side-effect, need for more vitamin D), it could have then become a visual cue for the algorithms that men use to recognize women and assess mate value.


-One theory states that all people came from Africa and migrated to other locations. Why would the migration of those groups lead to such radical differences between races, such as an African's wider nose, a European's fine hair texture, an Asian's almond shaped eyes and an Inuit's body structure?

Many of these physical differences do not seem to involve differences in adaptation to the physical environment. This is especially so for difference in hair and eye color and difference in hair length. It may also be partially true for differences in skin color. The Amerindians have inhabited North and South America from the Arctic to the Tropics for some 12,000-15,000 years, and yet they all have almost the same skin color.

Some people, including Darwin, have attributed these physical differences to the action of sexual selection (as opposed to natural selection by climate, sunlight, etc.). According to this hypothesis, criteria of beauty differ from one human population to the next and, over time, these differences have pushed sexual selection in different directions, with the result that human populations now differ considerably in appearance.

The problem with this hypothesis is that human populations don't seem to differ considerably in beauty criteria. A number of controlled studies have shown that people of varying ethnic/racial backgrounds share similar notions of beauty. In the most comprehensive of these studies, Cunningham et al. (1995) assessed criteria of female beauty among men of Taiwanese, White American, Black American, Asian, and Hispanic backgrounds. All of the subjects perceived a female face to be more attractive when possessing high eyebrows, widely spaced large eyes with dilated pupils, high cheekbones, small nose, narrow face with thin cheeks, large smile, full lower lip, small chin, and fuller hairstyle. There was some variation. The East Asian subjects tended to prefer more immature and inexpressive faces whereas the Black American subjects tended to prefer women with larger buttocks and heavier body build. These differences in beauty criteria seem to be minor and are certainly dwarfed by the differences in physical appearance that Darwin sought to explain.

I have argued that sexual selection has differed among human populations not only in its criteria but also in its intensity with respect to women and men. Intensity of sexual selection is determined by two factors: the overall ratio of men to women and the incidence of polygyny (which affects the ratio of men to women among those who are sexually available). These two factors varied among the different environments that humans inhabited during the hunter-gatherer and agricultural stages of prehistory.

Competition for mates correspondingly varied. In some environments, women competed against other women for a limited number of available men. In others, men competed against other men for a limited number of available women. Female-female competition for available men intensified with increasing distance from the equator. First, the demographic sex ratio became more female-biased because a lower density of game animals lengthened hunting distances and thus increased male mortality. Second, the operational sex ratio became more female-biased because polygyny became less frequent due to the longer winters that restricted food gathering and increased female dependence on male provisioning.

Women thus competed the most for mates in the 'continental Arctic,' i.e., steppe-tundra where wandering herds were almost the sole food supply. Inversely, men competed the most for mates in the Tropics, particularly where year-round agriculture replaced hunting and gathering, thereby allowing women to become primary food producers and freeing men to take more wives.
When too many of one sex compete for too few of the other, the sex in short supply can pick and choose. It can thus favor certain visible characteristics at the expense of others. If demand only somewhat exceeds supply, the characteristics will be those that matter beyond the immediate moment of selection, i.e., that indicate health, youth, or fecundity. Such criteria become relatively less important as demand outstrips supply. The more candidates there are for any one potential mate, the more they must vie for attention and the more success will depend on having characteristics that excite and retain attention.

This is the logic of advertising. 'Visual merchandising' matters most in saturated, highly competitive markets where the consumer is presented with many interesting and otherwise similar choices. When faced with a surplus of choice, an animal, like a consumer, is more easily swayed by eye-catching stimuli. For example, a secondary sexual characteristic may be larger or more vividly colored, thereby hyperstimulating the observer's sex-identification algorithms. Alternately, the visually enhanced feature may be used not for sex identification but for personal identification and communication (e.g., the eyes and other facial features). Either way, one's chances of mating are improved, as are the chances of passing on the eye-catching features.

Variation in mate competition may have influenced the evolution of highly visible and colorful physical traits, like hair length, eye/hair pigmentation, and perhaps even skin color. In particular, it may explain an unusual convergence of color traits in northern and eastern Europe. Within this region, both hair and eye color are highly polymorphic and skin is unusually white, almost at the physiological limit of depigmentation, and much whiter than in other populations at similar latitudes with similar exposure to solar UV at ground level. This 'European exception' constitutes a marked deviation from human geographic variation in hair, eye, and skin color.

The 'intensity of sexual selection' hypothesis predicts that among hunter-gatherers, and by extension early modern humans, female-female rivalry for mates should intensify with increasing distance from the equator, ultimately peaking in the steppe-tundra of the continental Arctic. Today, this environment is confined to the northern fringes of continental Eurasia and North America. During the last ice age, it lay further south and covered more territory, especially in Europe. The Scandinavian icecap had pushed the steppe-tundra zone far to the south and on to the plains stretching from southwestern France through northern Germany and into eastern Europe. This expanse of continental tundra-the largest during human prehistory-closely corresponds to the geographic distribution of hair/eye color dimorphism and extreme skin depigmentation.

References

Cunningham, M.R., Roberts, A.R., Barbee, A.P., Druen, P.B., and Wu, C-H. (1995). "Their ideas of beauty are, on the whole, the same as ours": consistency and variability in the cross-cultural perception of female physical attractiveness. Journal of Personality and Social Psychology, 68, 261-279.

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

4 comments:

Amir said...

Hey another great post! So have you read Robin Baker's "Sperm Wars". What do you think about his theories?

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Anonymous said...

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