Subjects identified the left-hand image as a woman and the right-hand one as a man. Yet the two images differ only in skin tone. Study by Richard Russell, Sinha Laboratory for Vision Research, MIT.
Skin color differs by sex: women are fairer and men browner and ruddier. Women also exhibit a greater contrast in luminosity between their facial skin and their lip and eye areas. These differences arise from differing concentrations of three skin pigments: melanin (brown); hemoglobin (red); and carotene (yellow). The cause is ultimately hormonal, as shown by studies on castrated and ovariectomized adults, on boys and girls during puberty, and on digit ratios (Edwards and Duntley, 1939; Edwards and Duntley, 1949; Edwards et al., 1941; Frost, 2010; Kalla and Tiwari, 1970; Manning et al., 2004; Mesa, 1983; Omoto,1965; Porcheron et al., 2013). Women are fairer than men in all human populations. The difference is greatest in people of medium color and least in very dark- or very fair-skinned people, apparently because of "floor" or "ceiling" effects (Frost, 2007).
This sex difference is used by the human mind for sex recognition. In fact, it's more important for this purpose than other visual cues, like face shape. When subjects are shown an image of a human face, they can tell whether it is male or female even if blurred and differing only in hue and luminosity. Hue provides a "fast channel" for sex recognition. If the facial image is too far away or the lighting too dim, the mind switches to the "slow channel" and relies on luminosity (Bruce and Langton, 1994; Dupuis-Roy et al., 2009; Frost, 2011; Hill et al., 1995; Russell and Sinha, 2007; Russell et al., 2006; Tarr et al., 2001; Tarr et al., 2002).
Skin color also differs by age. It can be used to distinguish younger from older women, since the contrast in luminosity between facial skin and the lip/eye areas decreases with age (Porcheron et al., 2013). It can also be used to recognize infants. All humans are born with very little melanin, and the resulting pinkish-white skin is often remarked upon in different cultures.
This is especially so where adults are normally dark-skinned, in striking contrast to newborns. In Kenya, the latter are often called mzungu ('European' in Swahili), and a new mother may ask her neighbors to come and see her mzungu (Walentowitz, 2008). Among the Tuareg, children are said to be whitened by the freshness and moisture of the womb (Walentowitz, 2008). The situation in other African peoples is summarized by a French anthropologist: "There is a rather widespread concept in Black Africa, according to which human beings, before 'coming' into this world, dwell in heaven, where they are white. For, heaven itself is white and all the beings dwelling there are also white. Therefore the whiter a child is at birth, the more splendid it is" (Zahan, 1974, p. 385). A Belgian anthropologist makes the same point: "black is thus the color of maturity [...] White on the other hand is a sign of the before-life and the after-life: the African newborn is light-skinned and the color of mourning is white kaolin" (Maertens, 1978, p. 41).
This infant/adult difference is evolutionarily old, being present in nonhuman primates. In langurs, baboons, and macaques, the newborn's skin is pink while the adult's is black. This visual cue not only helps adults to locate a wayward infant but also seems to induce a desire to defend and provide care (Jay, 1962). Humans may respond similarly to the lighter color of infants and women. This would be consistent with a tendency by the adult female body to mimic the newborn body in other ways: face shape; pitch of voice; amount of body hair; texture and pliability of the skin; etc. Over time, women may have come to resemble this 'infant schema' because it is the one that can best reduce aggressiveness in a male partner and induce him to assume a provider role.
The sun-tanning fad: An aesthetic revolution
The sex-specific aspects of skin color have influenced the development of cosmetics in many cultures. Even in ancient times, women would use makeup to increase the natural contrast in luminosity between their facial skin and their lip/eye areas (Russell, 2009; Russell, 2010). They would also make their naturally fair complexion even fairer by avoiding the sun and applying white powders or bleaching agents.
This feminine aesthetic changed radically in the 1920s with the sudden popularity of sun-tanning throughout the Western world, initially as a health fad. Tanned skin then entered women's fashion and became part of the flapper image, along with bobbed hair, broad shoulders, a relatively flat chest, narrow hips, and long legs. This fashion image was intended to be hermaphroditic, the aim being to energize the erotic appeal of the female body by masculinizing some of its key features (Bard, 1998; Segrave, 2005).
Has the tanning fad un-gendered skin color? Not wholly, to judge by the above studies on sex recognition. There still seems to be a tendency to prefer a lighter skin tone for women than for men. This was the conclusion of a recent study on how people perceive two skin pigments: melanin (brown) and carotene (yellow). When shown facial images with varying concentrations of melanin and carotene, the subjects had a greater preference for carotene than for melanin. This preference was stronger for female faces than for male faces, irrespective of the observer's sex. Nonetheless, for both male and female faces, the preferred skin color was much darker than it would have been a century ago (Lefevre and Perrett, 2014).
Again, this aesthetic norm has darkened only in the Western world. The tanned look had some popularity among Japanese women in the postwar era up to the 1970s, but it has since gone out of fashion (Ashikari, 2005). It never did catch on elsewhere in Asia (Li et al., 2008). In North America and Europe, the tanned look seems much more persistent, and this persistence suggests that it is locked into place by something else in our cultural environment.
Such as ...? One factor may be the conscious effort to promote images of dark-skinned people in advertising and, more generally, in popular culture. Another factor may be a half-conscious desire in popular culture for more aggressive, "harder" forms of eroticism. This is something that fair skin is less effective at delivering, having been originally part of the infant schema and thus less conducive to that kind of emotional response.
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