FWDM-2

Men have browner and ruddier complexions in comparison to the relative pallor of women, a result of differing melanin and hemoglobin levels in the skin's outer layers. This sex difference is universal and arises at puberty, as does differentiation in skin texture, body hair, fat distribution, and other cutaneous characteristics. Specifically, it arises because girls lighten in color much more than boys do during adolescence. It may widen even further in adulthood. Repeated tanning causes a gradual darkening of permanent, "constitutive" pigmentation and this effect seems to be stronger in men than in women, although women may also darken with successive pregnancies.

Human skin reflectance across the visible spectrum (Edwards and Duntley, 1939). Women's skin reflects more light than does men's across the entire visible spectrum. From top to bottom, the three curves represent measurements taken at the breast (woman only), the belly, and the pectoral. At these sites, the sex difference in skin color seems much larger than it is at the upper inner arm. This has long been ascribed to differences in suntanning, yet the sex difference seems to reflect differences in all of the skin's pigments, and not simply melanin.

Across a wide range of populations, women's skin color is lighter than men's at the upper inner arm (Jablonski & Chaplin 2000; van den Berghe & Frost 1986). At this relatively unexposed site, reflectance (the percentage of light reflected by the skin) has an 83% heritability and thus provides a good measure of constitutive pigmentation. The constitutive sex difference seems to decrease from strongly to weakly pigmented peoples and is not significant in the Dutch — the least pigmented sample yet examined. Because the Dutch are already close to the physiological limit of human depigmentation, it may be that the skin of their adolescent girls cannot lighten any further (Rigters-Aris 1973).

Facultative pigmentation, or tanning, likewise seems to differ between the sexes. In Papua New Guinea, Harvey (1985) found that men tanned more than women did even though both groups spent the same amount of time outdoors and covered their bodies similarly. In Japan, Hulse (1967) determined that male reflectance declined much more than female reflectance as he went from the northern to the southern parts of the country, i.e., with more intense solar radiation, the men tanned more readily than the women did. Another New Guinea study, found that sun exposure gradually increased constitutive pigmentation over time, again more so in men. He noted that foreheads darkened more in men than in women with increasing age, yet "the adult females are responsible for most of the food cultivation and are therefore exposed almost continuously to sunlight" (Walsh 1964)

On unexposed skin, the sex difference in reflectance is minimal in weakly pigmented peoples, probably because postpubescent female lightening encounters a limit to further depigmentation. On exposed skin, however, adult male darkening should continue unabated. This may explain why a study found Dutch men to be significantly darker than Dutch women at the forehead, though both were alike at the upper inner arm. As in New Guinea, the men darkened more than the women did with increasing age (Rigters-Aris 1973).

We are still unsure why the sexes differentiate in skin pigmentation during adolescence. The process is apparently genetic in origin. In a carefully controlled study at a residential school, Kalla and Tiwari (1970) found that the boys and girls steadily diverged in skin color after puberty despite their following the same regimen of indoor/outdoor activities and wearing uniforms that prevented tanning. In a study on monozygotic twins, Omoto (1965) found that intrapair variance for skin color did not change as girls lightened in complexion during adolescence, again suggesting a minimal role for non-genetic factors.

The occurrence of this sexual differentiation after puberty suggests that the sex hormones are responsible. Both androgens and estrogens increase skin pigmentation by promoting melanin synthesis and cutaneous blood flow; the effect of androgens, however, is stronger (Edwards & Duntley 1949). This stronger effect in itself might explain why the sexes differentiate in skin color during adolescence, except that this differentiation occurs because girls lighten in color and not because boys darken.

An answer to this puzzle might be provided by Mazess (1967) who found that skin reflectance in women correlates with their degree of adiposity, i.e., the more subcutaneous fat they have, the lighter-skinned they are. Fatty tissue is known to contain an enzyme, aromatase, that converts one of the androgens into an estrogen (Siiteri & MacDonald 1973). The thickening of subcutaneous fat in adolescent girls should therefore lower androgen levels within the tissues immediately beneath the skin (in women, the adrenal glands produce a certain quantity of circulating androgens). It is thus this localized elimination of androgen — and only secondarily the overall lower level of circulating androgens in women — that probably causes the sex difference in skin pigmentation. This effect might also account for the fair complexion of young infants, given their proportionately large volume of "baby fat".

Thickness of subcutaneous fat by sex and by age (Parizkova 1977). The similarity between these sex and age differences and those for skin color may reflect localized production of estrogen in fatty tissues. It is probably this estrogen source that feminizes women's skin, making it fairer, as well as smoother and less hairy.

If women are less pigmented because their subcutaneous fat decreases their skin's exposure to androgen, one would have to conclude that the scientific literature has underestimated the sex difference in skin color. Most studies use the upper inner arm for measurements, to reduce the effects of tanning. This site, however, is only slightly dimorphic for subcutaneous fat and it might be expected that male and female reflectance values would differ more at the breasts and the waist. In fact, they do — five to ten times more than at the upper inner arm. But it has always been assumed that differences in sun exposure were responsible.

Final note: It may be that women in recent times have narrowed this sex difference through dieting and sun-tanning, thereby helping to reduce its social significance in modern societies.

Tanned skin entered women's fashion in the mid-1920s at the same time as bobbed hair and a figure displaying narrow hips, a flat chest, and large shoulders. This frankly androgynous appearance was dubbed "the boyish look" or "la garçonne" (Vogue June 22, 1929).

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