4. Hormonal inputs into perception of human skin color by men and women

My wish list for research in 2024

Subjects identify the face on the left as female and the face on the right as male. The only difference is the lightness of the skin. Richard Russell, Sinha Laboratory for Vision Research, MIT.

“The fair sex” is paler than men, who conversely are ruddier and browner than women. This sex difference seems to play a role in gender recognition and in relations between men and women, particularly in female response to darker male skin.

Women are universally the fair sex. They are paler than men, who conversely are ruddier and browner (Frost, 2010; Frost, 2023; van den Berghe and Frost, 1986). This sex difference is due to the differing ways the skin’s pigments—melanin, hemoglobin, carotene—interact with the sex hormones, either androgens in men or estrogens in women. A hormonal cause has been shown by studies of normal, castrated, and ovariectomized individuals, by studies of skin reflectance at puberty, and by studies of digit ratios (Edwards and Duntley, 1939; Edwards et al., 1941; Edwards and Duntley, 1949; Frost, 1988; van den Berghe and Frost, 1986; Manning et al., 2004).

Gender recognition

This sex difference is used subconsciously to recognize male and female faces (Frost, 2011; Russell, 2003; Russell, 2009; Russell, 2010; Russell et al., 2006; Semin et al., 2018).

Specifically, gender is identified from two aspects of facial color:

• hue (men are ruddier and browner)

• brightness (facial skin is lighter in women and contrasts more with the darker lip/eye area).

Hue provides the observer with a fast channel for gender recognition. If a face is too far away or the lighting too dim, the observer will switch to the slower but more accurate channel of brightness (Dupuis-Roy et al., 2009; Dupuis-Roy et al., 2019; Jones et al., 2015; Nestor and Tarr, 2008a; Nestor and Tarr 2008b; Tarr et al. 2001; Tarr, Rossion, and Doerschner, 2002). We thus perceive skin color through the lens of a mental algorithm that arose for gender recognition. This algorithm may explain why lighter skin seems more feminine and darker skin more masculine (Semin et al., 2018).

Male-female relations

The differing complexions of men and women play a role not only in gender recognition but also in relations between men and women. In particular, it seems to play a role in attraction by women to men.

In one study, women were asked to optimize the attractiveness of facial pictures by varying the skin's darkness and ruddiness. They made the male faces darker and ruddier than the female faces (Carrito et al., 2016). In another study, women were asked to rate different levels of male ruddiness. They associated high levels with aggression, medium levels with dominance, and low levels with attractiveness. Unlike the participants of the first study, they may have understood the term “attractive” in an aesthetic or even feminine sense (Stephen et al., 2012).

Female attraction to darker, ruddier male skin seems to be mediated by the level of estrogen in brain tissues. This estrogenic effect is shown by two studies of women at different phases of their menstrual cycle and by a study of preschool children:

• Women were shown pairs of facial pictures that differed slightly in the lightness of the skin, and they were asked to choose the most pleasing one. When male faces were shown, the darker one was more strongly preferred by those women who were in the first two-thirds of their menstrual cycle than by those in the last third. During the first two-thirds of the cycle, the level of estrogen is high in relation to the level of progesterone (which acts as an anti-estrogen). During the last third, the ratio is reversed: the level of estrogen is low in relation to the level of progesterone. There was no cyclical effect among women judging female faces or taking oral contraceptives (Frost, 1994).

• Women had their brain activity measured by MRI while viewing pictures of male faces. Their brains showed a stronger response to masculinized male faces than to feminized ones, and the strength of their response correlated with the level of estrogen across the menstrual cycle. In a personal communication, the lead author stated that the faces had been masculinized by making them darker and more robust in shape (Rupp et al., 2009).

• Preschool boys and girls were presented with two dolls that differed slightly in skin color and asked to choose the “nicer” one. Their choices were recorded, as were measurements of their body mass index and their subcutaneous fat. Doll choice did not differ by sex. But it did differ by adiposity. Among children less than three years old, those who chose the darker doll had significantly more body fat than those who chose the lighter doll. In that age range, estrogen is produced mostly in the fatty tissues, which contain an enzyme (aromatase) that converts an androgen (androstenedione) into an estrogen (estrone) (Baird, 1976; Frost, 1989).

The doll on the right is slightly darker and ruddier than the one on the left. Among children below three years of age, those who chose the darker doll had significantly more body fat than those who chose the lighter doll. At such ages, estrogen is produced mainly in the body’s fatty tissues.

In other doll studies, boys and girls have similar preferences up to six years of age (Renninger and Williams, 1966; Williams and Roberson, 1967; Williams and Rousseau, 1971). At older ages, male and female preferences begin to diverge. When a group of American children, 3 to 8 years of age, were presented with a white-faced puppet and a brown-faced one, the latter puppet was more often chosen by girls than by boys, this finding being as true for Euro-American children as for African American children (Asher and Allen, 1969).

There are fewer controlled studies of male response to lighter female skin. It has been argued that the lighter skin of women mimics that of infants, whose pinkish color is especially noticeable in darker-skinned populations and, apparently, in other primate species. It seems to identify the primate infant as a vulnerable being in need of protection (Alley, 1980; Booth, 1962; Jay, 1962).

In our species, the adult female may have evolved a lighter complexion as a means to tap into the same behavioral response, the aim being not so much to increase male sexual arousal as to reduce male aggressiveness and stimulate feelings of care (Frost, 2010, p. 131-136; Frost, 2023; Guthrie, 1970).

Proposed study

First research aim: expand on Rupp et al. (2009) by using brain MRI to measure how women respond to male facial hue and luminosity in relation to the levels of estrogen and progesterone across the menstrual cycle. Male facial photos would be altered to produce different degrees of brownness, redness, and brightness.

Second research aim: repeat the doll study of Frost (1989) with direct measures of estrogen and androgen levels in preschool children. This may be difficult, given the low hormonal levels of early childhood (Baird, 1976; Klein et al., 1994).

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