My wish list for 2024: Hormonal inputs into perception of human skin color by men and women

 

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.

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“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.

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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).

 

 

References

 

Alley, T. R. (1980). Infantile colouration as an elicitor of caretaking behaviour in Old World primates. Primates 21(3): 416-429. https://doi.org/10.1007/BF02390470

 

Asher, S.R. and Allen, V.L. (1969). Racial preference and social comparison processes. Journal of Social Issues 25(1): 157-166. https://doi.org/10.1111/j.1540-4560.1969.tb02584.x    

 

Baird, D.T. (1976). Oestrogens in clinical practice. In: J.A. Loraine and E. Trevor Bell (eds.) Hormone assays and their clinical application (p. 408). Edinburgh: Churchill Livingstone.

 

Booth, C. (1962). Some observations on behavior of Cercopithecus monkeys. Annals of the New York Academy of Sciences 102(2): 477-487. https://doi.org/10.1111/j.1749-6632.1962.tb13654.x   

 

Bruce, V., and Langton, S. (1994). The use of pigmentation and shading information in recognising the sex and identities of faces. Perception 23(7): 803-822. http://dx.doi.org/10.1068/p230803   

 

Carrito, M.L., dos Santos, I.M.B., Lefevre, C.E., Whitehead, R.D., da Silva, C.F., and Perrett, D.I. (2016). The role of sexually dimorphic skin colour and shape in attractiveness of male faces. Evolution and Human Behavior 37(2): 125-133. https://doi.org/10.1016/j.evolhumbehav.2015.09.006    

Dupuis-Roy, N., Faghel-Soubeyrand, S., and Gosselin, F. (2019). Time course of the use of chromatic and achromatic facial information for sex categorization. Vision Research 157: 36-43. https://doi.org/10.1016/j.visres.2018.08.004   

 

Dupuis-Roy, N., Fortin, I., Fiset, D., and Gosselin, F. (2009). Uncovering gender discrimination cues in a realistic setting. Journal of Vision 9(2): 10, 1-8. https://doi.org/10.1167/9.2.10   

 

Edwards, E.A., and Duntley, S.Q. (1939). The pigments and color of living human skin. American Journal of Anatomy 65(1): 1-33. https://doi.org/10.1002/aja.1000650102   

 

Edwards, E.A., and Duntley, S.Q. (1949). Cutaneous vascular changes in women in reference to the menstrual cycle and ovariectomy. American Journal of Obstetrics & Gynecology 57(3): 501-509. https://doi.org/10.1016/0002-9378(49)90235-5   

 

Edwards, E.A., Hamilton, J.B., Duntley, S.Q., and Hubert, G. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men. Endocrinology 28(1): 119-128. https://doi.org/10.1210/endo-28-1-119   

 

Frost, P. (1988). Human skin color: A possible relationship between its sexual dimorphism and its social perception. Perspectives in Biology and Medicine 32(1): 38-58. https://doi.org/10.1353/pbm.1988.0010

 

Frost, P. (1989). Human skin color: the sexual differentiation of its social perception. Mankind Quarterly 30: 3-16. http://doi.org/10.46469/mq.1989.30.1.1   

 

Frost, P. (1994). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship. Perceptual and Motor Skills 79(1): 507-14. https://doi.org/10.2466/pms.1994.79.1.507   

 

Frost, P. (2010). Femmes claires, hommes foncés. Les racines oubliées du colorisme. Quebec City: Les Presses de l'Université Laval, 202 p. https://www.pulaval.com/livres/femmes-claires-hommes-fonces-les-racines-oubliees-du-colorisme    

 

Frost, P. (2011). Hue and luminosity of human skin: a visual cue for gender recognition and other mental tasks. Human Ethology Bulletin 26(2): 25-34. https://www.researchgate.net/publication/256296588_Hue_and_luminosity_of_human_skin_a_visual_cue_for_gender_recognition_and_other_mental_tasks    

 

Frost, P. (2023). The original meaning of skin color. Aporia Magazine, February 7.

 

Guthrie, R.D. (1970). Evolution of human threat display organs. In T. Dobzhansky, M.K. Hecht, and W.C. Steere (Eds.) Evolutionary Biology 4: 257-302. New York: Appleton-Century Crofts.

 

Hill, H., V. Bruce, and Akamatsu, S. (1995). Perceiving the sex and race of faces: The role of shape and colour. Proceedings of the Royal Society B: Biological Sciences 261(1362): 367-373. https://doi.org/10.1098/rspb.1995.0161   

 

Hill, R., and Barton, R. (2005). Red enhances human performance in contests. Nature 435: 293. https://doi.org/10.1038/435293a   

 

Jay, P.C. (1962). Aspects of maternal behavior among langurs. Annals of the New York Academy of Sciences 102(2): 468-476. https://doi.org/10.1111/j.1749-6632.1962.tb13653.x

 

Jones, A.L., Russell, R., and Ward, R. (2015). Cosmetics alter biologically-based factors of beauty: evidence from facial contrast. Evolutionary Psychology 13(1): https://doi.org/10.1177%2F147470491501300113    

 

Klein, K.O., Baron, J., Colli, M.J., McDonnell, D.P., and Cutler, G.B. Jr. (1994). Estrogen levels in childhood determined by an ultrasensitive recombinant cell bioassay. Journal of Clinical Investigation 94(6): 2475-2480. https://doi.org/10.1172/JCI117616

 

Manning, J.T., Bundred, P.E., and Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25(1): 38-50. https://doi.org/10.1016/s1090-5138(03)00082-5   

 

Nestor, A., and Tarr, M.J. (2008a). The segmental structure of faces and its use in gender recognition. Journal of Vision 8(7): 7, 1-12, https://doi.org/10.1167/8.7.7   

 

Nestor, A., and Tarr, M.J. (2008b). Gender recognition of human faces using color. Psychological Science 19(12): 1242-1246. https://doi.org/10.1111/j.1467-9280.2008.02232.x   

 

Renninger, C.A. and Williams, J.E. (1966). Black-white color connotations and racial awareness in preschool children. Perceptual and Motor Skills 22(3): 771-785. https://doi.org/10.2466/pms.1966.22.3.771   

 

Rupp, H.A., James, T.W., Ketterson, E.D., Sengelaub, D.R., Janssen, E., and Heiman, J.R. (2009). Neural activation in women in response to masculinized male faces: mediation by hormones and psychosexual factors. Evolution and Human Behavior 30(1): 1-10. https://doi.org/10.1016/j.evolhumbehav.2008.08.006   

 

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features. Perception 32(9): 1093-1107. http://dx.doi.org/10.1068/p5101   

 

Russell, R. (2009). A sex difference in facial pigmentation and its exaggeration by cosmetics. Perception 38(8): 1211-1219. https://doi.org/10.1068/p6331   

 

Russell, R. (2010). Why cosmetics work. In: R.B. Adams Jr., N. Ambady, K. Nakayama, and S. Shimojo (eds.) The Science of Social Vision, (pp. 186-203). New York: Oxford.

Russell, R., Sinha, P., Biederman, I., and Nederhouser, M. (2006). Is pigmentation important for face recognition? Evidence from contrast negation. Perception 35: 749-759. https://doi.org/10.1068%2Fp5490   

 

Semin, G.R., Palma, T., Acartürk, C., and Dziuba, A. (2018). Gender is not simply a matter of black and white, or is it? Philosophical Transactions of the Royal Society B Biological Sciences 373(1752):20170126. https://doi.org/10.1098/rstb.2017.0126    

 

Siiteri, P.K. and MacDonald, P.C. (1973). Role of extraglandular estrogen in human endocrinology. In: S.R. Geiger (ed.), Handbook of Physiology, vol. II, Part 1, (pp. 615-629). Washington D.C.: American Physiology Society, Section 7.

 

Stephen, I.D., Oldham, F.H., Perrett, D.I., and Barton, R.A. (2012). Redness enhances perceived aggression, dominance and attractiveness in men's faces. Evolutionary Psychology 10(3). https://doi.org/10.1177%2F147470491201000312   

 

Tarr, M.J., Kersten, D., Cheng, Y., and Rossion, B. (2001). It's Pat! Sexing faces using only red and green. Journal of Vision 1(3): 337, 337a. https://doi.org/10.1167/1.3.337   

 

Tarr, M. J., Rossion, B., and Doerschner, K. (2002). Men are from Mars, women are from Venus: Behavioral and neural correlates of face sexing using color. Journal of Vision 2(7): 598, 598a, https://doi.org/10.1167/2.7.598   

 

Trivers, R., Manning, J., and Jacobson, A. (2006). A longitudinal study of digit ratio (2D:4D) and other finger ratios in Jamaican children. Hormones and Behavior 49(2): 150-156. https://doi.org/10.1016/j.yhbeh.2005.05.023     

 

van den Berghe, P. L. and P. Frost. (1986). Skin color preference, sexual dimorphism, and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies 9(1): 87-113. https://doi.org/10.1080/01419870.1986.9993516

 

Williams, J.E. and Roberson, J.K. (1967). A method for assessing racial attitudes in preschool children. Educational and Psychological Measurement 27(3): 671-689. https://doi.org/10.1177/001316446702700310   

 

Williams, J.E. and Rousseau, C.A. (1971). Evaluation and identification responses of Negro preschoolers to the colors black and white. Perceptual and Motor Skills 33(2): 587-599. https://doi.org/10.2466/pms.1971.33.2.587   

Red is beautiful: Perceived femininity of skin color in an African population

 

Perceived masculinity and femininity of facial skin color. Cameroonian women rated faces of Cameroonian men, and Cameroonian men rated faces of Cameroonian women (Fiala et al. 2022, Supplementary Material).

 

 

 

Women are the fair sex. They are paler than men, who conversely are ruddier and browner. Today, that sexual dimorphism is hardly noticed in Western societies, having been overwhelmed by much larger differences of race and ethnicity and further obscured since the 1920s by the tanning fad (Segrave 2005). But it was noticed earlier. Wherever the visual arts developed—ancient Egypt, the Greco-Roman world, early South and East Asia, Mesoamerica—female figures were given a lighter hue and male figures a darker one (Capart 1905, pp. 26-27; Eaverly 2013; Frost 2010, pp. 35-81; Pallottino 1952, pp. 34, 45, 73, 76-77, 87, 93, 95, 105, 107, 115; Soustelle 1970, p. 130; Tegner 1992; Wagatsuma 1967).

 

Sexual dimorphism in skin pigmentation

 

Skin color was first measured objectively in the 1930s, when spectrophotometers became commercially available. By measuring how much light the skin reflects across the visible spectrum, and how much it absorbs, one could identify its pigments and quantify their relative importance. Edwards and Duntley (1939) concluded that male and female complexions differ because of differing concentrations of melanin (brown), hemoglobin (red), and carotene (yellow).

 

Castration keeps men from acquiring their distinctive complexion:

 

One of the outstanding characteristics of a human male castrate is the paleness of the skin. After treatment with androgenic hormone, however, the individual takes on a darker and more ruddy hue. This observation suggests that the skin of the castrate is deficient in melanin and blood, and that the androgenic hormone increases the content of these substances in the integument. (Edwards et al. 1941)

 

Estrogen has similar but much weaker effects, which are further reduced by the other female hormone, progesterone. Ovariectomy thus has much less impact on female skin than castration has on male skin (Edwards and Duntley 1949). The sex hormones seem to alter skin pigmentation not only through ongoing transient effects but also through permanent organizational effects before birth and at puberty.

 

A hormonal causation is also suggested by the digit ratio. This is the length of the index finger divided by the length of the ring finger, and it tells us the relative proportions of estrogens to androgens in body tissues during development. In adults, the digit ratio correlates with lightness of female skin but not with lightness of male skin (Manning et al. 2004).

 

Sexual dimorphism in tanning capacity

 

Men and women likewise differ in tanning capacity. Men tan more than women even when both are equally exposed to the sun. This was shown in a New Guinea study of three body sites: skin on the unexposed upper inner arm; skin on the exposed forearm; and time spent in the sun. Despite identical sun exposure, the men were darker than the women, and more so on exposed skin (Harvey 1985). The same finding appears in another New Guinea study, whose author ruled out the possibility of the women being less exposed, "as in most parts of New Guinea the adult females are responsible for most of the food cultivation and are therefore exposed almost continuously to sunlight." (Walsh 1964).

 

Differences between human populations

 

Skin color is more sexually dimorphic where people are medium-colored and less so where they are very fair or very dark (Frost 2007; Madrigal and Kelly 2007). This sexual dimorphism cannot fully express itself in a very fair population because female skin encounters a physiological limit when it lightens after puberty. In a very dark population, male skin likewise encounters a physiological limit to further darkening.

 

Are there other population differences? Has this sexual dimorphism evolved differently in different populations?

 

Apparently. A recent paper shows that this sex difference differs qualitatively between Europeans and sub-Saharan Africans. When Fiala et al. (2022) measured the skin color of individuals from the Czech Republic and Cameroon, they found that women had fairer skin in both groups. But their skin was fairer in different ways. Among the Czechs, female skin was less red than male skin, in line with previous studies on European or Euro-American subjects. Among the Cameroonians, however, female skin was redder than male skin, and also more yellow.

 

Adaptation to the natural environment?

 

Is the redder complexion of African women an adaptation to the natural environment? If the skin is better supplied with blood, does it better cope with UV radiation, heat load, skin injuries, or some other aspect of a tropical environment? Let’s examine these three factors, while keeping in mind that they would have to be more fitness-reducing for African women than for African men.

 

UV radiation. The yellow pigment of skin (carotene) does provide some protection from UV (Stahl et al. 2012). So it’s plausible that African women compensate for having less melanin in their skin by having more carotene. On the other hand, there is no evidence that the red pigment of skin (hemoglobin) provides UV protection.

 

Heat load. When more blood is flowing to the skin, heat is radiated away more easily from the body (Hertzman 1959). It may be, then, that the increased redness of African female skin serves to disperse body heat in warmer climates. Nonetheless, we still have to explain why heat load is more fitness-reducing for African women than for African men.

 

Skin injuries. When more blood is flowing to the skin, wounds heals faster because more leukocytes can reach skin tissues and fight potential infections (Mathieu et al. 2006). Again, we still have to explain why this factor would matter more for African women than for African men. Coetzee et al. (2012) raise a similar objection: if ruddiness is attractive because it indicates physical health, why is it considered unattractive in the case of European women or African men?

 

Adaptation to the social environment?

 

Alternatively, the redder complexion of African women may have evolved as an adaptation to the social environment, specifically for gender recognition. In a study using Euro-American participants, people could tell whether a facial photo was male or female, at a rate much higher than chance, even when the image was blurred and provided no useful information other than the degree of redness (Tarr et al. 2001). Sexual dimorphism in skin color has two components: hue (degree of brownness and redness) and luminosity (degree of contrast between lightness of facial skin and darkness of lip/eye area). Hue is the fast channel for gender recognition. If the face is too far away or the lighting too dim, the mind will switch to the slower but more accurate channel of luminosity (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). This gender cue may serve not only to tell men and women apart but also to modify male behavior by reducing aggressiveness and stimulating feelings of care and protection (Frost 2011).

 

African women maintain this gender cue through a different mix of skin pigments. They have more carotene in their skin, and thus a yellower complexion, to offset the loss of UV protection due to having less melanin. Unlike European women, they also have more blood in their skin and thus a redder complexion. Why is this? Perhaps increased redness does not visually alter dark skin in the same way that it visually alters light skin. When redness is increased, the dark skin of African women may look lighter and the light skin of European women may look darker. The social environment has thus favored lighter female skin in both populations, but the mix of pigments is different.

 

This gender cue was studied by Fiala et al. (2022) in their Czech/Cameroonian study. Cameroonian women were asked to rate facial photos of Cameroonian men, and Cameroonian men were asked to rate facial photos of Cameroonian women. The results showed a significant correlation between skin color and perceived masculinity/femininity:

 

The slope between perceived masculinity and colour (higher scores along all three CIELab dimensions, meaning basically lighter skin that allows both redness and yellowness to stand out) of Cameroonian men was negative - 0.29 (CI: - 0.52, - 0.05). In Cameroonian women, the slope between perceived femininity and colour was conclusively positive 0.52 (CI: 0.27, 0.76).

 

[…] More masculine men in the Cameroonian sample have therefore darker, less red, and less yellow skin colour.

[…] More feminine women thus have a lighter, yellower, and redder skin than less feminine women.

 

There was no such correlation among the Czechs. This second finding seems to contradict previous findings that facial skin color is used for gender recognition (see above). In those studies, however, the participants were Euro-American or Euro-Canadian, and they were not necessarily conscious of the visual cues they were using. At least on a conscious level, the sex difference in skin color has lost its social significance within the Western world, largely because of the growing importance of racial/ethnic differences in real life and in the virtual life of advertising and the mass media. In addition, the naturally lighter complexion of women has often been reduced or eliminated through deliberate tanning.

 

Ethnographic data

 

When we were preparing our joint paper on skin color preference, Pierre van den Berghe examined the Human Relations Area Files, a cross-cultural database. He found a strong association in traditional societies between femininity and lightness of skin color: the ideal woman was described as “white” in Europe, the Middle East, and East Asia, as “golden” in Southeast Asia, and as “red” in sub-Saharan Africa (van den Berghe and Frost 1986). We were somewhat surprised to see this idealization of female redness in different ethnographic accounts:

 

Tallensi (Ghana) ‑

“In skin colour they vary from black through chocolate brown to bronze, which the natives call “red” (bon‑ze'e) and regard as the most attractive bodily hue.” (Fortes 1945, p. 7)

 

Hausa (Nigeria) ‑

“Light skin colour, referred to as “red”, ranks high in the Hausa criteria of beauty; many variations of colour, from black to a very light reddish brown are seen.” (Smith 1965, p. 264)

 

Igbo (Nigeria) ‑

“In Ibo culture, however, these yellowish or reddish complexions are considered more beautiful than the darker, ‘blacker,’ complexions.” (Ardener 1954, pp. 71-72)

 

Somali (Somalia) ‑

“Men appreciate women of good height and stature, with good hips and breasts, and plump but not fat.  A reddish tinged skin is thought highly of in preference to a dark dull black.” (Lewis 1962, p. 13)

 

This ideal is explained at some length by Lugira (1970, pp. 34-35) with respect to the Ganda people of Uganda:

 

The Ganda concept of skin pigmentation considers light coloured complexions to be differing shades of white.  A dark brown skin colour is said to be — eruyeru, that is, somewhat white.  A really brown‑reddish‑yellow person is said to be mweru = white, which in comparison would be considered to be blonde; and this in the Ganda aesthetic language is considered as red = myufu, the most perfect skin pigmentation. (Lugira 1970, pp. 34‑35)

 

So the question remains open. Female skin may be redder in Africa because of selection by the natural environment, perhaps as a means to reduce heat load or facilitate wound healing. There is also evidence, however, for selection by the social environment.

 

References

 

Ardener, E.W. (1954). Some Ibo attitudes to skin pigmentation. Man 54: 71-73. http://www.jstor.org/stable/2793760

 

Capart, J. (1905). Primitive Art in Egypt. London: H. Grevel.

 

Coetzee, V., S.J. Faerber, J.M. Greeff, C.E. Lefevre, D.E. Re, and D.I. Perrett. (2012). African Perceptions of Female Attractiveness. PLoS ONE 7(10): e48116. https://doi.org/10.1371/journal.pone.0048116

 

Dupuis-Roy, N., S. Faghel-Soubeyrand, and F. Gosselin. (2019). Time course of the use of chromatic and achromatic facial information for sex categorization. Vision Research 157: 36-43. https://doi.org/10.1016/j.visres.2018.08.004

 

Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting. Journal of Vision 9(2): 10, 1-8. https://doi.org/10.1167/9.2.10

 

Eaverly, M.A. (2013). Tan Men/Pale Women. Color and Gender in Archaic Greece and Egypt, a Comparative Approach. Ann Arbor: The University of Michigan Press. https://doi.org/10.3998/mpub.3080238

 

Edwards, E.A., and S.Q. Duntley. (1939).The pigments and color of living human skin. American Journal of Anatomy 65(1): 1-33. https://doi.org/10.1002/aja.1000650102

 

Edwards, E.A., and S.Q. Duntley. (1949). Cutaneous vascular changes in women in reference to the menstrual cycle and ovariectomy. American Journal of Obstetrics & Gynecology 57(3): 501-509. https://doi.org/10.1016/0002-9378(49)90235-5

 

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men. Endocrinology 28(1): 119-128. https://doi.org/10.1210/endo-28-1-119

 

Fiala, V., P. Ture?ek, R.M. Akoko, Š. Pokorný, and K. Kleisner. (2022). Africans and Europeans differ in their facial perception of dominance and sex-typicality: a multidimensional Bayesian approach. Scientific Reports 12(1): 6821. https://doi.org/10.1038/s41598-022-10646-6. 

 

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Puberty and skin color

Skin color differentiates between boys and girls after puberty. Before puberty, girls are actually darker-skinned than boys (Kalla 1973; Mesa 1983)

Complexions differ between the sexes: women are paler and men ruddier and browner. Today, this sex difference seems hardly noticeable in Western societies, having been overwhelmed by much larger differences of race and ethnicity and further obscured since the 1920s by the tanning fad (Segrave 2005). Nonetheless, it was noticed in earlier times. A lighter hue was traditionally given to female figures and a darker hue to male figures in the visual arts of all early civilizations, including those of Italy, Greece, Egypt, China, Japan, and Mesoamerica (Capart 1905, pp. 26-27; Eaverly 1999; Pallottino 1952, pp. 34, 45, 73, 76-77, 87, 93, 95, 105, 107, 115; Siepe 2004; Soustelle 1970, p. 130; Tegner 1992; Wagatsuma 1967). This sex difference also appears in ancient Greek poetry, where women are described as “white” and men as “black” (Irwin 1974, pp. 121, 129-155). “White” skin is still key to female identity in many non-Western societies, as shown by interviews with Japanese men: "Whiteness is a symbol of women, distinguishing them from men." "One's mother-image is white" (Wagatsuma 1967, pp. 417-418).

Spectrophotometric studies

With the advent of the spectrophotometer, researchers could study skin color by measuring the percentage of light reflected by the skin, most often at the upper inner arm—where tanning is minimal. An American team thus attributed the differing complexions of men and women to differing concentrations of the three main skin pigments: melanin, hemoglobin, and carotene (Edwards and Duntley 1939). The same team showed that this sex difference was reduced by ovariectomy and even more so by castration (Edwards and Duntley 1949; Edwards et al. 1941). Later research identified puberty as the time when boys and girls diverge in skin color (van den Berghe and Frost 1986). The best controlled studies are those by Kalla (1973) and Kalla and Tiwari (1970) on South Asians and Tibetans and by Mesa (1983) on Spanish participants. The samples are large enough to measure this sexual differentiation by year and by sex. In addition to showing that girls become progressively lighter-skinned than boys during adolescence, these studies also show that girls are actually darker-skinned than boys just before puberty.

These sex and age differences thus seem to be innately programmed, specifically via the sex hormones. This hypothesis is further supported by a study of skin color in monozygotic and dizygotic twins from three age groups: 12 year olds; 13 to 15 year olds; and 16 to 18 year olds. Variance within the twin pairs differed significantly on average between monozygotic and dizygotic twins, thus “indicating a strong genetic component in the variability of skin lightness.” As in other studies, puberty had a stronger effect on female skin color than on male skin color, with girls becoming progressively lighter-skinned. For both sexes, mean within-pair variance did not differ significantly from one age group to the next, further indicating that these age changes are under genetic control (Omoto 1965).

Nonetheless, most people, including academics, have continued to ascribe the differing complexions of men and women to differences in lifestyle (see for example Eaverly 1999 and Irwin 1974). Perhaps girls become lighter-skinned after puberty because they are less free to go outside unaccompanied, as used to be the case in many cultures. The sex difference in skin color should therefore disappear as women come to resemble men in terms of lifestyle. This alternative hypothesis is supported by a recent study of young adults from Ireland, Poland, Italy, and Portugal, which found women to be darker-skinned than men on the upper inner arm, the body site most often used to measure the color of untanned skin (Candille et al. 2012). This finding contradicts findings from earlier studies on Europeans or European-descended participants (van den Berghe and Frost 1986).

Digit ratio studies (2D:4D)

Recently, the innate causation hypothesis has received support from two "digit ratio" studies. This ratio is index finger length divided by ring finger length, and it corresponds to the ratio of androgens to estrogens in the fluids of the developing fetus. A higher ratio indicates more feminization, and a lower ratio more masculinization.

A British team led by John Manning (2004) examined adults of both sexes. Lightness of skin color was found to correlate in women but not in men with digit ratio, i.e., women are lighter-skinned if their body tissues have been exposed to higher estrogen levels. This finding was true for both the left hand and the right hand, although the correlation was stronger for the left hand.

A Polish team led by Aneta Sitek (2018) looked at children just before puberty, when girls are actually darker-skinned than boys. Darkness of skin color was found to correlate in girls but not in boys with digit ratio, i.e., pre-pubertal girls are darker-skinned if their body tissues have been exposed to higher estrogen levels. This finding was true only for the right hand.

For reasons still unclear, the digit ratio of the right hand is more responsive to the sex hormones than the digit ratio of the left hand, as shown by a greater sex difference in digit ratio for the right hand than for the left (Honekopp and Watson 2010). In reviewing the literature, Honekopp and Watson (2010) argue that right-hand digit ratio is a better indicator of prenatal exposure to the sex hormones. Left-hand digit ratio seems to be more affected by hormonal exposure later in life. This is suggested by the findings of a longitudinal study: digit ratio increases in children with age, and this effect is greater for the left hand than for the right hand (Trivers et al. 2006).

Discussion

Digit ratio studies point to a hormonal cause, and not to differences in lifestyle, as the reason why skin color differentiates between boys and girls at puberty. This is consistent with earlier spectrophotometric studies on normal, castrated, and ovariectomized individuals (Edwards and Duntley 1939; Edwards and Duntley 1949; Edwards et al. 1941). Furthermore, most spectrophotometric studies have shown that women are lighter-skinned than men even at the upper inner arm—a body site normally unaffected by tanning (van den Berghe and Frost 1986).

But why were women darker-skinned than men at this body site in a recent study of young adults from Ireland, Poland, Italy, and Portugal? (Candille et al. 2012). One can only conclude that the upper inner arm is no longer a reliable site for measuring the color of untanned skin. Perhaps young Western women now make a point of tanning their underarms because they increasingly shave this part of their body and expose it to view.

References

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http://www.jstor.org/stable/20027045