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   

My wish list for research in 2024: Why does estrogen make my brown eyes blue? Sex linkage of hair and eye colors

 

Eye colors (R.A. Sturm, University of Queensland)

Estrogen seems to favor the expression of non-black hair and non-brown eyes during fetal development. The “new” hair and eye colors are not only more frequent among women but also associated, in the case of blue eyes, with feminization of male face shape, female shoulder width, and female waist-to-hip ratio ... and with shyness in young boys.

 

Europeans have a surprising variety of hair and eye colors. Their hair is not only black but also brown, flaxen, golden, or red. Their eyes are not only brown but also blue, gray, hazel, or green (Frost, 2006; Frost, 2022). This differentiation from the original black hair and brown eyes seems to have begun among women and gone farther among them.

 

Hair color - Women more often have the new hair colors, particularly red and blond. Conversely, their hair is less often black—three to five times less often. This sex difference is natural (Hysi et al., 2018; Shekar et al., 2008). Among Czechs, 19% of women and 11% of men have the highest gradation of hair redness (Frost et al., 2017).

 

Eye color - Women more often have the new eye colors, particularly green and hazel (Frost et al., 2017). Conversely, their eyes are less often brown. The first new eye color seems to have been blue, which then differentiated to create gray, green, and hazel. Thus, “blue” in its narrow sense has lost ground among women to the derived variants of green and hazel.

Population frequencies of eye colors, for men and women (Frost et al., 2017)

The new hair and eye colors are unusual in two ways. First, they are brighter than the original black and brown. They thus reflect more light and have a higher chance of standing out against the visual landscape. Second, they are “purer”—they occupy thinner slices of the visible spectrum than the original black and brown. In nature, pure colors are typically found in situations where an animal or a plant has to catch attention, such as to get pollinated, to warn predators, or to attract a mate.

 

This need for attention may explain how a single hair or eye color evolved into a diverse palette of hues. A color gets noticed not only for its brightness and purity but also for its novelty. The last quality is frequency-dependent. If a noticeable color becomes too frequent in a population, it thereby becomes less noticeable and, hence, less novel. The desire for novelty is now reoriented toward less frequent colors, including those that have recently appeared through mutation. Thus, over successive generations, the population will accumulate more and more color variants. This is likely how hair and eye color became polymorphic (see Note #1).

 

Again, the evidence seems to point to women being the main target of this selection for brighter, purer, and more novel colors. One piece of evidence is the higher frequency of the new hair and eye colors in the female population. Another is the role of estrogen in this sex-linkage. The female hormone seems to favor the expression of non-black hair and non-brown eyes during fetal development.

Red is the hair color that differs the most in frequency between women and men. Red hair should therefore be most clearly associated with increased exposure to estrogen during fetal development. This hypothesis is supported by the higher incidence of estrogen-dependent diseases in redhaired women. According to a health survey of over seven thousand people, male redheads are as healthy as other men, doing better on average in three categories and worse in three. Female redheads, however, do worse on average than other women in ten categories and better in only three. They are especially prone to four types of cancer: colorectal, cervical, uterine, and ovarian—three of which are estrogen-dependent (Frost et al., 2017). Being both female and red-haired therefore generates the highest level of risk for estrogen-dependent diseases, probably because of the combined effect of these two risk factors.

 

In sum, the new hair and eye colors were favored by a selection pressure that acted primarily on European women, with European men acquiring them as a side-effect (since the new alleles are only partly sex-linked). The selection was specifically for eye-catching qualities—brightness, spectral purity, and relative novelty.

 

This looks like sexual selection, but why would women have a greater need to get noticed on the mate market? Usually, it is the other way around, both for humans and for mammals in general. Females are less available for mating because of the limitations of pregnancy, lactation, and early infant care. Conversely, males are more available, and thus often have more than one mate at any one time. That was, in fact, the situation of most humans in prehistory. But that situation changed as they expanded their range out of the tropics and into more seasonal environments. At higher latitudes, proportionately fewer men were available for mating at any one time. There were two reasons:

 

·         Polygyny was more costly for men. With men specializing in hunting and women in gathering, women became dependent on men during winter—since there was little food to be gathered. Men thus had to bear a greater share of food provisioning, with the result that polygyny became impossible for all but the ablest hunters.

·         Death rates were higher for men than for women. Because men had to hunt for more food and over longer distances, they suffered a higher death rate at younger ages. They were thus fewer in number overall.

 

Male scarcity was most acute in an environment that no longer exists: the steppe-tundra of the last ice age, essentially the vast plains stretching from the Baltic to western Siberia. That environment supported large herds of reindeer and other herbivores, which could in turn support a large human population. But at a cost: women depended almost entirely on their hunting husbands for food, and those hunters had to cover long distances without alternative food sources, thus risking death from starvation or exposure. The result was an imbalance in the operational sex ratio: too many women for too few men, and strong selection for women with eye-catching features (Frost, 2006; Frost, 2022; Frost, 2023).

 

Proposed study

 

The aim here is to determine whether the ratio of estrogens to androgens in fetal tissues influences the development of hair and eye color. One way would be to measure the “digit ratio”—the length of the index finger divided by the length of the ring finger. This measure of fetal exposure to the sex hormones is relatively inexpensive, though disputed by some researchers. The lower your digit ratio, the more you have been masculinized by androgens during fetal development; the higher your digit ratio, the more you have been feminized by estrogens during fetal development. The left-hand digit ratio is associated with prenatal and postnatal exposure to the sex hormones. The right-hand ratio is associated much more with prenatal exposure (see Note #2).

 

An unpublished study, using a sample of 644 British participants, found that the left-hand digit ratio was significantly higher on average among individuals with blond hair than among those with brown, red, or other hair colors. For eye color, there was a similar but weaker relationship: the left-hand digit ratio was higher on average among individuals with blue eyes than among those with other eye colors.

 

That study was not published because of two objections from the referees: hair dyeing could not be excluded as a possible factor; and identification of hair and eye color was too subjective. Yet it is difficult to see how hair dyeing or misidentification can explain the digit ratio differences. Such methodological problems would introduce more noise into the data and make any differences less significant.

 

I wish to see that study replicated with a more rigorous experimental design, specifically a larger sample and narrower age range. Age interacts with the effects of the sex hormones, i.e., prenatal effects on hair color are the opposite of pubertal effects. Whereas women are lighter-haired than men from 17 onward, they are actually darker-haired up to the age of 14 (Steggerda, 1941). The right-hand digit ratio should thus be better at predicting the darkening of hair color before puberty, and the left-hand digit ratio better at predicting the lightening of hair color after puberty.

 

In addition, I wish to see whether the relationship between fetal estrogenization and eye color explains three other relationships between non-brown eyes and certain behavioral/physical traits:

 

·         Blue-eyed boys tend to be shy. This is the “little boy blue” effect. A study of preschoolers found more social wariness in blue-eyed boys than in brown-eyed boys. The difference was greatest at the extremes of wariness. Among the very inhibited boys, 13 out of 14 were blue-eyed. Among the very uninhibited, only 4 out of 10 were. There was no such relationship among the girls, whose eyes were blue in 5 out of 9 among the very inhibited and in 6 out of 11 among the very uninhibited (Coplan et al., 1988).

·         Blue-eyed women tend to have narrower shoulders and lower waist-to-hip ratios. A Latvian study found small but significant correlations between female eye color and certain sexually dimorphic features. Shoulders were narrower and waist-to-hip ratios lower in blue-eyed women than in brown-eyed women (Kažoka and Vetra, 2011).

·         Blue-eyed men tend to have more feminine faces. This was an unintended finding of two Czech studies whose participants were asked to rate male and female facial photos. Initially, the brown-eyed male faces were rated as more dominant than the blue-eyed male faces. When, as a control, the brown-eyed faces were photoshopped to make them blue-eyed, they were still rated as more dominant. On careful examination, the originally brown-eyed faces were found to be more masculine with broader and more massive chins, broader mouths, larger noses, larger eyebrows, and closer-set eyes. The originally blue-eyed faces had smaller and sharper chins, narrower mouths, smaller noses, and greater distance between the eyes. Blue eyes were associated with a more feminine face shape only in male participants. This is perhaps because a male fetus normally does not have enough estrogen to feminize the face. If enough estrogen is present to feminize the face, there is probably enough to influence the development of eye color (Kleisner et al., 2010; Kleisner et al., 2013).

      

      Were brown eyes associated with a different face shape because some of the brown-eyed men were partly Jewish or Roma and had a more Mediterranean appearance? In that case, face shape would have been more variable in the brown-eyed men. It was not. This explanation also fails to explain the effect of gender: why were blue eyes associated with facial feminization in men but not in women?

 

 

Averaged faces: blue-eyed men (left), brown-eyed men (right), Czech population (Kleisner et al., 2010). 

The above studies suggest that the association between the "new" colors and physical/behavioral feminization is largely confined to men. (There is only a weak association between them and shoulder breadth or waist-to-hip ratio). This is probably because the feminization effects are triggered when the estrogen level has risen above a certain threshold. That threshold would already be surpassed by almost all female fetuses.

Notes

 

1. Preference for rare hair colors was demonstrated by Thelen (1983), who showed pictures of attractive women to male participants and then asked them to choose the one they most wanted to marry. There were three series of pictures: the first had equal numbers of brunettes and blondes; the second had one brunette for every five blondes; and the third had one brunette for every eleven blondes. The scarcer the brunettes were in a series, the more attractive they seemed, i.e., each brunette had a better chance of being chosen.

 

Thelen’s findings were not replicated by Janif et al. (2015), whose male participants made their choices online, i.e., in private and on their home computers. There was thus no control over the female images they may have previously viewed on the same computer screen or might still be viewing on an alternate screen or split screen. This source of unwanted female imagery introduces noise into the data, thus increasing the minimum number of online raters to produce replicable ratings of female facial attractiveness. Devcic et al. (2010) report that their mean ratings of facial attractiveness did not become stable until they had recruited 857 online raters. Popenko et al. (2012) state that they needed a minimum of 992 online raters to achieve stable ratings. By comparison, Janif et al. (2015) used 658 male raters, while making their data even noisier by recruiting an ethnically diverse pool of raters, i.e., over a third were of non-European descent. Those raters would have tended to perceive female faces with black hair as ethnic insiders and female faces with non-black hair as ethnic outsiders.

 

2. Using a meta-study, Sorokowski and Kowal, 2023) concluded that the digit ratio indicates only an individual’s prenatal exposure to testosterone (and only in amniotic fluid, not in core blood). The authors, however, did not look at the ratio of estrogens to androgens. Their exclusion of data on estrogen levels is puzzling, since fetal exposure to estrogens is no less important than fetal exposure to androgens.

 

References

 

Coplan, R., B. Coleman, and K. Rubin. (1998). Shyness and little boy blue: Iris pigmentation, gender, and social wariness in preschoolers. Developmental Psychobiology 32(1): 37-44. https://doi.org/10.1002/(SICI)1098-2302(199801)32:1<37::AID-DEV4>3.0.CO;2-U

 

Devcic, Z., Karimi, K., Popenko, N., and Wong, B.J.F. (2010). A web-based method for rating facial attractiveness. Laryngoscope 120(5), 902-906. https://doi.org/10.1002/lary.20857

 

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior 27(2): 85-103. https://doi.org/10.1016/j.evolhumbehav.2005.07.002

 

Frost, P. (2022). European Hair, Eye, and Skin Color: Solving the Puzzle. Washington: Academica Press, 169 pp., ISBN 9781680538724 https://www.academicapress.com/node/549

 

Frost, P. (2023). A people of many colors. Peter Frost’s Newsletter. January 24. https://peterfrost.substack.com/p/a-people-of-many-colors

 

Frost, P., K. Kleisner, and J. Flegr. (2017). Health status by gender, hair color, and eye color: Red-haired women are the most divergent. PLoS One 12(12): e0190238. https://doi.org/10.1371/journal.pone.0190238   

 

Hysi, P.G., A.M. Valdes, F. Liu, N.A. Furlotte, D.M. Evans, V. Bataille, et al. (2018). Genome-wide association meta-analysis of individuals of European ancestry identifies new loci explaining a substantial fraction of hair color variation and heritability. Nature Genetics 50(5): 652-656. https://doi.org/10.1038/s41588-018-0100-5

 

Janif, Z.J., R.C. Brooks, and B.J. Dixson. (2015). Are preferences for women's hair color frequency-dependent? Adaptive Human Behavior and Physiology 1(1): 54-71. https://doi.org/10.1007/s40750-014-0008-y

 

Kažoka, D. and J. Vetra. (2011). Variations in some anthropometrical parameters of the women with the different iris color in Latvia. Papers on Anthropology XX: 160-170. https://doi.org/10.12697/poa.2011.20.17

 

Kleisner, K., T. Kocnar, A. Rubešová, and J. Flegr. (2010). Eye color predicts but does not directly influence perceived dominance in men. Personality and Individual Differences 49(1): 59-64. https://doi.org/10.1016/j.paid.2010.03.011

 

Kleisner, K., L. Priplatova, P. Frost, and J. Flegr. (2013). Trustworthy-looking face meets brown eyes. PLoS One 8(1): e53285. https://doi.org/10.1371/journal.pone.0053285

Popenko, N.A., Devcic, Z., Karimi, K., and Wong, B.J.F. (2012). The virtual focus group. A modern methodology for facial attractiveness rating. Plastic and Reconstructive Surgery 130(3), 455e-461e. https://doi.org/10.1097/PRS.0b013e31825dcb48

 

Shekar, S.N., D.L. Duffy, T. Frudakis, G.W. Montgomery, M.R. James, R.A. Sturm, and N.G. Martin. (2008). Spectrophotometric methods for quantifying pigmentation in human hair-Influence of MC1R genotype and environment. Photochemistry and Photobiology 84(3): 719-726. https://doi.org/10.1111/j.1751-1097.2007.00237.x   

 

Sorokowski, P., and M. Kowal. (2023). Relationship between the 2D:4D and prenatal testosterone, adult level testosterone, and testosterone change: Meta-analysis of 54 studies. American Journal of Biological Anthropology. 183(1): 20-38. https://doi.org/10.1002/ajpa.24852

 

Steggerda, M. (1941). Change in hair color with age. Journal of Heredity 32(11): 402-403. https://doi.org/10.1093/oxfordjournals.jhered.a104977

 

Thelen, T.H. (1983). Minority type human mate preference. Social Biology 30(2): 162-180. https://doi.org/10.1080/19485565.1983.9988531

 

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. 

 

Fortes, M. (1945). The Dynamics of Clanship among the Tallensi. London: Oxford University Press.

 

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis. American Journal of Physical Anthropology 133(1): 779-781. https://doi.org/10.1002/ajpa.20555

 

Frost, P. (2010). Femmes claires, hommes foncés. Les racines oubliées du colorisme. Quebec City: Les Presses de l'Université Laval, 202 p. https://doi.org/10.7202/006424ar

 

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

 

Harvey, R. G. (1985). Ecological factors in skin color variation among Papua New Guineans, American Journal of Physical Anthropology 66(4): 407-416. https://doi.org/10.1002/ajpa.1330660409

 

Hertzman, A.B. (1959). Vasomotor regulation of cutaneous circulation. Physiological Reviews 39(2):280-306. doi: 10.1152/physrev.1959.39.2.280. PMID: 13645236.

 

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

 

Lewis, I.M. (1962). Marriage and the Family in Northern Somaliland. Kampala: East African Institute of Social Research.

 

Lugira, A.M. (1970). Ganda Art. Kampala: Osasa pub.

 

Madrigal, L., and W. Kelly. (2006). Human skin-color sexual dimorphism: A test of the sexual selection hypothesis. American Journal of Physical Anthropology 132(3): 470-482. https://doi.org/10.1002/ajpa.20453

 

Manning, J.T., P.E. Bundred, and F.M. Mather. (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

 

Mathieu, D., J.C. Linke, and F. Wattel. (2006). Non-Healing Wounds. Handbook on Hyperbaric Medicine. Springer, pp. 401-428. https://doi.org/10.1007/1-4020-4448-8

 

Nestor, A., and M.J. Tarr. (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 M.J. Tarr. (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

 

Pallottino, M. (1952). Etruscan Painting. Lausanne: Skira.

 

Segrave, K. (2005). Suntanning in 20th Century America. Jefferson (North Carolina): McFarland & Company.

 

Smith, M.F. (1965). Baba of Karo: A Woman of the Muslim Hausa. London: Faber & Faber.

 

Soustelle, J. (1970). The Daily Life of the Aztecs. Stanford, California: Stanford University Press.

 

Stahl, W., and H. Sies. (2012). β-Carotene and other carotenoids in protection from sunlight. The American Journal of Clinical Nutrition 96(5): 1179S–1184S https://doi.org/10.3945/ajcn.112.034819

 

Tarr, M.J., D. Kersten, Y. Cheng, and B. Rossion. (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., B. Rossion, and K. Doerschner. (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

 

Tegner, E. (1992). Sex differences in skin pigmentation illustrated in art. The American Journal of Dermatopathology 14(3): 283-287.  https://doi.org/10.1097/00000372-199206000-00016

 

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

 

Wagatsuma, H. (1967). The social perception of skin color in Japan. Daedalus 96(2): 407-443. http://www.jstor.org/stable/20027045

 

Walsh, R. J. (1964). Variation in the melanin content of the skin of New Guinea natives at different ages. Journal of Investigative Dermatology 42(3): 261-265. https://doi.org/10.1038/jid.1964.59