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

 

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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

 

Sex differences in human eye morphology

 

Women have rounder-looking eyes with narrower fissures, but only in Europeans. Eyes are not sexually dimorphic in other human populations. (Petr Novak, Wikicommons)

 

 

The exposed white of the eye is larger in men than in women among Europeans but not in other human groups. This sexual dimorphism is due to the white of the eye being more horizontally exposed in men, with the result that female eyes look rounder. In addition, eye fissures are narrower and less rectangular in women (Danel et al. 2018; Danel et al. 2020).

 

This is analogous to what we see with eye color and hair color. Eyes are brown in most humans with the exception of Europeans, whose eyes may also be blue, gray, or green. Hair is black in most humans with the exception of Europeans, whose hair may also be blonde, red, or brown. In both cases, the palette of colors is more evenly balanced in women than in men. Women are less likely to have the more common hues, like blue or brown eyes and black hair. Conversely, they are more likely to have the less common hues, like green eyes and red hair.

 

There is no common genetic cause of these sex differences in eye morphology, eye color, and hair color. The genes are different in each case. The common cause seems to be some kind of selection among ancestral Europeans. Something favored the reproduction of women with rounder-looking eyes and less common eye and hair colors.

 

Was that "something" a someone? Were men selecting women through a process of sexual selection? That has been my explanation: in northern Eurasia until the end of the last ice age, women outnumbered men and had to compete for them, as a result of high male mortality and the high cost of polygyny. There was thus strong selection for women with an eye-catching appearance, and this selection ultimately changed the appearance of both sexes. The new phenotype eventually died out in northern Asia but survived in parts of Europe, which had a larger and more continuous human presence. It then spread throughout the rest of Europe almost at the dawn of history (Frost 2006; Frost 2014; Frost et al. 2017).

 

Danel et al. (2020) consider this explanation but reject it because female eye morphology does not correlate with two other aspects of female attractiveness: face shape and facial averageness. That lack of correlation, however, simply shows that each of these aspects has different constraints on the direction of sexual selection:

 

Eye morphology - the direction of sexual selection seems open-ended. Women are more attractive if they have rounder eyes.

 

Face shape - the direction of sexual selection goes into reverse beyond a certain point. Women are more attractive if they have smaller chins and smaller noses, but only up to a certain point. Excessively small chins and noses are not attractive either.

 

Facial averageness - the constraints are again different. Women become less attractive on each side of a narrow median.

 

References

 

Danel, D.P., S. Wacewicz, Z. Lewandowski, P. Zywiczynski, and J.O. Perea-Garcia. (2018). Humans do not perceive conspecifics with a greater exposed sclera as more trustworthy: a preliminary cross-ethnic study of the function of the overexposed human sclera. Acta Ethologica 21: 203-208.

https://doi.org/10.1007/s10211-018-0296-5

 

Danel, D.P., S. Wacewicz, K. Kleisner, Z. Lewandowski, M.E. Kret, P. Zywiczynski, and J.O. Perea-Garcia. (2020). Sex differences in ocular morphology in Caucasian people: a dubious role of sexual selection in the evolution of sexual dimorphism of the human eye. Behavioral Ecology and Sociobiology 74(115)

https://doi.org/10.1007/s00265-020-02894-1

 

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. (2014). The puzzle of European hair, eye, and skin color. Advances in Anthropology 4(2): 78-88.

https://doi.org/10.4236/aa.2014.42011

 

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

 

Red-haired women are special

The Damsel of the Sanct Grael – Dante Gabriel Rossetti (1828-1882)

It's known that red-haired women, but not red-haired men, are more sensitive to pain. Red hair is also associated with a higher risk of developing endometriosis, Parkinson's disease, and decreased platelet function.

A study in the latest issue of PLoS One has confirmed that red hair, especially in women, is linked to certain health issues. According to a survey of over seven thousand participants, red-haired women do worse than other women in ten health categories and better in only three, being especially prone to colorectal, cervical, uterine, and ovarian cancer. Red-haired men seem to be as healthy as other men, doing better in three categories and worse in three. Reproductive success, i.e., number of children, is the only category where redheads of both sexes do better than other participants.

This study has also confirmed that red hair is naturally more frequent in women than in men. To a lesser degree, the same is true for blond hair and green eyes. These bright colors seem to result from a selection pressure that mainly targeted women, i.e., sexual selection. In other words, among early Europeans there were too many women and not enough men; hence, competition between women for mates favored those who could better catch the attention of men, such as through a palette of bright hair and eye colors.

Because women are overrepresented among redheads, it may be that estrogen promotes synthesis of red pigments by hair follicles, particularly during fetal development. Thus, if a baby is born red-haired and female, estrogenization of its body tissues should be, on average, near the top end of the normal range. It will therefore be more at risk of developing certain health issues.

Another hypothesis can be put forward. If red hair was the last hair color to evolve, the underlying alleles may not have finished adapting to the rest of the genome, and vice versa. This reciprocal adaptation is all the more necessary because one of the five alleles for red hair seems to be of Neanderthal origin. The hypothesis of incomplete adaptation does not exclude the hypothesis of high estrogenization. In fact, there may be interaction between the two factors. Although it's likely that sexual selection did produce new hair and eye colors, we must still explain why, in this palette of colors, red hair seems to show the greatest difference between men and women both in population frequency and in associated health effects. 

Reference

Frost P, Kleisner K, Flegr J (2017) Health status by gender, hair color, and eye color: Red-haired women are the most divergent. PLoS ONE 12(12): e0190238. 

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0190238

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La rousse est particulière

On sait que les rousses, mais pas les roux, sont plus sensibles que les autres à la douleur. La rousseur est également associée à un plus grand risque de développer l'endométriose, la maladie de Parkinson, ainsi que des troubles de l'agrégation plaquettaire.

Une étude publiée dans le dernier numéro de la revue PLoS ONE confirme que la rousseur, surtout chez la femme, est reliée à certains problèmes de santé. Selon une enquête menée auprès de plus de sept mille participants, les rousses se classent pire que les autres femmes dans dix catégories de santé et mieux dans seulement trois, étant surtout susceptibles de développer des cancers du gros intestin, du col utérin, de l'utérus ou des ovaires. Quant aux roux, leur état de santé ressemble à celui des autres hommes : mieux dans trois catégories et pire dans trois. Le succès reproducteur, soit le nombre d'enfants, est la seule catégorie où les têtes rouges des deux sexes font mieux que les autres participants.

Cette étude confirme également que la rousseur est naturellement plus fréquente chez la femme que chez l'homme. Dans une moindre mesure,  c'est le même constat avec la blondeur et les yeux verts. Ces couleurs vives semblent être le résultat d'une pression de sélection visant surtout la femme, soit la sélection sexuelle. Autrement dit, il y aurait eu trop de femmes et pas assez d'hommes chez les premiers Européens, avec pour résultat une concurrence entre les femmes favorisant celles qui attiraient mieux les regards des hommes, comme par exemple par une palette de couleurs vives décorant les cheveux et les yeux.

Les femmes étant surreprésentées parmi les têtes rouges, on peut émettre l'hypothèse que l'œstrogène favorise la synthèse de pigments rouges dans les follicules pileux, particulièrement au cours du développement fœtal. Alors, si un enfant nait à la fois roux et de sexe féminin, l'œstrogénisation de ses tissus organiques doit être, en moyenne, vers la limite supérieure de la normale. La rousse sera donc plus à risque de connaitre certains problèmes de santé.

Une autre hypothèse est possible. Si la rousseur a été la dernière couleur de cheveux à paraître, il se peut que les allèles sous-jacents n’aient pas encore fini de s'adapter au reste du génome et vice versa. Cette adaptation réciproque est d'autant plus nécessaire parce que l'un des cinq allèles pour la rousseur semble être d'origine néandertalienne. Notons que l'hypothèse d'adaptation incomplète n'exclut pas celle de forte œstrogénisation. En fait, il pourrait y avoir une interaction entre les deux facteurs. S'il est vraisemblable que la sélection sexuelle ait produit de nouvelles couleurs des cheveux et des yeux, il faudra toujours expliquer pourquoi, dans cette palette de couleurs, la rousseur semble montrer la plus grande différence entre les hommes et les femmes, autant en termes de fréquence dans la population que sur le plan de la santé.