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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

 

My wish list for research in 2024: Recent cognitive evolution in West Africa

 

Drainage basin of the Niger River (Wikicommons - Wizardist)

Social complexity was more advanced, and cognitive demands higher, in West African societies that benefited from trade via the Niger River. This was especially true for the Igbo of the Niger Delta, who dominated trade between the coast and the interior.

 

We have identified thousands of genes whose alleles are associated with cognitive ability, i.e., the capacity to process information, to recognize patterns, and to solve problems (Lee et al., 2018). By finding out which alleles are present on an individual’s genome, we can make an estimate of that person’s cognitive ability, and that estimate will show a high correlation with performance on tests in mathematics, reading, and science (r = 0.8). The same method can provide an estimate of a population’s mean cognitive ability, and that estimate will show a high correlation with the population’s mean IQ (r = 0.9) (Piffer, 2019).

 

Using this method, the anthropologist Davide Piffer has estimated the mean cognitive ability of several West African populations. Mean cognitive ability seems to increase as you go from west to east, being lowest among the Mende of Sierra Leone and progressively higher among Gambians, the Esan of Nigeria, and the Yoruba of Nigeria. The Yoruba have almost the same mean as do African Americans, who are nonetheless 20% European by ancestry (Piffer, 2021, see Figure 7).

 

This geographic pattern seems to reflect differences in societal development. From the fourth century onward, West African societies became more complex in the north and the east, i.e., within the drainage basin of the Niger. As trade along that river increased in volume and value, villages grew into towns, and social relations became more varied and complex. This social complexity was both a cause and effect of trade. As powerful individuals acquired the materials they needed to erect buildings, create works of art, and hold ceremonies to legitimize their rule, they became even more powerful and, thus, better able to purchase such materials. Social complexity was thus driven by a positive feedback loop: elite buying power led to an increase in trade, which in turn led to an increase in elite buying power (Frost, 2022; McIntosh and McIntosh, 1988, p. 123).

 

As social relations became more varied and complex in settlements along the Niger, those populations had to cope with a heavier cognitive workload. The demands of farming were giving way to those of craft production, urban architecture, and long-distance trade. Numeracy and literacy were becoming important, as were skills for manipulation and assemblage of various materials. Did that new social environment select for an increase in cognitive ability?

Evidence of high cognitive ability is especially strong among the Igbo people (formerly the Ibo), who live at the Niger’s mouth and who have historically dominated trade between the coast and the interior (Frost, 2022). Their children excel at school not only in Nigeria but also in overseas communities, such as those of the United Kingdom. They do exceptionally well on the GCSE (Chisala, 2015).

 

In addition to high cognitive ability, the Igbo are said to have a certain mindset: “the Ibo have a greater achievement motivation and are more willing to explore new avenues of power than either the Yoruba or the Hausa.” They have “a general belief in the possibility, indeed necessity, of manipulating one’s world; of determining one’s own destiny; of ‘getting up’ in the world” (Slater, 1983).  The earliest European observers, from the eighteenth century, described them as “competitive, individualistic, status-conscious, antiauthoritarian, pragmatic, and practical—a people with a strongly developed commercial sense” (Mullin, 1994, p. 286).

 

Trade thus seems to select for higher cognitive ability, either directly through new cognitive demands (i.e., pricing, bargaining, accounting) or indirectly through a resulting increase in social complexity. This has been the case not only among the Igbo but also among the Ashkenazi Jews, the Parsis, and other trading peoples (Cochran et al., 2006; Frost, 2012; Frost, 2021). As these peoples became specialized in trade, over the past millennium or so, they appear to have experienced a sharp rise in mean cognitive ability. These examples of recent evolutionary change support the view that mental and behavioral evolution did not stop back in the Pleistocene, anymore than the evolution of outward physical traits like skin color or body shape. Cognitive ability continued to evolve into the time of recorded history, albeit to different extents in different human groups (Cochran and Harpending, 2009; Hawks et al., 2007; Rinaldi, 2017).

 

Shell vessel with leopard from Igbo-Ukwu, Nigeria, ninth century (Wikicommons). This bronze artefact, like others from the same site, has an unusually high silver content with only traces of zinc, an alloy not used in Europe or the Middle East at that time. Ancestral Igbo thus seem to have developed metallurgy on their own (McIntosh and McIntosh, 1988, pp. 120-121).

 

Proposed study

 

The aim is to test the hypothesis that mean cognitive ability increased to a greater extent in those populations that were closer to the Niger, particularly the Igbo at the Niger’s mouth, where trade led to greater social complexity and higher cognitive demands during precolonial times.

 

For this study, mean cognitive ability can be estimated from genomic data, specifically from alleles associated with educational attainment (Edu PGS). The alleles identified to date are only a fraction of all those that play a role in cognitive ability, but we have identified enough of them to produce reliable estimates of mean cognitive ability within a population. With the help of data from history and prehistory, we could then outline the trajectories that mean cognitive ability has followed in different West African populations.

 

Finally, these hypothetical trajectories could be verified by retrieving and examining aDNA from archaeological sites throughout West Africa. It would be particularly interesting to determine when mean cognitive ability began to increase among ancestral Igbo, and how fast it increased. That research aim may be unrealistic, however, given the degradation of DNA in hot climates.

 

References

 

Chisala, C. (2015). The IQ gap is no longer a black and white issue. The Unz Review, June 25. http://www.unz.com/article/the-iq-gap-is-no-longer-a-black-and-white-issue/

 

Cochran, G., J. Hardy, and H. Harpending. (2006). Natural history of Ashkenazi intelligence. Journal of Biosocial Science 38(5): 659-693. https://doi.org/10.1017/S0021932005027069

 

Cochran, G. and H. Harpending. (2009). The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books: New York.

 

Frost, P. (2012). Tay-Sachs and French Canadians: A case of gene-culture co-evolution? Advances in Anthropology 2(3): 132-138. http://dx.doi.org/10.4236/aa.2012.23016     

 

Frost, P. (2021). Commentary on Fuerst et al: Do Human Populations Differ in Their Mental Characteristics? Mankind Quarterly 62(2). http://doi.org/10.46469/mq.2021.62.2.9

 

Frost, P. (2022). West Africa and recent cognitive evolution. Peter Frost’s Newsletter, November 14. https://peterfrost.substack.com/p/west-africa-and-recent-cognitive   

 

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA) 104: 20753-20758. https://doi.org/10.1073/pnas.0707650104

 

Lee, J. J., R. Wedow, A. Okbay, E. Kong, O. Maghzian, M. Zacher, et al. (2018). Gene discovery and polygenic prediction from a genome-wide association study of educational attainment in 1.1 million individuals. Nature Genetics 50(8): 1112-1121. https://doi.org/10.1038/s41588-018-0147-3    

 

McIntosh, S.K., and R.J. McIntosh. (1988). From stone to metal: New perspectives on the later prehistory of West Africa. Journal of World Prehistory 2(1): 89-133. https://doi.org/10.1007/BF00975123    

 

Mullin, M. (1994). Africa in America: Slave Acculturation and Resistance in the American South and the British Caribbean, 1736-1831. University of Illinois Press.

 

Piffer, D. (2019). Evidence for Recent Polygenic Selection on Educational Attainment and Intelligence Inferred from Gwas Hits: A Replication of Previous Findings Using Recent Data. Psych 1: 55-75. https://doi.org/10.3390/psych1010005

 

Piffer, D. (2021). Divergent selection on height and cognitive ability: evidence from Fst and polygenic scores. OpenPsych. April 3 https://doi.org/10.26775/OP.2021.04.03   

 

Rinaldi, A. (2017). We're on a road to nowhere. Culture and adaptation to the environment are driving human evolution, but the destination of this journey is unpredictable. EMBO reports 18: 2094-2100. https://doi.org/10.15252/embr.201745399   

 

Slater, R. (1983). Bureaucracy, Education and the Ibo: A Review. Journal of Educational Administration and History 15(1): 46-49. https://doi.org/10.1080/0022062830150106    

 

My wish list for research in 2024: 1. Genetic pacification in Western Europe from late medieval to early modern times

Men fighting with daggers and a sword (Wikicommons)

The State consolidates its power by monopolizing the use of violence. To this end, it will eliminate those men who use violence for personal ends. The gene pool is thus changed: directly, through the execution of violent men; and indirectly, through a new environment that makes it harder for them to live and reproduce.

What do I get for Christmas? Usually a box of chocolates. Or socks. If I had the choice, I’d rather get new scientific findings that interest me. Here is the first of several research proposals I hope will come to fruition in 2024.

Violence for me but not for thee

The State consolidates its power by monopolizing the use of violence. To this end, it will eliminate those men who use violence for personal ends, thus creating a new environment where social relations are peaceful and where disputes are normally settled through nonviolent means.

Pacification occurred slowly in Western Europe during the early Middle Ages. The obstacles were many: the rudimentary nature of law enforcement; the belief in a man's right to settle personal disputes as he saw fit; and the Church's opposition to the death penalty.

Those obstacles began to disappear in the eleventh century, when Church and State agreed on the need to punish the wicked so that the good may live in peace. Courts imposed the death penalty more and more often. By the late Middle Ages, 0.5 to 1.0 per cent of all men were put to death in each generation, with perhaps just as many dying at the scene of the crime or in prison while awaiting trial. Meanwhile, the homicide rate fell from a high of 20 to 40 homicides per 100,000 in the late Middle Ages to a low of 0.5 to 1 per 100,000 in the mid-twentieth century. The pool of violent men dried up until most murders occurred under conditions of jealousy, intoxication, or extreme stress (Frost and Harpending, 2015).

Did the high execution rate remove not only violent men but also their propensities for violence? Did the gene pool actually change? That scenario is plausible, given the moderate to high heritability of aggressive/antisocial behavior. Heritability is estimated at 40% by a meta-analysis of twin and adoption studies (Rhee and Waldman, 2002) and 96% by a twin study of 9 to 10 year-olds of diverse ethnic backgrounds (Baker et al., 2007). The higher figure has been attributed to the narrow age range and the use of a panel of evaluators to rate each subject. According to the latest twin study, the heritability is 40% among twins with different evaluators and 69% among those with the same evaluator (Barker et al., 2009). Finally, a review paper concludes that about half of the variance in aggressive behavior is genetic in origin (Veroude et al., 2015).

To what degree, then, could the high execution rate of late medieval and early modern times reduce the average man’s propensity for personal violence? I and Henry Harpending tried to model the impact of this selection pressure over successive generations. We concluded that the execution rate could explain a little over half of the decline in the homicide rate (Frost and Harpending, 2015). The rest could have less direct causes. Cultural norms must have shifted toward a more negative view of violent men, causing them to suffer rejection as marriage partners, discrimination in employment, exclusion from community activities, and “accidents.” By confining such men to the lower classes of society, which reproduced at a below-replacement rate in premodern times, this social exclusion would have reduced the propensity for male violence from one generation to the next.

It is possible that we underestimated the direct impact of the death penalty on homicides. Specifically, we may have erred in assuming that a propensity for homicide would cause a man to murder only once in his lifetime. It is more likely that the average executed offender had already killed more than one person and would have gone on to kill even more if allowed to live. Bandits often killed not only their victims but also any witnesses (Frost, 2015).

On the other hand, it is also possible that we overestimated the direct impact of the death penalty on the gene pool. Many violent men would have reproduced before running afoul of the law. Furthermore, men were not executed solely for acts of personal violence (Frost, 2015).

On this last point, we should understand the logic of medieval justice. The aim was not so much to find culprits for specific major crimes as to profile the criminally minded, often on the basis of minor offences. If you had crossed the behavioral threshold for a petty crime, it was assumed that you would more easily commit, or may have already committed, a crime of greater importance. Qui vole un œuf vole un bœuf.

In Western Europe, the homicide rate fell dramatically during late medieval and early modern times (Wikicommons - Max Roser, 2003)

Proposed study

The aim is to verify the above model of “genetic pacification” by examining existing data on human DNA retrieved from medieval and post-medieval sites. The datasets would be examined for changes over time in the gene pool, specifically changes in the population frequencies of alleles associated with aggressive male behavior. This evolution would be charted from the eleventh century to the early twentieth for Western Europe or for a more limited geographic area within Western Europe, such as England.

There would be three tasks:

- locate relevant aDNA datasets (Western Europe, 11th century to early 20th)

- identify alleles that are associated with male violence, e.g., alleles at MAO-A, DAT1, DRD3, DRD4, etc. (Waldham and Rhee, 2006)

- identify changes over time in the population frequencies of such alleles 

The last task would have two lines of enquiry:

How did genetic pacification relate to social class?

Today, personal violence is associated with low SES, but that association seems to be relatively recent. At first, the lower classes were the ones pacified through frequent use of the death penalty. Only later, as the death penalty gained public acceptance, did the courts impose it more and more equally on the entire population.

Initially, the courts were hindered by a widespread view of male violence as normal and virile. It was nonviolence that seemed abnormal and unmanly. Thus, in fourteenth century England, jurors were reluctant to convict murderers despite good evidence of guilt:

Because they preferred direct action and applauded self-help as a way of settling disputes, jurors were willing to acquit murderers even more freely than thieves. Most murders grew out of arguments that the villagers knew about. They assumed, along with the participants in the homicidal drama, that a good fight was an acceptable way of resolving the conflict. If someone got killed, that was understandable. (Hanawalt, 1979, p. 269)

The death penalty fell overwhelmingly on the poor and on outsiders without friends. Prominent people were more often prosecuted for homicide but seldom hanged (Hanawalt, 1979, pp. 53-54).

A similar situation prevailed in early Renaissance Venice, where noblemen felt entitled to use violence. They were the ones most likely to commit speech offenses, assaults, and rapes. Though making up only 4% of the population, they committed nearly a quarter of the recorded assaults. This positive correlation between SES and violent crime has been noted elsewhere in late medieval and post-medieval Europe (Eisner, 2003; Recueil, 2023; Ruggiero, 1980).

It is only from the nineteenth century onward that we begin to see a negative correlation between SES and violent crime, apparently due to in-migration from less pacified societies. With the shift from family workshops to industrial capitalism, the workforce was no longer confined to family members or even local people. Employers could cut labor costs by recruiting farther afield, typically from regions of labor surplus on the periphery of the Western world … or outside it entirely. Such workers came from regions where men had to use violence on a regular basis because the State was weak or nonexistent.

If the aDNA datasets include information on social class, it would be interesting to see how and when the correlation between SES and violent crime “flipped.”

How did genetic pacification relate to other trajectories of mental/behavioral evolution?

In this line of enquiry, the aim is to determine whether genetic pacification went hand in hand with other mental/behavioral changes, particularly an increase in cognitive ability (as measured by alleles associated with educational attainment - Edu PGS). Cognitive evolution may have required the establishment of social peace, notably to facilitate trade and other aspects of social complexity. Alternatively, the relationship may have been less direct, with social complexification acting as a shared selection pressure.

This subject has already attracted the interest of at least four researchers:

Davide Piffer

This archaeogeneticist is examining aDNA to reconstruct the cognitive evolution of Western Europe during medieval and post-medieval times (Kirkegaard, 2023).

Gregory Clark

This economic historian has documented the growth of the English middle class, which, from the twelfth century onward, made up a growing proportion of the English population. He argues that this demographic change led to a behavioral change: "Thrift, prudence, negotiation, and hard work were becoming values for communities that previously had been spendthrift, impulsive, violent, and leisure loving” (Clark, 2007, p. 166; Clark, 2009; Clark, 2023; Frost, 2022).

Georg Oesterdiekhoff 

This sociologist has argued for a similar mental and behavioral evolution across Western Europe during late medieval and post-medieval times. He believes that this evolution recapitulated Jean Piaget’s stages of cognitive development:

Medieval times - most people fail to develop mentally beyond the stage of preoperational thinking. They can learn language and social norms but their ability to reason is hindered by cognitive egocentrism, anthropomorphism, finalism, and animism.

Sixteenth century onward - more and more people reach the stage of operational thinking. They can better understand probability, cause and effect, and the perspective of another person, whether real or hypothetical (Oesterdiekhoff, 2023).

Université catholique de Louvain

A research team at this institution is seeking to “determine the role of elite knowledge and upper-tail human capital in the rise of the West between the period 1000 and 1800 CE.” During that period, the smart fraction grew in size until it reached a critical mass where thinkers were no longer isolated individuals but rather communities of people who could interact in clubs, salons, coffeehouses, and debating societies. This intellectual ferment, “the Enlightenment,” occurred across all domains of intellectual production, not only the sciences but also literature, music, and the arts (de Courson et al., 2023). 

The above research team “will study a variety of published source materials to create a geographical grid of the density, composition and quality of upper-tail human capital across time. [The team] will also develop a new theory to understand how codified knowledge and practical skills interact and how ideas spread” (Université catholique de Louvain, 2023).

References

Baker L. A., K.C. Jacobson, A. Raine, D.I. Lozano, and S. Bezdjian. (2007). Genetic and environmental bases of childhood antisocial behavior: A multi-informant twin study. Journal of Abnormal Psychology 116: 219–235. https://psycnet.apa.org/doi/10.1037/0021-843X.116.2.219   

Barker E. D., H. Larsson, E. Viding, B. Maughan, F. Rijsdijk, N. Fontaine, and R. Plomin. (2009). Common genetic but specific environmental influences for aggressive and deceitful behaviors in preadolescent males. Journal of Psychopathology and Behavioral Assessment 31: 299–308. https://doi.org/10.1007/s10862-009-9132-6   

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World. Princeton University Press: Princeton.

Clark, G. (2009). The domestication of man: the social implications of Darwin. ArtefaCToS 2: 64-80.

 https://www.researchgate.net/publication/277275046_The_Domestication_of_Man_The_Social_Implications_of_Darwin 

Clark, G. (2023). The inheritance of social status: England, 1600 to 2022. Proceedings of the National Academy of Sciences 120(27): e2300926120 https://doi.org/10.1073/pnas.2300926120

de Courson, B., V. Thouzeau, and N. Baumard. (2023). Quantifying the scientific revolution. Evolutionary Human Sciences, 5, E19. https://doi.org/10.1017/ehs.2023.6

Eisner, M. (2003). Long-term historical trends in violent crime. Crime and Justice 30: 83-142. https://doi.org/10.1086/652229

Frost, P. (2015). Supplement to: Western Europe, State Formation, and Genetic Pacification. March. https://www.researchgate.net/publication/274251025_Supplement_to_Western_Europe_State_Formation_and_Genetic_Pacification

Frost, P. (2022). Europeans and recent cognitive evolution. Peter Frost’s Newsletter, December 12.

Frost P., and H. Harpending. (2015). Western Europe, state formation, and genetic pacification. Evolutionary Psychology 13(1): 230-243. https://doi.org/10.1177%2F147470491501300114 

Hanawalt, B.A. (1979). Crime and Conflict in English Communities 1300-1348. Cambridge (Mass.): Harvard University Press.

Kirkegaard, E.O.W. (2023). Ancient genomes discussion with Davide Piffer and Emil at Lake Como. YouTube

Oesterdiekhoff, G.W. (2023). Was pre-modern man a child? The quintessence of the psychometric and developmental approaches. Intelligence 40: 470-478. https://doi.org/10.1016/j.intell.2012.05.005

Recueil, C. (2023). Jailbirds of a feather flock together. Aporia Magazine, October 5.

Rhee S.H., and I.D. Waldman. (2002). Genetic and environmental influences on antisocial behavior: A meta-analysis of twin and adoption studies. Psychological Bulletin 128: 490–529. https://psycnet.apa.org/doi/10.1037/0033-2909.128.3.490   

Ruggiero, G. (1980). Violence in Early Renaissance Venice. Rutgers University Press.

Université catholique de Louvain. (2023). Did elite human capital trigger the rise of the West? Insights from a new database of European scholars. European Research Council. Grant agreement ID: 883033 https://doi.org/10.3030/883033

Veroude, K., Zhang-James, Y., Fernàndez-Castillo, N., Bakker, M.J., Cormand, B., and Faraone, S.V. (2016). Genetics of aggressive behavior: an overview. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics 171(1): 3-43. https://doi.org/10.1002/ajmg.b.32364

Waldman, I.D., and S.H. Rhee. (2006). Genetic and Environmental Influences on Psychopathy and Antisocial Behavior. In C. J. Patrick (Ed.), Handbook of psychopathy (pp. 205–228). The Guilford Press.