Thursday, April 30, 2009

Paternal investment and agriculture

In sub-Saharan societies, female-dominated agriculture is associated with low paternal investment and high polygyny rates. Why? The short answer is that year-round tropical agriculture enables women to meet their food needs and those of their children without a male provider. Paternal investment thus tends to fall to zero and men are free to maximize their reproductive fitness by mating with as many women as possible.

Long answer: this difference between tropical and non-tropical humans was already present before agriculture. Even during the hunter-gatherer stage, men provided more food for their mates and offspring with increasing distance from the equator, apparently because the longer winters made food gathering impossible for women during much of the year. Thus, when agriculture became the new mode of food production, men were much more predisposed to exploit its possibilities outside the tropics.

For instance, most livestock were domesticated in Europe and Asia, the guinea fowl being the only food-producing animal to have been domesticated in sub-Saharan Africa. Livestock domestication thus seems to have been limited not only by technical constraints (availability of wild animals with the right characteristics) but also by psychological constraints (relative predispositions of men and women to take part in food production).

Wherever men had previously provisioned their families with food from hunting, they were more inclined to do the same by domesticating game animals—often the same ones they used to hunt.

I discuss some of these points in my latest article: "Sexual Selection and Human Geographic Variation."


Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.

Thursday, April 23, 2009

Skin color and Egyptian/Nubian encounters

In history, when was the first encounter between human groups that greatly differed in skin color? The first recorded contacts were probably between the copper-skinned Egyptians and the black-skinned Nubians, known as nehesy, who had pushed down the Nile valley from sub-Saharan Africa to the land of Kush, just south of the second cataract. How did the Egyptians view the darker skin of the nehesy? Did they see it as being of no importance? Or did they react more emotionally, even pathologically?

Adams (2006) sees this darker skin as contributing to a negative image:

The earliest image of Kush of which we have any historical record was a negative one. The southerners were the “inferior other” against whom ancient Egyptians chose to measure their own superiority; hence they commonly appended hieroglyphic characters to the name of Kush that have been variously translated as "miserable," "wretched," or "abominable." For Egyptians, "wretched Kush" clearly had the same symbolic meaning as had “darkest Africa” for Europeans and Americans of the Victorian era.

In contrast, Snowden (1983) sees an absence of color prejudice, noting that many Nubians entered Egypt as laborers or soldiers. They were reputed for their military prowess, particularly as archers. Nonetheless, there seems to have been at least one attempt to limit their entry into Egypt, as attested by a boundary stele on the Nubian frontier:

Southern boundary that was made in year 8 under the King of Upper and Lower Egypt Khakaure, granted life for ever and ever, in order to prevent any Negro from passing it in faring downstream or journeying (?) with a boat, (and likewise) any cattle belonging to Negroes; excepting such Negro as may come to do barter in Iken or else on an embassy. Every good thing shall be done with them, yet without suffering any boat belonging to Negroes to pass downstream by Heh for ever. (Gardiner, 1916)

This text is problematic for two reasons. First, the word ‘Negro’ is used to translate nehesy (modern translations use ‘Nubian’). Although negative connotations were attached to nehesy, they may have been purely cultural in nature—the kind of scorn that is often felt toward rival peoples. It is not clear that ancient Egyptians reacted negatively to black skin per se.

Second, the motive is equally unclear. It is tempting to see the travel ban as a sort of influx control measure, such as once existed in South Africa. Yet there may have been other reasons. Snowden (1983, p. 22) simply sees an attempt to regulate traffic on the Nile.

Contemporary writings shed little light on this question. For one thing, the corpus of ancient Egyptian literature is quite limited. We know much more about how early Christians thought and felt because they were part of a cultural tradition that copied and recopied their writings up to the present day. The same was not true for the ancient Egyptians. Their cultural tradition was forgotten and even demonized by its Christian and Muslim successors, with the result that much detective work was needed to make their surviving hieroglyphic texts once more comprehensible.

For another thing, the ancient Egyptian language, like Sumerian and other early languages, functioned largely as a memory aid for key events and transactions. Not until the advent of Homeric Greek did people have a language that could truly express their inner thoughts and feelings.

We can gain more insight into this question by studying how Nubians were treated when significant numbers of them entered Egypt as laborers. Did spatial segregation persist between them and the indigenous working class? Is there evidence of assimilation? The anthropologist K. Godde (2009) tried to find an answer by studying cemeteries at Hierakonpolis in Egypt. The cemeteries were divided into three social classes, including one for the working class (HK43). Other cemeteries appeared to contain Nubian burials. When Godde analyzed the skeletal remains, he found no evidence of Nubian burials in the working-class cemetery or Egyptian burials in the Nubian cemeteries. A certain degree of spatial segregation thus seems to have persisted throughout the lives of these individuals. As Godde (2009) notes:

This biological analysis supports the differences in archaeological data between the Nubian cemeteries and HK43 and further substantiates the idea that Nubian workers were buried in separate cemeteries from Egyptians, regardless of Egyptian social status.


Adams, W.Y. (2006). The Kingdom and Civilization of Kush in Northeast Africa. In J.M. Sasson (Editor in chief) Civilizations of the Ancient Near East. (Vol. 2, pp. 775-789). Hendrickson Publishers.

Gardiner, A.H. (1916). An ancient list of the fortresses of Nubia. The Journal of Egyptian Archaeology, 3, 184-192.

Godde, K. (2009) The working class at Hierakonpolis. Nubian or Egyptian? American Journal of Physical Anthropology, 138 (S48), 201, 78th Annual Meeting of the AAPA.

Snowden Jr., F.M. (1983). Before Color Prejudice. The Ancient View of Blacks. Cambridge (Mass.): Harvard University Press.

Thursday, April 16, 2009

Is ruddy beautiful?

Are ruddy faces attractive? Apparently they are—if we believe the findings of a study from the University of St. Andrews, Scotland (Stephen et al., 2009).

The subjects were presented with photos of male or female faces, one at a time, and asked to give the faces an optimal healthy appearance. This was done by manipulating the facial complexion on a continuum ranging from red (color of oxygenated blood) to dark blue (color of deoxygenated blood). To varying degrees, the subjects preferred to redden the faces, but this effect was stronger with female faces than with male ones.

At first glance, this sex difference seems counterintuitive. Women are the ‘fair sex’ while men are ruddier and browner in complexion (Edwards & Duntley, 1939). Why, then, would a ruddy complexion be considered more appropriate for a female face than for a male one?

The problem may be that the researchers measured responses only to colors ranging from red to dark blue. No other colors were tested. So the visual cue could be luminosity rather than hue. In other words, the subjects may have more strongly associated red with femininity because red is lighter than dark blue. This point is hinted at in the press release:

Professor Dave Perrett, head of the Perception Lab commented, "Our evaluators all thought that bright red blood with lots of oxygen looked healthier than darker, slightly bluer blood with lower oxygen levels. It is remarkable that people can see this subtle difference.

Curiously, the same press release describes the ‘deoxygenated’ face as ‘pale’. To my eyes, the ‘oxygenated’ face is clearly the one with the lighter hue.

There is in fact a large body of cross-cultural and psychological research showing that people associate lighter skin tones with women and darker ones with men (Feinman & Gill, 1978; Frost, 1994; Osgood, 1960; Russell, 2003; Russell & Sinha, 2007; Russell et al., 2006; Tarr et al., 2001, van den Berghe & Frost, 1986). Could this mental association explain why the feminine ideal is more pink than blue?

I suspect the researchers were surprised by the sex difference in their findings. In the ‘Discussion’ section, they suggest that ruddiness, and hence greater blood circulation, is more adaptive for women than for men, given that female blood has a lower oxygen content. If this were so, however, wouldn’t natural selection cause women to become ruddier than men? This is the opposite of what we see.

It would be more parsimonious to attribute this sex difference in response to a sex-identification algorithm, rather than to a health-assessment algorithm.


Edwards, E.A., & Duntley, S.Q. (1939). The pigments and color of living human skin. American Journal of Anatomy, 65, 1-33.

Feinman, S. & G.W. Gill. (1978). Sex differences in physical attractiveness preferences, Journal of Social Psychology, 105, 43-52.

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, 507-514.

Osgood, C.E. (1960). The cross-cultural generality of visual-verbal synthesthetic tendencies, Behavioral Science, 5, 146-169.

Russell, R. 2003. Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.

Russell, R. & P. Sinha. 2007. Real-world face recognition: The importance of surface reflectance properties, Perception, 36, 1368-1374.

Russell, R., P. Sinha, I. Biederman & M. Nederhouser. 2006. Is pigmentation important for face recognition? Evidence from contrast negation. Perception, 35, 749-759.

Stephen, I.D., Coetzee, V., Law Smith, & M., Perrett, D.I. (2009) Skin Blood Perfusion and Oxygenation Colour Affect Perceived Human Health. PLoS ONE, 4(4), 1-7
e5083. doi:10.1371/journal.pone.0005083

Tarr, M.J., D. Kersten, Y. Cheng & B. Rossion. (2001). It's Pat! Sexing faces using only red and green, Journal of Vision, 1(3),

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

Thursday, April 9, 2009

Origins of male homosexuality - Conclusion

What causes exclusive male homosexuality? This is the question I’ve addressed in the last few posts. The answer is still elusive although there seems to be consensus on some points.

One such point is the relative importance of inborn causation versus environmental causation. In men, an exclusively homosexual orientation has a heritability of 30-45%. A genetic cause must therefore be interacting with an unknown but more important environmental cause (Bailey et al., 2000). The genes in question are likewise unknown but may be located near the ones for RH factor (Ellis et al., 2008).

This raises another question. Why would natural selection create a genetic predisposition, however minor, to become exclusively gay? The answer is probably the one put forward by Ed Miller (2000). Human evolution has seen a relatively recent increase in provisioning by men of their mates and offspring, together with a corresponding decrease in polygyny. Even among present-day humans, this evolutionary trend has gone further in some populations than in others. How, then, did natural selection change male behavior over so little time? Miller’s answer: by partially feminizing the male mind (i.e., by impeding cerebral masculinization during prenatal and neonatal development). This is the fastest way with the least genetic change, the only downside being an increased risk that some boys will grow up with the wrong sexual orientation.

For several reasons, I disagree with Miller on one point: I believe that the downside of this rapid natural selection has not been a certain proportion of exclusively homosexual men but rather a larger proportion of weakly heterosexual men. First, as noted earlier, the genetic predisposition is not acting alone. It seems to be interacting with a more important cause of environmental origin. Second, I have trouble believing that a balanced polymorphism could maintain 3-5% of all men in a state of sexual indifference to women. (I incidentally feel the same way about the ‘gay uncle’ theory, where the reproductive cost of indifference to women is balanced by care given to related children—the cost seems too high to offset the presumed benefit). Third, I have trouble believing that 3-5% of all men were sexually indifferent to women before the late 19th century. Male homosexuality is attested in earlier time periods but usually in a facultative form, i.e., older heterosexual men having sex with boys or with males of servile status.

In my opinion, the likeliest scenario is one where a genetic predisposition weakens male heterosexuality but is not enough, in itself, to cause exclusive male homosexuality. Something in the environment has to push some of these heteros over the borderline. If so, there must be a large population of weakly heterosexual men, certainly much larger than the 3-5% who end up being exclusively homosexual.

Alongside this scenario would be a residual of various other causes—random genetic mutations, psychological or environmental stresses during pregnancy, chimerism, etc.—that each occur at such a low rate that natural selection cannot effectively counter any one of them. These residual causes might account for a baseline of exclusive male homosexuality that has always been with us, perhaps less than 1% of all men.

The current level of 3-5%, however, is much harder to ascribe to longstanding causes. I suspect it’s recent, essentially since the late 19th century, and due either to a pathogen that is co-evolving with its host population or to a recent environmental factor that humans have not yet overcome through natural selection.

If a pathogen is responsible, it may have become more prevalent because of the great increase in urbanization at that time or perhaps because some other factor had increased transmissibility. The main problem so far with the ‘gay germ’ theory is simply lack of evidence. Where is the smoking gun?

If a new environmental factor is responsible, it may be some kind of estrogen or estrogen-like compound in the neonatal environment. The late 19th century, however, is too early for most candidates. In fact, there seem to be only two credible ones. One is borax, which was used as a food preservative until the 1950s. The other is estrogen-rich drinking and bathing water from sources contaminated by untreated wastewater. Such effluent greatly increased in volume with the introduction of modern sewer systems in the late 19th century and decreased only with conversion to secondary and tertiary wastewater treatment during the 1970s.

How should we test these different theories? First, we should do what researchers normally do: conduct controlled studies and debate the findings in academic journals. Unfortunately, very little of either is going on. J. Michael Bailey, Lee Ellis, and the people around them, seem to account for much of the current research (and even Bailey runs into a great deal of flak). Most real debate actually seems to be happening in the blogosphere. This may or not be a bad thing, but it does say a lot about the climate surrounding this subject.


Bailey, J.M., M.P. Dunne, & N.G. Martin. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78, 524-536.

Ellis, L, Ficek, C, Burke, D, & Das, S. (2008). Eye color, hair color, blood type, and the rhesus factor: exploring possible genetic links to sexual orientation. Archives of Sexual Behavior, 37(1), 145-9.

Miller, E.M. (2000). Homosexuality, birth order and evolution: Toward an equilibrium reproductive economics of homosexuality. Archives of Sexual Behavior, 29, 1-34.

Thursday, April 2, 2009

The chimerism theory of male homosexuality

Steve Sailer has entered this debate with a post on the chimerism theory, i.e., the idea that male homosexuality arises when a male fetus absorbs cells from a female twin that has died during early fetal development. Chimerism appears to be more widespread than previously thought, but its actual incidence remains unknown (also see this post).

What do I think? I’m frankly skeptical. For one thing, to keep the male brain from masculinizing during prenatal or neonatal development, it isn't enough to have a ‘female cell’ somewhere in the male body. The cell must be positioned at a critical point on this developmental pathway. Otherwise, the brain will develop normally.

But I have a more basic objection: chimerism is nothing new. So the human organism has had eons of evolutionary time to adjust. We come back to Greg Cochran’s point: natural selection tends to remove any condition that seriously hinders reproduction. And ‘tends to’ is an understatement. If an organism cannot reproduce, its characteristics will not survive into the next generation. Game over.

Finally, if the chimerism theory is true, exclusive homosexuality should be more common in populations with a higher incidence of twinning, such as sub-Saharan Africans. In southwest Nigeria, twin births are 3 to 5 times more common than in Europe (Akinboro et al., 2008). Presumably, there would also be proportionately more ‘phantom twins’, i.e., those that die in the womb. Is exclusive homosexuality likewise 3 to 5 times more common in Nigeria?

The post’s comments brought up another point: there is nothing evolutionarily puzzling about male homosexuality in and of itself:

I'm still not convinced that human homosexual activity is anything that needs to be 'explained' any more than whistling or square dancing or checkers need to be explained.When you strip away the family values or evolutionary or gay rights hysteria, homosexual activity is just the co-occurrence of two traits that are separate but widely dispersed in humanity:

a) enjoyment of recreational sex

b) emotional bonding with members of the same sex.

The evolutionary puzzle lies elsewhere, as another commenter pointed out (while citing a previous comment):

I don't think that the issue is homosexual behavior as much as exclusive homosexual orientation. --Glaivester
Yes, it's not so much the homophilia, but the heterophobia. Or, as I once saw scribbled on a university (men's) bathroom stall, "I'd rather die than go to bed with a woman." I doubt this was John the Baptist speaking.

It is thus exclusive homosexuality, and not homosexuality per se, that needs explaining. Facultative homosexuality does not preclude reproduction, nor does it preclude normal heterosexual development of the male brain. It may arise simply because access to women is limited (as in prisons or in polygynous societies where older men monopolize the pool of fertile women). It may also arise in hypersexual men who have low thresholds for sexual excitement, i.e., who are turned on by anyone or anything that remotely resembles a woman.


Akinboro, A., Azeez, M.A., and Bakare, A.A. (2008). Frequency of twinning in southwest Nigeria, Indian Journal of Human Genetics, 14, 41-47.