Saturday, November 21, 2015

Evolution of long head hair

Swan princess, John Bauer (1882-1918). Human head hair is of relatively recent origin, reaching incredible lengths in some groups but not in others.


I've published an article on the evolution of long head hair in humans. The following is the abstract:

In many humans, head hair can grow to a much greater length than hair elsewhere on the body. This is a "derived" form that evolved outside Africa and probably in northern Eurasia. The ancestral form, which is frizzier and much shorter, survives in sub-Saharan Africans and in other groups whose ancestors never left the tropics. This original hair form is nonetheless relatively straight and silky during infancy. Head hair thus seems to have lengthened in two stages: 1) retention of the infant hair form at older ages; and 2) further lengthening to mid-back and even waist lengths. These changes seem to have gone farther in women, whose head hair is thicker and somewhat longer. The most popular evolutionary explanations are: 1) relaxation of selection for short hair; and 2) sexual selection for women with long hair. Neither hypothesis is satisfactory. The first one cannot explain why head hair lengthened so dramatically over so little time. The second hypothesis suffers from the assumption that some populations have remained naturally short-haired because they consider long-haired women undesirable. Almost the opposite is true in traditional African cultures, which have a long history of lengthening and straightening women's hair. It is argued here that sexual selection produced different outcomes in different populations not because standards of beauty differed but because the intensity of sexual selection differed. In the tropical zone, sexual selection acted more on men than on women and was thus too weak to enhance desirable female characteristics. This situation reversed as ancestral humans spread northward into environments that tended to limit polygyny while increasing male mortality. Because fewer men were available for mating, women faced a more competitive mate market and were selected more severely.


Frost, P. (2015). Evolution of long head hair in humans, Advances in Anthropology, 5, 274-281.


Saturday, November 14, 2015

The fellowship instinct

Grace Fellowship Assembly of God, Bloomington, Indiana – Fellowship is what primarily draws people to religion (Wikicommons - Vmenkov).


Religiosity is moderately heritable—25 to 45% according to twin studies (Bouchard, 2004; Lewis and Bates, 2013). These figures are of course underestimates, since any noise in the data gets classified as ‘non-genetic’ variability. So the estimates would be higher if we could measure religiosity better.

But what does it mean to be religious? Does it mean adhering to a single organized religion with a clergy, a place of worship, and a standardized creed? This definition works fairly well in the Christian and Muslim worlds, but not so well farther afield. In East Asia, people often have more than one faith tradition: “If one religion is good, two are better.” Moreover, 'religion' has never controlled East Asian societies to the extent that Christianity and Islam have controlled theirs, as Francis Fukuyama notes in The Origins of Political Order. This word becomes even more problematic in simple societies. Did hunter-gatherers have religion? If we take the example of the Inuit, they believed in spirits of various kinds, but those spirits were indifferent to humans and their concerns, being not at all like the fellow in the Christmas jingle: 

He sees you when you're sleeping.
He knows when you're awake.
He knows if you've been bad or good,
So be good for goodness sake!

Simple hunter-gatherers had no idea that a moral God exists. Nor did they see morality as being absolute or universal. A human action could be good or bad, depending on who was doing what to whom. Morality could not be separated from kinship. Your first moral obligation was to yourself, then to your family, then to your close kin. Beyond, who cares?

So what exactly is the heritable component of religiosity? Or should we say components? These questions were addressed by a recent twin study, which concluded that "religiosity is a biologically complex construct, with distinct heritable components" (Lewis and Bates, 2013). The most important one seems to be 'community integration,' which is the desire to be among people who befriend each other and help each other on a regular basis. Much research shows that religious people have stronger social needs than the rest of us, and they tend to lose interest in religion when such needs are no longer met. When former Methodist church members were asked why they left their church, the most common response was their failure to feel accepted, loved, or wanted by others in the congregation (Lewis and Bates, 2013).

The second most important component seems to be 'existential certainty'—belief in a controlling God who will ultimately take care of everything. Belief in divine control reduces anxiety and actually increases one's sense of personal control. As such, it provides "an epistemic buffer from a range of factors such as unpredictability, instability, and concerns over mortality that exist in this world."

In sum, this study found that community integration accounts for 45% of innate religiosity and existential certainty for 11%. These two components represent most of the genetic variability.

Just one thing. The study was done with a sample of Americans who were 85.1% Christian, the rest being mostly atheist, agnostic, or ''no religious preference." Would the results have been similar with participants from the Middle East, Africa, or East Asia?

I don’t think so. Religiosity, by its very nature, should be very sensitive to gene-culture coevolution. It's moderately heritable and serves different purposes in different cultural environments. Any one religion will favor its own ways of being and acting, and people who conform will do better than those who don’t. Thus, over successive generations, the gene pool of believers will become characterized by certain predispositions, personality traits, and other heritable aspects of mental makeup. These characteristics will tend to persist even if the believers cease to believe and become secularized.

This point is made by the authors, albeit indirectly. On the one hand, a community of believers will modify their religion to suit their social and existential needs:

[...] religion per-sé may not be the sole organization or system able to fill the niche created by human needs for community and existential meaning. The succession, displacement, and evolution of religions can be viewed in this light as the shaping of religious systems by their adherents to maximize the extent to which their needs are met.

On the other hand, a religion will modify its community of believers by favoring the survival of those with the "right" mindset and by removing those with the “wrong” mindset:

[...] this ''exchangeable goods'' notion of religion may fail to acknowledge the tight fit between religious belief and human psychology: ''religious practices and rituals co-evolved with religiously inclined minds, so that they now fit together extremely well."

In short, Man has made religion in his own image, but religion has returned the favor. In a very real sense, it has made us who we are.


Bouchard, T. J. Jr., (2004). Genetic influence on human psychological traits: A survey. Current Directions in Psychological Science, 13, 148-151. 

Lewis, G.J. and T.C. Bates. (2013). Common genetic influences underpin religiosity, community integration, and existential uncertainty, Journal of Research in Personality, 47, 398-405.

Saturday, November 7, 2015

The missing hour of sleep

Rêverie, Adrien de Witte (1850-1935), Wikicommons

African Americans sleep on average almost an hour less than do Euro Americans. The two groups have mean sleep times of 6.05 hours and 6.85 hours. This finding has recently been discussed by Brian Resnick in National Journal and by our Steve Sailer.

Researchers reject a genetic explanation: "There is a consensus that innate biological differences between blacks and whites are not a factor" (Resnick, 2015). So what is the cause?

One study points the finger at racism:  "If you can take out that discrimination piece, the average African-American and the average Caucasian look at lot more similar. [...]  "It's not perfect, but in terms of sleep, a lot of the disparity goes away" (Resnick, 2015).

The study is by Tomfohr et al. (2012). It found that duration of deep sleep and duration of Stage 2 of light sleep correlated in African Americans with perceived discrimination, which is defined as "the extent to which an individual believes that members of his or her ethnic group have been discriminated against in society."

Nonetheless, as the authors note, sleep duration still differs significantly between African and Euro Americans even when the difference is adjusted for the effects of perceived discrimination. So we are left with a curious finding: two separate causes, one genetic and the other environmental, are producing the same pattern of effects. Both are reducing deep sleep and Stage 2 light sleep in African Americans while not affecting Stage 1 light sleep.

Whenever I see this kind of finding, I start looking for confounds. Is one cause a sock puppet for the other? It may be that perceived discrimination increases with African ancestry. Perhaps African Americans who feel conscious of discrimination also tend to be darker-skinned and more visibly African than those who don’t. This confound has actually been shown by several studies, such as the following:

This study tested the extent to which skin color is associated with differential exposure to discrimination for a sample of 300 Black adults. Results revealed that dark-skinned Blacks were 11 times more likely to experience frequent racial discrimination than their light-skinned counterparts; 67% of subjects reporting high discrimination were dark-skinned and only 8.5% were light-skinned. (Klonoff and Landrine, 2000; see also Keith and Herring, 1991)

Even if perceived discrimination could fully explain the race difference in sleep duration, we still couldn’t exclude a genetic explanation, since the degree of perceived discrimination is confounded with the degree of African ancestry.

In reality, perceived discrimination accounts for only part of the race difference, and since this difference remains significant even if we factor out that putative cause, the most parsimonious explanation is a genetic cause. Only that cause can fully account for shorter sleep duration in African Americans.

Studies in Africa

Another way to solve this puzzle is to look at Africans living in Africa. Do they show the same pattern we see in African Americans?

We know less about sleep patterns in Africa, but what we do know suggests that Africans, too, have shorter sleep duration. When Friborg et al. (2012) studied sleep in Ghanaians and Norwegians, they found that Ghanaians slept about an hour less than do Norwegians on weekends and between a quarter and half an hour less on weekdays. Oluwole (2010) studied sleep in Nigerian undergraduates and found they slept an average of 6.2 hours plus another 70 minutes in the afternoon. This pattern is actually typical in the tropical zone. People prefer to get some sleep when the temperature is at its peak and spend more time awake when it's more bearable.

But why would this pattern persist in African Americans? Perhaps it’s hardwired to some degree. When siestas become the cultural norm, there is selection for those individuals who enjoy being normal (and against those who don't).

Sleep patterns are heritable:

Assessed self-reported sleep data from 2,238 monozygotic (MZ) and 4,545 dizygotic (DZ) adult twin pairs born in Finland before 1958. Results indicate a significant hereditary effect on sleep length and on sleep quality. When the data were examined in subgroups defined by sex, age (18-24 yrs and 25+ yrs), and cohabitation status of the twin pair, the highest heritability estimates for sleep length were for Ss living together aged 25 yrs or older. For Ss living apart, the heritability estimates were statistically significant in all Ss aged 25 yrs or older. For sleep quality, significant heritability estimates were found for all groups except women living together. Results indicate that a significant proportion of the variance in sleep length and quality was due to factors that make MZ Ss more similar than DZ Ss. (Partinen et al., 1983)

A single genetic polymorphism seems to explain much of the variability between individuals in sleep patterns, particularly deep sleep and slow wave activity (SWA):

Here we show in humans that a genetic variant of adenosine deaminase, which is associated with the reduced metabolism of adenosine to inosine, specifically enhances deep sleep and SWA during sleep. In contrast, a distinct polymorphism of the adenosine A2A receptor gene, which was associated with interindividual differences in anxiety symptoms after caffeine intake in healthy volunteers, affects the electroencephalogram during sleep and wakefulness in a non-state-specific manner. Our findings indicate a direct role of adenosine in human sleep homeostasis. Moreover, our data suggest that genetic variability in the adenosinergic system contributes to the interindividual variability in brain electrical activity during sleep and wakefulness. (Retey et al., 2005)


So African Americans are getting enough sleep at night. It’s just that they're not getting enough afternoon naps. But aren't naps for kids? Or old fogeys? Actually, they’re quite normal for adults in much of the world. In the Nigerian study, 82% of the participants regularly took afternoon naps.

It’s ironic that the “r word” has been injected into this debate. If a behavior deviates from the white American norm, and if racism is held responsible either directly or indirectly, one is assuming that this deviation is pathological. It is “deviant.” It shouldn’t exist and something should be done about it. The white American norm thus becomes a norm for all humans, and all humans—if they want to be fully human—should strive toward it.

In reality, there is no single human nature. Genetic evolution didn’t slow down when humans began to split up and settle the different continents. It accelerated. And not just because our ancestors were adapting to different natural environments. Most of the acceleration took place long after the globe had been settled from the equator to the arctic. It happened when humans began to adapt to an increasingly diverse range of cultural environments. And those adaptations were mostly behavioral and psychological.

One of them is the way we sleep. The African sleep pattern is normal in its native environment. It is simply an adaptation to a particular set of circumstances, just as the northern European sleep pattern is an adaptation to another set of circumstances.


Friborg, O., B. Bjorvatn, B. Amponsah, and S. Pallesen. (2012), Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°). Journal of Sleep Research, 21: 176-184.

Keith, V. M., and Herring, C. (1991). Skin Tone and Stratification in the Black-Community. American Journal of Sociology, 97(3), 760-778.

Klonoff, E. A., and Landrine, H. (2000). Is skin color a marker for racial discrimination? Explaining the skin color-hypertension relationship. Journal of Behavioural Medicine, 23(4), 329-338.

Oluwole, O. S. A. (2010), Sleep habits in Nigerian undergraduates. Acta Neurologica Scandinavica, 121, 1-6.

Partinen, M., J. Kaprio, M. Koskenvuo, P. Putkonen, and H. Langinvainio (1983). Genetic and environmental determination of human sleep. Sleep: Journal of Sleep Research & Sleep Medicine, 6(3), 79-185.

Resnick, B. (2015). The Black-White sleep gap, National Journal, October 23

Retey, J.V., M. Adam, E. Honegger, R. Khatami, U.F.O. Luhmann, H.H. Jung, W. Berger, and H.P. Landolt. (2005). A functional genetic variation of adenosine deaminase affects the duration and intensity of deep sleep in humans, Proceedings of the National Academy of Sciences U.S.A., 102, 15676-15681

Sailer, S. (2015). Racism never sleeps: "The Black-White Sleep Gap: An Unexpected Challenge in the Quest for Racial Justice",  The Unz Review, October 29

Tomfohr, L., M.A. Pung, K.M. Edwards, and J.E. Dimsdale. (2012). Racial differences in sleep architecture: The role of ethnic discrimination, Biological Psychology, 89, 34-38.

Saturday, October 31, 2015

The contradictions of polygyny

Chief Makwira and his wives, Malawi, 1903 (Wikicommons). Older men had first priority. Younger men could gain access to women only through war or adultery.


In my last column, I reviewed the findings of Butovskaya et al. (2015) on testosterone and polygyny in two East African peoples:

- Testosterone levels were higher in the polygynous Datoga than in the monogamous Hadza. This difference is innate.

- Datoga men were more aggressive than Hadza men on all measures used (physical aggression, verbal aggression, anger, and hostility)

- Datoga men were larger and more robust than Hadza men

- All of these characteristics seem to be adaptive under conditions when men have to compete against other men for access to women

Testosterone levels were not only higher in the Datoga but also more variable. Alvergne et al. (2009) studied this variability in Senegalese men, finding that the monogamous ones differed from the polygynous ones in the way testosterone levels changed with age. The levels were higher in the polygynous men than in the monogamous men between the ages of 15 and 30. After 45, this pattern reversed: the monogamous men had the higher levels. At all ages, the polygynous men were more extraverted than the monogamous ones, this quality being defined as "pro-social behavior which reflects sociability, assertiveness, activity, dominance and positive emotions." Extraversion may assist a reproductive strategy of seducing women, rather than providing for them.

Thus, when Africans gave up hunting and gathering for farming, there was selection for a new package of male traits. Some of these traits are physiological (higher testosterone levels), some anatomical (denser bones, greater arm and leg girth; changes to muscle fiber properties, etc.), and some behavioral (polygyny, aggressiveness, extraversion, etc.). But this selection didn't eliminate older genotypes, at least not wholly. There seems to be a balanced polymorphism that allows a minority of quieter, monogamous men to thrive in a high-polygyny society like Senegal. When polygynous men become too numerous, they may spend too much time looking for mating opportunities and not enough checking up on their current wives to avoid being cuckolded. It might be better for some to live continuously with one wife.

African Americans versus Euro Americans

The above differences within sub-Saharan Africa (Datoga vs. Hadza, polygynous Senegalese vs. monogamous Senegalese) are also seen between African Americans and Euro Americans. In all these cases, the differences are of degree and proportion, rather than absolute and non-overlapping.

Testosterone reaches high levels in young African American adults (Pettaway, 1999; Ross et al., 1986; Ross et al., 1992; Winters et al., 2001). African Americans are also likelier to have alleles for high androgen-receptor activity (Kittles et al.,2001). Lifetime exposure to testosterone is reflected in development of prostate cancer, with African American men having the world's highest incidences (Brawley and Kramer, 1996). It was once thought that lower incidences prevail among black West Indians and sub-Saharan Africans, but underreporting is now thought to be responsible (Glover et al., 1998; Ogunbiyi and Shittu, 1999; Osegbe, 1997).

In African Americans, blood testosterone levels peak during adolescence and early adulthood, being higher than those of Euro Americans of the same age. Levels decline after 24 years of age, and by the early 30s are similar to those of European Americans (Gapstur et al., 2002; Nyborg, 1994, pp. 111-113; Ross et al., 1986; Ross et al., 1992; Tsai et al., 2006; Winters et al., 2001). This is the same pattern we saw in polygynous Senegalese men versus monogamous Senegalese men. In short, polygyny seems associated with a more exaggerated pattern of variation with age.

The demographic contradictions of a high-polygyny society

Testosterone levels are normally higher in all young men, but why are they higher still when polygyny is common? The reason seems to be the scarcity of available women. High-polygyny societies generate a shortage of mateable women, and this shortage is managed by giving priority to men who are at least ten years past puberty. For instance, among the Nyakyusa: "[...] there is a difference of ten years or more in the average marriage-age of girls and men, and it is this differential marriage-age which makes polygyny possible" (Wilson, 1950, p. 112).

By concentrating celibacy among young men, this age rule compels them to seek sex through warfare or illicit means. According to Pierre van den Berghe (1979, pp. 50-51):

Typically, the more men are polygynous in a given society, the greater the age difference between husbands and wives. [...] The temporary celibacy of young men in polygynous societies is rarely absolute, however. While it often postpones the establishment of a stable pair-bond and the procreation of children, it often does not preclude dalliance with unmarried girls, adultery with younger wives of older men, or the rape or seduction of women conquered in warfare. Thus, what sometimes looks like temporary celibacy is, in fact, temporary promiscuity. These young men often devote themselves to warfare during their unmarried years and sometimes homosexuality is tolerated during that period.

For young men in a high-polygyny society, warfare—typically raids against neighboring communities—is the main way to gain access to women. In a sense, war becomes a means of resolving the demographic contradictions of a high-polygyny society. Polygyny creates a wife shortage among young men, and this contradiction is resolved by turning it outward. As warriors, young men are encouraged to satisfy their sexual urges through raids against neighboring peoples. Warfare thus becomes endemic.

This relationship between polygyny and war has often been noted in studies of African societies:

Dorjahn (1959) says African warfare emphasized taking captives, rather than killing the enemy. Kelly's discussion of Nuer warfare provides an interesting perspective on this phenomenon. In Nuer warfare the main casualties were younger men and older women, with male and female mortality being almost equal. Younger women and children were captured. Female captives were valued because they could be used to generate bridewealth when they were married to other Nuer, whereas captive boys were adopted into the lineage of their captor and would require bridewealth payment when they married. Consequently, few males were taken captive (Kelly 1985:56-57). (White and Burton, 1988)

In their cross-cultural study of the causes of polygyny, White and Burton (1988) conclude that "polygyny is associated with warfare for plunder and/or female captives":

[...] polygyny is seen as associated with the expansion of male-oriented kin groups through favorable environments, facilitated by capture of women or bridewealth via warfare. Following this analysis, it is difficult to see polygyny as having benign effects upon the lives of all women. Rather, polygyny produces benefits for senior wives, who have sons and can mobilize the labor of junior wives and children (Hartung 1982); it has negative effects on women who become slaves, captives, or junior wives, or who do not have sons.

We now come to a leading cause of the African slave trade. Polygyny led to warfare, which led to a surplus of unwanted male captives. These captives could be sold as slaves, but local markets would soon be saturated. The excess supply had to be sold farther away, with the result that slave trading networks began to reach the Middle East as early as the time of Christ (Frost, 2008).

While outsiders from the Middle East and Europe would later get more and more involved, becoming not only traders but also captors, it was Africans themselves who initially controlled the supply chain. In its early stages, and even later, this trade was driven by factors internal to Africa.

Contrary evidence

Whenever I discuss this subject, some people will counter that certain studies have shown an absence of racial/ethnic differences in testosterone levels. Let me discuss these studies at some length.

This meta-analysis concluded: "After adjustment for age, black men have a modestly but significantly 2.5 to 4.9% higher free testosterone level than white men." Here, “adjustment for age” means comparing black and white men of the same age. The conclusion isn't surprising, since African American men have a testosterone advantage only from puberty to their early 30s. At other ages, their testosterone levels are either equal to or less than those of Euro American men. 

This meta-analysis has two other flaws. First, it included only studies on "men," thus excluding studies on teenagers, among whom the race difference is greatest.

Second, it included Rohrmann et al. (2007). This study suffers from serious methodological problems, as I will now explain. 

This study concluded that "contrary to the postulated racial difference, testosterone concentrations did not differ notably between black and white men."

This study also found that 45-69 year old black men have higher testosterone levels (5.62 ng/ml) than do 20-44 year old black men (5.35 ng/ml). Such a finding is paradoxical and indicates a faulty dataset. The authors used serum samples from the National Center for Health Statistics that had been earlier collected for its Third National Health and Nutrition Examination Survey (NHANES III). The authors state they used 1,479 samples that remained out of an initial total of 1,998. Over 25% of the original samples were missing. The authors state that some samples were missing because they were being used for another study.

The same serum bank had in fact been used for research on a sexually transmitted disease. This was the study by Fleming et al. (1997), who reported that more than 25% of adults between 30 and 39 years of age were positive for HSV-2 (Herpes Simplex virus type 2). Those samples may have been set aside either for further testing or for legal reasons. The serum bank would have thus lost some of its most polygynous donors.

This study measured salivary testosterone in young men (15-30 years) from the United States, Congo, Nepal, and Paraguay. Americans had the highest levels (335 pmol/l), followed by Congolese (286 pmol/l), Nepalese (251 pmol/l), and Paraguayans (197 pmol/l).

Who were these Americans? They are simply identified as ... young Americans—a demographic that is now less than 60% of European descent. In Boston, where the study was conducted, public schools in 2005 were 46% black and 31% Latino (mainly Puerto Ricans and Dominicans). The authors also state that "the USA participants were recruited by public advertisement." The pool of participants may have therefore resembled that of people who give blood in exchange for payment, i.e., it may have been disproportionately poor and non-white. In any event, the results are unusable without any information on racial background.

The Congolese participants were likewise unrepresentative of Congolese in general. They were Lese who inhabit the Ituri forest in proximity to the Efe pygmies. Many Lese are, in fact, partly of pygmy ancestry. As such, their testosterone levels would be closer to that of hunter-gatherers with lowers levels of polygyny and less male-male competition for mates.

This study concluded that testosterone levels did not differ between African American and Euro American boys between the ages of 6 and 18. Such a finding is to be expected for the first few years of this age range, when no difference should exist between the two groups. The main flaw, however, is that the participants were compared not by age but by Tanner stage. Since African Americans enter puberty earlier, this study compared younger African American boys with older Euro American boys.

Testosterone levels may differ between the two groups because of earlier maturation by African American boys. But why would this difference persist beyond adolescence and into the mid-twenties? This question remains unresolved because none of the participants were older than 18.

Various African studies

Several studies have found lower testosterone levels in African populations than in North Americans. This difference might be partly due to the effects of malnutrition or infectious diseases, notably among the Zimbabwean subjects studied by Lukas et al. (2004). The main reason, however, is that these studies mostly had middle-aged or even elderly participants. Lukas et al. (2004) report a mean age of 42.18. The scatter plot (Fig. 2) suggests a logarithmic decline in testosterone with age, but there were too few participants below 25 for analysis of that age group. The same criticism applies to Campbell et al. (2003), a study of testosterone levels in Ariaal pastoralists from northern Kenya. The mean age was 46.8.

In addition, some of these studies concern hunter-gatherers, like the !Kung of Namibia and the Ituri Forest pygmies of the Congo, who have low polygyny rates and weak male-male competition for mates (e.g., Winkler and Christiansen, 1993). Their low testosterone levels are thus to be expected.


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Alvergne, A., M. Jokela, and V. Lummaa. (2010). Personality and reproductive success in a high-fertility human population, Proceedings of the National Academy of Sciences, 107, 11745-11750.

Alvergne, A., C. Faurie, and M. Raymond. (2009). Variation in testosterone levels and male reproductive effort: Insight from a polygynous human population, Hormones and Behavior, 56, 491-497.

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Frost, P. (2008). The beginnings of black slavery, Evo and Proud, January 25

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Osegbe, D.N. (1997). Prostate cancer in Nigerians: facts and non-facts, Journal of Urology, 157, 1340-1343.

Pettaway, C.A. 1999. Racial differences in the androgen/androgen receptor pathway in prostate cancer, Journal of the National Medical Association, 91, 653-660.

Richard, A., S. Rohrmann, L. Zhang, M. Eichholzer, S. Basaria, E. Selvin, A.S. Dobs, N. Kanarek, A. Menke, W.G. Nelson, and E.A. Platz. (2014). Racial variation in sex steroid hormone concentration in black and white men: a meta-analysis, Andrology, 2(3), 428-35

Richards, R.J., F. Svec, W. Bao, S.R. Srinivasan, and G.S. Berenson. (1992). Steroid hormones during puberty: racial (black-white) differences in androstrenedione and estradiol. The Bogalusa heart study, The Journal of Clinical Endocrinology & Metabolism, 75, 624-631.

Rohrmann, S., Nelson, W.G., Rifai, N., Brown, T.R., Dobs, A., Kanarek, N., Yager, J.D., Platz, E.A. (2007). Serum estrogen, but not testosterone levels differ between Black and White men in a nationally representative sample of Americans, The Journal of Clinical Endocrinology & Metabolism, 92, 2519-2525

Ross, R.K., Bernstein, L., Lobo, R.A., Shimizu, H., Stanczyk, F.Z., Pike, M.C. and Henderson, B.E. (1992). 5-apha-reductase activity and risk of prostate cancer among Japanese and US white and black males, Lancet, 339, 887-889. 

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Saturday, October 24, 2015

Polygyny makes men bigger, tougher ... and meaner

Hadza men are smaller, less robust, and less aggressive than the more polygynous Datoga (Wikicommons - Idobi).


Humans differ in paternal investment—the degree to which fathers help mothers care for their offspring. They differ in this way between individuals, between populations, and between stages of cultural evolution.

During the earliest stage, when all humans were hunter-gatherers, men invested more in their offspring with increasing distance from the equator. Longer, colder winters made it harder for women to gather food for themselves and their children. They had to rely on meat from their hunting spouses. Conversely, paternal investment was lower in the tropics, where women could gather food year-round and provide for themselves and their children with little male assistance.

This sexual division of labor influenced the transition to farming. In the tropics, women were the main providers for their families as gatherers of fruits, berries, roots, and other wild plant foods. They were the ones who developed farming, thereby biasing it toward domestication of wild plants.

This may be seen in sub-Saharan Africa, where farming arose near the Niger's headwaters and gave rise to the Sudanic food complex—a wide range of native crops now found throughout the continent (sorghum, pearl millet, cow pea, etc.) and only one form of livestock, the guinea fowl (Murdock, 1959, pp. 44, 64-68). Many wild animal species could have been domesticated for meat production, but women were much less familiar with them. Men knew these species as hunters but had little motivation to domesticate them. Why should they? Women were the main providers. 

And so women shouldered even more the burden of providing for themselves and their offspring. Men in turn found it easier to go back on the mate market and get second or third wives. Finally, men had to compete against each other much more for fewer unmated women.
There was thus a causal chain: female dominance of farming => female reproductive autonomy => male polygyny => male-male rivalry for access to women. Jack Goody (1973) in his review of the literature says: "The desire of men to attract wives is seen as correlated with the degree of women's participation in the basic productive process." The more women produce, the lower the cost of polygyny.

In sub-Saharan Africa, the cost was often negative. Goody quotes a 17th century traveler on the Gold Coast: the women till the ground "whilst the man only idly spends his time in impertinent tattling (the woman's business in our country) and drinking of palm-wine, which the poor wives are frequently obliged to raise money to pay for, and by their hard labour maintain and satisfie these lazy wretches their greedy thirst after wines."

Goody cites data from southern Africa showing that the polygyny rate fell when the cost of polygyny rose:

In Basutoland one in nine husbands had more than one wife in 1936; in 1912, it was one in 5.5 (Mair 1953: 10). Hunter calculates that in 1911 12 per cent of Pondo men were plurally married and the figure was slightly lower in 1921. In 1946, the Tswana rate was 11 per cent; according to a small sample collected by Livingstone in 1850 it was 43 per cent. The figures appear to have changed drastically over time and the reasons are interesting. 'The large household is now not a source of wealth, but a burden which only the rich can bear' (Mair 1953: 19). Not only is there a specific tax for each additional wife, but a man's wives now no longer give the same help in agriculture that they did before. One reason for this is that the fields are ploughed rather than hoed. Among the Pondo, 'the use of the plough means that the amount of grain cultivated no longer depends on women's labour' (Goody, 1973)

Although polygynous marriage has become less common in southern Africa, polygynous behavior seems as frequent as ever. To a large degree, polygynous marriage has given way to more transient forms of polygyny: prostitution and other informal arrangements.

Goody also notes that women are much less self-reliant in the northern savannah of West Africa:

In savannah regions where water is scarce and trees scattered, their collection may make great demands on a woman’s time. So too does the grinding of hard grain, in the absence of mills. In all these domestic pursuits the savannah is more demanding on a woman’s time than the forest and consequently she can often make less contribution to agriculture. (Goody, 1973)

Yet polygyny rates have remained high. Goody gives the example of Ghana. Polygyny rates are about the same in the north and the south, yet in the north men participate much more in farming.

So what is going on? Goody concludes that "female farming and polygyny are clearly associated in a general way" but ultimately the "reasons behind polygyny are sexual and reproductive rather than economic and productive." It would be more parsimonious to say that the polygyny rate increases when the cost of providing for a woman and her children decreases for men. Over time, low-cost polygyny selects for men who are more motivated to exploit sexual opportunities. This new mindset influences the subsequent course of gene-culture coevolution.

Such gene-culture coevolution has gone through four stages in the evolutionary history of sub-Saharan Africans:

First stage

Tropical hunter-gatherers were already oriented toward low paternal investment. Men had a lesser role in child rearing because year-round food gathering provided women with a high degree of food autonomy. Women were thus selected for self-reliance and men for polygyny. Pair bonding was correspondingly weak in both sexes.

Second stage

This mindset guided tropical hunter-gatherers in their transition to farming. In short, female-dominated food gathering gave way to female-dominated horticulture—hoe farming of various crops with almost no livestock raising. Women became even more autonomous, and men even more polygynous. There was thus further selection for a mindset of female self-reliance, male polygyny, and weak pair bonding.

Third stage

A similar process occurred with the development of trade. Female-dominated horticulture tended to orient women, much more than men, toward the market economy. This has particularly been so in West Africa, where markets are overwhelmingly run by women. Trade has thus become another means by which African women provide for themselves and their children.

Fourth stage

Female-dominated horticulture has given way to male-dominated farming (pastoralism, cereal crops) in some regions, such as the northern savannah regions of West Africa. Despite higher male participation in farming, the pre-existing mindset has tended to maintain high polygyny rates. We see a similar tendency in southern Africa, where polygyny rates have fallen over the past century, and yet polygynous behavior persists in the form of prostitution and less formal sexual arrangements.

The Hadza and the Datoga

Mode of subsistence, mating system, and mindset are thus interrelated. These interrelationships are discussed by Butovskaya et al. (2015) in their study of two peoples in Tanzania: the largely monogamous Hadza (hunter-gatherers) and the highly polygynous Datoga (pastoralists). In their review of previous studies, the authors note:

In hunter-gatherer societies, such as the monogamous Hadza of Tanzania (Africa), men invest more in offspring than in small-scale pastoralist societies, such as the polygynous Datoga of Tanzania [12-14]. Polygyny and between-group aggression redirect men's efforts from childcare toward investment in male-male relationships and the pursuit of additional mates [15]. When men participate in childcare, their testosterone (T) level decreases [15-18]. Muller et al. [19] found that, among the monogamous, high paternally investing Hadza, T levels were lower for fathers than for non-fathers. This effect was not observed among the polygynous, low paternally investing Datoga. (Butovskaya et al., 2015).

Butovskaya et al. (2015) confirmed these previous findings in their own study:

Datoga males reported greater aggression than Hadza men—a finding in line with previous reports [29,30]. It is important to mention several striking differences between these two cultures. There is a negative attitude toward aggression among the Hadza but not among the Datoga. In situations of potential aggression, the Hadza prefer to leave [30]. In contrast, aggression is an instrument of social control—both within the family and in outgroup relations in Datoga society. Datoga men are trained to compete with each other and to act aggressively in particular circumstances [30]

The authors also confirmed differences in reproductive behavior between the two groups: 

Our research indicates a difference in the number of children in Hadza and Datoga men achieved after the age of 50. This may be interpreted as differences attributable to different life trajectories and marriage patterns. Beginning in early childhood, boys in the two societies are subjected to different social and environmental pressures (e.g., it is typical for Datoga parents to punish children for misbehavior, while parental violence is much less typical for Hadza parents). Hadza men start reproducing in the early 20s, but their reproductive success later in life is associated with their hunting skills [15]. In the Datoga, men marry later, typically in their 30s. Male status and, consequently, social and reproductive success in the Datoga are positively correlated with fighting abilities and risk-taking in raiding expeditions among younger men, and with wealth, dominance, and social skills among older men. In the Datoga, as in other patrilineal societies, fathers do not invest directly in child care, but children do benefit from their father's investment in the form of wealth and social protection, as well as various services provided by father's patrilineal male relatives [56]. In polygynous societies, spending resources on attracting additional wives may be more beneficial [40,57,58]. It would be difficult for some men to invest directly in providing for all their children, given that men with multiple wives can father a considerable number of children, and that households with wives may be located at substantial distance from one another.

This behavioral difference seems to be mediated by differing levels of androgens, such as testosterone:

The effect of androgens, such as T, operates through stimulation of androgen receptors [21-23]. The androgen receptor (AR) gene contains a polymorphic and functional locus in exon 1, comprising two triplets (CAG and GGN). This locus supports a regulatory function that responds to T, with fewer CAG repeat clusters being more effective in transmitting the T signal [22]. Moreover, the length of the GGN repeat predicts circulating and free T in men.

At the androgen receptor gene, the authors found fewer CAG repeats in the Datoga than in the Hadza. The number of repeats was also more variable in the Datoga. The Datoga's higher and more variable polygyny rates thus seem to correlate with higher and more variable levels of testosterone.

The authors also wished to see whether these differing levels of testosterone correlate with differing levels of aggressiveness. To this end, they interviewed the Hadza and Datoga participants:

They were asked to provide information including their age, sex, marital status, number of children, ethnicity and aggression history (especially fights with other tribal members). All questions were read aloud in one-to-one dialogues and further explanations were provided, if necessary. Self-reported aggression was assessed with the Buss-Perry Aggression Questionnaire (BPAQ; [48]). The BPAQ includes 29 statements, grouped into four subscales—physical aggression (9 items), verbal aggression (5 items), anger (7 items), and hostility (8 items)—answered on aLikert scale anchored by 1 (extremely uncharacteristic of me) and 5 (extremely characteristic of me).

Total aggression was found to correlate negatively with CAG repeat number. Age group did not predict aggression.

More polygyny = stronger sexual selection of men

Finally, the authors suggest that Datoga men, with their higher polygyny rate and fiercer competition for access to women, have undergone greater sexual selection. They have thus become bigger and more masculine than Hadza men. Although this selection pressure also exists among the Hadza, the driving force of sexual selection has been weaker because Hadza men are more monogamous and less sexually competitive:

Our findings are in concordance with other research, demonstrating that even among the relatively egalitarian Hadza there is selection pressure in favor of more masculine men [59-62]. At the same time, preference for more masculine partners, with greater height and body size, is culturally variable and influenced by the degree of polygyny, local ecology, and other economic and social factors [59-62]. Many Datoga women commented that they would like to avoid taller and larger men as marriage partners, as they may be dangerously violent [44,62]. Only 2% of Hadza women listed large body size as an attractive mate characteristic [63]. Hadza marriages in which the wife is taller than the husband are common, and as frequent as would be expected by chance [64]. (Butovskaya et al., 2015)

This is consistent with what we see in nonhuman polygynous species. Successful males tend to be the ones that are better not only at attracting the opposite sex but also at fighting off rivals. They thus become bigger, tougher, and meaner.

This is also consistent with what we see generally in the highly polygynous farming peoples of sub-Saharan Africa. They and their African-American descendants exceed European-descended subjects in weight, chest size, arm girth, leg girth, muscle fiber properties, and bone density (Ama et al., 1986; Ettinger et al.,1997; Himes, 1988; Hui et al., 2003; Pollitzer and Anderson, 1989; Todd and Lindala, 1928; Wagner and Heyward, 2000; Wolff and Steggerda, 1943; Wright et al., 1995).


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