Did women jumpstart recent cognitive evolution?

Scatter plots of frequencies of CASC5 variants by sex (Shi et al. 2017). During the last ice age, natural selection favored an increase in the gray matter of ancestral East Asians ... primarily in women.

Back in 2005 there was much interest in genes that regulate brain size, particularly in the ways they varied geographically within our species. It was found that two of these genes, Microcephalin and ASPM, continued to evolve as modern humans spread out of Africa. The latest variant of Microcephalin arose some 37,000 years ago in Eurasia and is still largely confined to the indigenous peoples of Eurasia and the Americas (Mekel-Bobrov et al. 2005). The latest variant of ASPM appeared even later, some 5,800 years in the Middle East (Evans et al. 2005).

Interest fell off when no association could be shown between the new variants and IQ or brain size (Mekel-Bobrov et al. 2007; Rushton et al. 2007; see also Frost 2020). Since then, we have learned that the new ASPM variant is associated with a larger cerebral cortex, and not a larger brain as a whole. Overall brain volume seems to be constrained in modern humans, perhaps by the breadth of a woman's pelvis during childbirth or simply by the high metabolic costs of brain tissue (Ali and Meier 2008; Frost 2020). As for the lack of an association with IQ, we now know that IQ correlates poorly or not at all with some cognitive abilities, like executive function and face recognition.

But what do the new variants actually do? Perhaps a specialized mental task.  It has been suggested that the new ASPM variant assists the brain in processing non-tonal language or alphabetical script (Dediu and Ladd 2007; Frost 2007).

CASC5, another gene for brain growth

Interest has since grown in another gene that regulates brain growth, CASC5. Like Microcephalin and ASPM, it has undergone recent evolution in the modern human lineage:

[...] the CASC5 gene contains mutations in modern humans, but not in Denisovans (Meyer et al. 2012) and this gene also shows distinct sequence divergence between modern humans and Neanderthals (Prufer et al. 2014). These data suggest that CASC5 is an important gene for human neurogenesis, and may harbor modern human specific mutations contributing to the recent evolutionary change of the human brain. (Shi et al. 2017)

Shi et al. (2017) found evidence of recent evolutionary change. Specifically, two nucleotides of CASC5 have been replaced with a new variant in all modern humans. Six other nucleotides have become polymorphic, with some people having the new variants and others not. These polymorphisms show regional differences:

- In four of the polymorphisms, the new variant has a much higher frequency in East Asians than in Europeans or Africans.

- In one polymorphism, it has a much higher frequency in Europeans than in the other two regional groups.

- The remaining polymorphism shows no differences in frequency between the three regional groups.

By and large, the new variants have been under strong positive selection, particularly among East Asians. When the authors examined the six polymorphisms, they found signals of selection for five of them in East Asians and for one in Europeans.

The new variants and brain characteristics

The authors then looked for correlations between the new variants and certain characteristics of the brain, specifically total brain volume, gray matter volume, and white matter volume. To this end, 267 healthy participants were recruited for brain imaging (Han Chinese, 178 females and 89 males, mean age 35.4 ± 12.5 years). All of them were free from mental disorders, drug abuse, alcohol dependence, and brain injury.

Gray matter was significantly larger in participants with the new variant than in those with the ancestral variant at five of the nucleotide sites, including the four polymorphic ones—the same ones that showed differences in variant frequency between East Asians and Europeans. When the authors examined the one polymorphism whose variants were equally common in East Asians, Europeans, and Africans, they found no brain differences between participants with the new variant and those with the ancestral one.

When the authors broke their data down by sex, they found that the new variants were significantly associated with a higher volume of gray matter only in women, not in men, although men seemed to trend in the same direction. The authors suggest that this effect would be significant in men if the number of male participants were larger. Probably. But it seems to me there would still be a sex difference, the number of participants being already large enough.

Ice age origin of the new variants

The authors say the new variants became prevalent "after modern humans migrated out of Africa less than 100,000 years ago." We can narrow down the time range further. The new variants are also present at high frequencies among the indigenous peoples of North and South America; therefore, they must have become prevalent before ancestral Amerindians crossed into North America some 12,000 years ago, apparently in a population that was ancestral both to Amerindians and to East Asians. That would be long before the time of recorded history and even before the Holocene, at a time when northern Eurasia was experiencing glacial conditions.

Did those conditions select for cognitive ability? Cold, seasonal environments did impose new cognitive demands on early modern humans, first by increasing their need to plan ahead over a yearly cycle and second by providing them with new tasks: garment making, needlework, weaving, leatherworking, and kiln operation. Women performed those tasks because the environment offered them few opportunities for food gathering—the usual female activity before the advent of farming. They thus moved into artisanal tasks that not only required greater cognitive ability but also offered much potential for further development. This was the "original industrial revolution" and it was led by women (Frost 2019a).

We can better understand this sexual division of labor by studying northern hunter-gatherers of recent times. According to a cross-cultural study, if women are less involved in food gathering, they specialize in activities unrelated to food procurement, i.e., house building, leatherworking, and burden carrying (Waguespack 2005). A study of two Inuit groups found the highest degree of technological complexity in garment making and shelter building, both of which are wholly or largely women's work (Oswalt 1976). Cold environments thus change the sexual division of labor among hunter-gatherers in a crucial way: while men continue to be food providers, women develop new technologies.

These findings may explain the recent evolution of CASC5: women were the focus of selection for cognitive ability during Ice Age times. But why was the selection stronger among ancestral East Asians than among ancestral Europeans? It looks like the climate at that time was more severe in northern Asia than in northern Europe. Europe benefited from the moderating influence of the Atlantic, which made for a milder and moister climate. Conditions were much colder and drier in northern Asia.

The evolution of human intelligence cannot be reduced to a single unified theory. Cold environments emancipated women from the mental straitjacket of food gathering, thus putting humans on the path to social complexity. That path, however, would take them to latitudes farther south in temperate and even tropical environments where they would be exposed to new cognitive demands. With the end of hunting, men moved not only into farming but also into the artisanal activities that women had developed. The same period saw a decline in brain volume that was greater in women than in men—an indication that cognitive demands were particularly high before the Holocene, and even more so for women (Frost 2019b).

The Holocene thus saw northern populations expand southward and eventually cover almost all of Eurasia, North Africa, Oceania, and the Americas. Those populations had a cognitive advantage that made them better able to exploit the social complexity emerging farther south. This point was made by Darwin's colleague Alfred Russel Wallace:

So when a glacial epoch comes on, some animals must acquire warmer fur, or a covering of fat, or else die of cold. Those best clothed by nature are, therefore, preserved by natural selection. Man, under the same circumstances, will make himself warmer clothing, and build better houses; and the necessity of doing this will react upon his mental organisation and social condition [...] a hardier, a more provident, and a more social race would be developed, than in those regions where the earth produces a perennial supply of vegetable food, and where neither foresight nor ingenuity are required to prepare for the rigours of winter. And is it not the fact that in all ages, and in every quarter of the globe, the inhabitants of temperate have been superior to those of tropical countries? All the great invasions and displacements of races have been from North to South, rather than the reverse.

References

Ali, F. and R. Meier. (2008). Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size. Molecular Biology and Evolution 25(11): 2247-2250.

https://www.researchgate.net/profile/Rudolf_Meier/publication/23185977_Positive_Selection_in_ASPM_Is_Correlated_with_Cerebral_Cortex_Evolution_across_Primates_but_Not_with_Whole-Brain_Size/links/00b7d53ad237958edd000000.pdf

Dediu, D., and R. Ladd. (2007). Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin. Proceedings of the National Academy of Sciences 104(26): 10944-10949

https://langev.com/pdf/dediu07linguisticTonePNAS.pdf

Evans, P. D., Gilbert, S. L., Mekel-Bobrov, N., Vallender, E. J., Anderson, J. R., Vaez-Azizi, L. M., et al. (2005). Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans. Science 309: 1717-1720.

http://web.stanford.edu/class/cs273a/papers.spr07/lecture15/Microencephalin.pdf

Frost, P. (2007). The spread of alphabetical writing may have favored the latest variant of the ASPM gene. Medical Hypotheses 70: 17-20.

http://www.sciencedirect.com/science/article/pii/S0306987707003234

Frost, P. (2019a). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181

https://doi.org/10.3390/psych1010012

Frost, P. (2019b). Why did brain size decrease after the ice age? Evo and Proud, July 6

https://evoandproud.blogspot.com/2019/07/why-did-brain-size-decrease-after-ice.html

Frost, P. (2020). A second look at ASPM. Evo and Proud, April 14

https://evoandproud.blogspot.com/2020/04/a-second-look-at-aspm.html

Mekel-Bobrov, N., S.L. Gilbert, P.D. Evans, E.J. Vallender, J.R. Anderson, R.R. Hudson, S.A. Tishkoff and B.T. Lahn. (2005). Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens. Science 309: 1720-1722

https://www.researchgate.net/publication/7611130_Ongoing_Adaptive_Evolution_of_ASPM_a_Brain_Size_Determinant_in_Homo_Sapiens

Mekel-Bobrov, N., D. Posthuma, S.L. Gilbert, P. Lind, M.F. Gosso, et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Human Molecular Genetics 16(6): 600-608.

https://academic.oup.com/hmg/article/16/6/600/610971

Oswalt, W.H. (1976). An Anthropological Analysis of Food-Getting Technology, 1st ed.; John Wiley and Sons: New York, NY, USA.

Rushton, J.P., P.A. Vernon, and T.A. Bons. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK 3(2): 157-60.

https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2006.0586

Shi, L., Hu, E., Wang, Z. et al. (2017). Regional selection of the brain size regulating gene CASC5 provides new insight into human brain evolution. Human Genetics 136: 193-204. https://doi.org/10.1007/s00439-016-1748-5

http://ir.ioz.ac.cn/bitstream/000000/12812/1/Regional%20selection%20of%20the%20brain%20size%20regulati1.pdf

Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist 107: 666-676.

https://pdfs.semanticscholar.org/8e50/02b23a25d1a97054f8bf9b8b852d2151a093.pdf

Wallace, A.R. (1864). The Origin of Human Races and the Antiquity of Man deduced from the Theory of "Natural Selection." Journal of the Anthropological Society of London, 2, clviii-clxxxvii, Alfred Russel Wallace Classic Writings. Paper 6. http://digitalcommons.wku.edu/dlps_fac_arw/6

The Original Industrial Revolution

Cro-Magnon woman (Wikicommons) – At northern latitudes, women had fewer opportunities for food gathering, so they were free to specialize in new and more cognitively demanding tasks, like garment making, needlework, weaving, leatherworking, pottery, and kiln operation.





I've published an article on the theory that cold Paleolithic winters selected for intelligence. This theory is often attributed to J. Philippe Rushton and Arthur Jensen but actually goes much further back. The article is open access (see link), and the abstract is provided below. Comments are welcome.




Rushton and Jensen argued that cognitive ability differs between human populations. But why are such differences expectable? Their answer: as modern humans spread out of Africa and into northern Eurasia, they entered colder and more seasonal climates that selected for the ability to plan ahead, in order to store food, make clothes, and build shelters for winter. This cold winter theory is supported by research on Paleolithic humans and recent hunter-gatherers. Tools become more diverse and complex as effective temperature decreases, apparently because food has to be obtained during limited periods and over large areas. There is also more storage of food and fuel and greater use of untended traps and snares. Finally, shelters have to be sturdier, and clothing more cold-resistant. The resulting cognitive demands are met primarily by women because the lack of opportunities for food gathering pushes them into more cognitively demanding tasks, like garment making, needlework, weaving, leatherworking, pottery, and kiln operation. The northern tier of Paleolithic Eurasia thus produced the "Original Industrial Revolution"—an explosion of creativity that preadapted its inhabitants for later developments, i.e., farming, more complex technology and social organization, and an increasingly future-oriented culture. Over time, these humans would spread south, replacing earlier populations that could less easily exploit the possibilities of the new cultural environment. As this environment developed further, it selected for further increases in cognitive ability. Indeed, mean intelligence seems to have risen during recorded history at temperate latitudes in Europe and East Asia. There is thus no unified theory for the evolution of human intelligence. A key stage was adaptation to cold winters during the Paleolithic, but much happened later.



Reference



Frost, P. (2019). The OriginalIndustrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 2019, 1(1), 166-181

https://doi.org/10.3390/psych1010012

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).

References 

Ama, P.F.M., J.A. Simoneau, M.R. Boulay, O. Serresse, G. Thériault, and C. Bouchard. (1986). Skeletal muscle characteristics in sedentary Black and Caucasian males, Journal of Applied Physiology, 61, 1758-1761.

http://www.educadorfisicoadinis.com.br/download/artigos/Skeletal%20muscle%20characteristics%20in%20sedentary%20black%20and%20Caucasian%20males.pdf  

Butovskaya M.L., O.E. Lazebny, V.A. Vasilyev, D.A. Dronova, D.V. Karelin, A.Z.P. Mabulla, et al. (2015). Androgen receptor gene polymorphism, aggression, and reproduction in Tanzanian foragers and pastoralists. PLoS ONE 10(8): e0136208. 

https://www.researchgate.net/publication/281170838_Androgen_Receptor_Gene_Polymorphism_Aggression_and_Reproduction_in_Tanzanian_Foragers_and_Pastoralists  

Ettinger, B., S. Sidney, S.R. Cummings, C. Libanati, D.D. Bikle, I.S. Tekawa, K. Tolan, and P. Steiger. (1997). Racial differences in bone density between young adult black and white subjects persist after adjustment for anthropometric, lifestyle, and biochemical differences, Journal of Clinical Endocrinology & Metabolism, 82, 429-434.

http://press.endocrine.org/doi/abs/10.1210/jcem.82.2.3732  

Goody, J. (1973). Polygyny, economy and the role of women, in J. Goody (ed.) The Character of Kinship, Cambridge: Cambridge University Press.

https://books.google.ca/books?hl=fr&lr=&id=TFjf4mUHqv4C&oi=fnd&pg=PA175&dq=Polygyny,+economy+and+the+role+of+women&ots=Lf9w6wxY9D&sig=3d0RBnoMbGUd2OYqghzhwdsO3BA#v=onepage&q&f=false

Himes, J. H. (1988). Racial variation in physique and body composition, Canadian Journal of Sport Sciences, 13, 117-126.

http://europepmc.org/abstract/med/3293730  

Hui, S.L., L.A. DiMeglio, C. Longcope, M. Peacock, R. McClintock, A.J. Perkins, and C.C. Johnston Jr. (2003). Difference in bone mass between Black and White American children: Attributable to body build, sex hormone levels, or bone turnover? Journal of Clinical Endocrinology & Metabolism, 88, 642-649. 

https://www.researchgate.net/profile/Conrad_Johnston/publication/10911942_Difference_in_bone_mass_between_black_and_white_American_children_attributable_to_body_build_sex_hormone_levels_or_bone_turnover/links/548f36af0cf225bf66a7fdc1.pdf

Murdock, G.P. (1959). Africa. Its Peoples and Their Culture History, New York: McGraw-Hill. 

Pollitzer, W.S. and J.J. Anderson. (1989). Ethnic and genetic differences in bone mass: a review with a hereditary vs environmental perspective, American Journal of Clinical Nutrition, 50, 1244-1259

http://ajcn.nutrition.org/content/50/6/1244.short 

Todd, T.W., and A. Lindala. (1928). Dimensions of the body: Whites and American Negroes of both sexes, American Journal of Physical Anthropology, 12, 35-101.

http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330120104/abstract  

Wagner, D.R. and V.H. Heyward. (2000). Measures of body composition in blacks and whites: a comparative review, American Journal of Clinical Nutrition, 71, 1392-1402.

http://ajcn.nutrition.org/content/71/6/1392.short

Wolff, G. and M. Steggerda. (1943). Female-male index of body build in Negroes and Whites: An interpretation of anatomical sex differences, Human Biology, 15, 127-152.

Wright, N.M., J. Renault, S. Willi, J.D. Veldhuis, J.P. Pandey, L. Gordon, L.L. Key, and N.H. Bell. (1995). Greater secretion of growth hormone in black than in white men: possible factor in greater bone mineral density-a clinical research center study, Journal of Clinical Endocrinology & Metabolism, 80, 2291-2297.

http://press.endocrine.org/doi/abs/10.1210/jcem.80.8.7543111  

Did the slave trade make Africans polygynous?

Slave exports to the Americas from different parts of Africa (Dalton & Leung, 2011). Did the slave trade create patterns of behavior that today exist throughout sub-Saharan Africa, such as generalized polygyny?

Why is polygyny so frequent in sub-Saharan Africa? As Goody (1973, p. 177) noted, the differences with Eurasia are striking:

In Europe and Asia, polygyny is largely but not exclusively an heir-producing device; often it is a way of replacing a barren wife. In Africa, plural marriage is far more generalised; according to Dorjahn, about 35 per cent of married men have more than one wife. Hence a large percentage of the population is likely to be part of a polygynous unit at some point in the life-cycle. Most men will be polygynously married at some time or other; women are yet more likely to be so. And most siblings will have sets of half siblings, both because of the plural marriage of their fathers and because of the remarriage of their mothers — since polygyny inevitably involves a large differential in the age of marriage, men will be older when they beget children than women are when they bear them. Hence there will be a higher proportion of widows and fatherless children.

Goody (1973) attributes this generalized polygyny partly to female self-reliance in food production. Year-round farming enables women to provide for their own needs and those of their children. A wife thus costs little in terms of upkeep, and this low maintenance cost encourages men to have as many wives as possible.

This rule nonetheless has interesting exceptions. In the savannah regions of Ghana, women plant grain and help with the harvest, but they leave yam cultivation to men and do not engage in hoeing for cereal agriculture. Yet polygyny rates are somewhat higher there than elsewhere in Ghana, where women contribute more to food production. Polygyny is also less frequent in East Africa than in West Africa, yet women contribute more to food production in East Africa than in West Africa.

Goody (1973, p. 185) concludes that “hoe agriculture, female farming and polygyny are clearly associated in a general way” but there must be other explanatory factors. But what?

For Dalton and Leung (2011), one big factor is the slave trade—the mass exportation of African laborers that ended only two centuries ago. West Africa tended to export male slaves while East Africa tended to export female slaves. This pattern reflected differences in market demand: on the one hand, the Americas wanted farm labor; on the other, the Middle East and South Asia wanted domestics or concubines. These differing sex ratios might therefore explain why polygyny is less frequent in East Africa than in West Africa:

The slave trades existed for hundreds of years, and, as a result, Africa experienced abnormal sex ratios for long periods of time. Polygyny could have emerged or been strengthened during the long period of abnormal sex ratios. Figures 1 and 2 suggest the Western Coast should have contained more polygynous marriages, whereas the Eastern Coast should have contained fewer.

These abnormal sex ratios returned to normal once the slave trade had ended in the early 19th century. Why, then, didn’t polygyny rates follow this return to normal? Dalton and Leung (2011, p. 8) blame cultural conservatism: “Once these cultural traits are established, polygyny can become self-sustaining.”

This hypothesis is interesting and might explain some of the variation in polygyny rates within sub-Saharan Africa. But it fails to explain why polygyny rates are in the double-digit range throughout sub-Saharan Africa. East Africa’s rates are lower but still high by world standards. Goody (1973, p. 181) states that East African cattle societies have a rate of 24.7%, i.e., the percentage of married men with more than one wife. Pebley and Mbugua (1989) similarly write: “The frequency is somewhat lower in East and South Africa, although 15 to 30 percent of husbands are reported to be polygynists in Kenya and Tanzania.”

Admittedly, the above figures run counter to the ones offered by Dalton and Leung (2011, p. 1):

[…] the percentage of men in polygynous marriages in Western African countries like Guinea, Togo, and Benin is 35.037, 29.793, and 29.679, whereas in Eastern African countries like Ethiopia, Kenya, and Malawi the percentage is 6.131, 9.206, and 10.101.

These figures come from the latest “Demographic and Health Surveys” and are twenty to forty years more recent than the other figures. As such, they reflect the decline in female farming and the growing urbanization of African societies. In this new context, institutionalized polygyny has given way to looser arrangements with multiple girlfriends and/or prostitutes. And why bring Ethiopia into the comparison? It is not a sub-Saharan country and differs from both West and East Africa in many ways, notably the long-established influence of the Coptic Church and Christian sexual norms.

Dalton and Leung also err in assuming that sub-Saharan Africa had low polygyny rates before the slave trade. Several lines of evidence argue otherwise:

1. The ratio of Y-chromosome to X-chromosome variability is much higher among sub-Saharan Africans, New Guineans, and Aboriginal Australians than among other human populations. This suggests a long-lasting trend of fewer men than women contributing to the gene pool (Dupanloup et al., 2003; see also Torroni et al., 1990; Scozzari et al., 1997).

2. Proto-Bantu, spoken approximately 3,000 years ago, has a specific term for “taking a second wife” (Polome, 1977).

3. A high level of male-male competition for females is suggested by the increased sexual dimorphism of African Americans for weight, chest size, arm girth, and leg girth (Todd & Lindala, 1928; Wolff & Steggerda, 1943). In contrast, a small, gracile, and almost childlike body characterizes Khoisans and Pygmies, the only sub-Saharan populations with low polygyny rates.

After arguing that the slave trade caused generalized polygyny, Dalton and Leung see therein a leading cause of Africa’s lag in economic and social development.

Among similar countries, polygynous countries are poorer than nonpolygynous countries. Polygynous countries have higher fertility and lower savings. The calibrated model in Tertilt (2005) suggests banning polygyny decreases fertility by 40 percent, increases savings by 70 percent, and increases GDP per capita by 170 percent. (Dalton & Leung, 2011, p. 3)

There is undoubtedly some kind of relationship between generalized polygyny and Africa’s stubbornly high fertility and economic poverty, but it’s not a simple one of cause and effect. Banning polygyny will not cause immediate changes to reproductive and economic behavior. Dalton and Leung themselves argue that cultural conservatism alone has maintained high polygyny rates in Africa for the past two centuries. Wouldn’t the same be true for other behaviors?

Remember also that the time depth of generalized polygyny is not the four or five centuries that Dalton and Leung claim. In sub-Saharan Africa, high polygyny rates are associated with ‘female farming’ societies, and such societies began to spread outward from a point of origin near the Niger’s headwaters some 6,000 to 7,000 years ago (Murdock, 1959, pp. 44, 64-68).

Behavioral predispositions have significant heritability, especially in relation to sexual behavior (Comings et al., 2002; Mendle et al., 2006; Belsky et al., 2007). If generalized polygyny has existed in sub-Saharan Africa for six to seven thousand years, wouldn’t it have favored certain predispositions and not others? And wouldn’t those predispositions survive the banning of polygyny?

The question doesn’t seem to have crossed the authors’ minds. They seem to believe, a bit naïvely, that there is only cultural conservatism to worry about. No less naïve are the authors they cite:

Nunn and Wantchekon (2010) examines a particular channel through which the slave trades impact current African economic performances, namely the levels of trust across individuals within Africa. Trust supports economic exchange in well-functioning markets and would have plausibly been affected within groups living in the capture and export economies participating in the slave trades. Nunn and Wantchekon (2010) shows those individuals whose ancestral groups experienced higher slave exports exhibit lower levels of trust even to this day. Our paper contributes to these findings by suggesting an additional channel through which the slave trades have had a long-term impact on current African society. (Dalton & Leung, 2011, p. 2)

Low trust is typical of all simple, clan-based societies. Papua-New-Guinea was not affected by the slave trade, yet it has very low levels of trust. The slave trade may indeed have made people less trusting in some parts of sub-Saharan Africa than in others. But this does not explain why levels of trust are so low in sub-Saharan Africa as a whole.

And questions can be raised about the study by Nunn and Wantchekon (2010). Those authors found that willingness to trust members of other ethnic groups correlated with an ethnic group’s historical importance as a source of slaves. Most slaves, however, were taken during inter-tribal wars, and such wars generally occurred in regions already prone to interethnic conflict.

In short, the correlation is valid, but it doesn’t prove the causal relationship that Nunn and Wantchekon infer. In fact, the line of causality probably runs in the opposite direction.

References

Belsky, J., L.D. Steinberg, R.M. Houts, S.L. Friedman, G. DeHart, E. Cauffman, G.I. Roisman, B.L. Halpern-Felsher. (2007). Family rearing antecedents of pubertal timing, Child Dev., 78, 1302-1321.

Comings, D.E., D. Muhleman, J.P. Johnson, J.P. MacMurray. (2002). Parent-daughter transmission of the androgen receptor gene as an explanation of the effect of father absence on age of menarche. Child Dev., 73, 1046-1051.

Dalton, J.T., & T.C. Leung. (2011). Why is Polygyny More Prevalent in Western Africa?
An African Slave Trade Perspective,

http://www.wfu.edu/~daltonjt/PolygynySlaveTrade.pdf

Demographic and Health Surveys (2011): www.measuredhs.com

Dupanloup, I., L. Pereira, G. Bertorelle, F. Calafell, M.J. Prata, A. Amorim, & G. Barbujani. (2003). A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity. Journal of Molecular Evolution, 57, 85-97.

Goody, J. (1973). The Character of Kinship, Cambridge: Cambridge University Press.

Mendle, J. E. Turkheimer, B.M. D’Onofrio, S.K. Lynch, R.E. Emery, W.S. Slutske, & N.G. Martin. (2006). Family structure and age at menarche: a children-of-twins approach, Dev Psychol., 42, 533-542.

Murdock, G.P. (1959). Africa. Its Peoples and Their Culture History. New York: McGraw-Hill.

Nunn, N., & L. Wantchekon. (2010). The Slave Trade and the Origins of Mistrust in Africa, American Economic Review, forthcoming.

Pebley, A. R., & W. Mbugua. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (ed.), Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

Polome, E.C. (1977). The reconstruction of Proto-Bantu culture from the lexicon. In L. Bouquiaux (Ed.) L'Expansion bantoue, 2, 779-791. Centre national de la recherche scientifique.

Scozzari, R., F. Cruciani, P. Malaspina, P. Santolamazza, B.M. Ciminelli, A. Torroni, D. Modiano, D.C. Wallace, K.K. Kidd, A. Olckers, P. Moral, L. Terrenato, N. Akar, R. Qamar, A. Mansoor, S.Q. Mehdi, G. Meloni, G. Vona, D.E.C. Cole, W.W. Cai, & A. Novelletto. (1997). Differential structuring of human populations for homologous X and Y microsatellite loci. American Journal of Human Genetics, 61, 719-733.

Todd, T.W. & A. Lindala. (1928). Dimensions of the body: Whites and American Negroes of both sexes. American Journal of Physical Anthropology, 12, 35-101.

Torroni, A., O. Semino, R. Scozzari, G. Sirugo, G. Spedini, N. Abbas, M. Fellous, et al. (1990). Y-chromosome DNA polymorphisms in human populations: differences between Caucasoids and Africans detected by 49a and 49f probes. Annals of Human Genetics, 54, 287-296.

Wolff, G. & M. Steggerda. (1943). Female-male index of body build in Negroes and Whites: An interpretation of anatomical sex differences. Human Biology, 15, 127-152.

Are African women oppressed?

Guess who does the farmwork? In sub-Saharan Africa, women do most of the labor. On the other hand, they have more control over the fruits of their labor.

Are African women oppressed? For many, the answer is ‘yes’:

Much popular writing and many popular ideas about gender in Africa continue to rest on the belief that women in all societies in the entire world are oppressed and that African women are particularly oppressed. Even in scholarly studies, the assumption of universal male dominance of African social and economic relations persists. (Saidi, 2010, p. 12)

It’s true that women do most of the labor in sub-Saharan Africa. They are, in fact, largely self-reliant in providing for themselves and their children:

In sub-Saharan Africa, the labor of women was usually the work of daily subsistence. […] Hunting and warfare, usually the activities of men, could in contrast produce either a big windfall or nothing at all in terms of male production. […] Therefore, in a very simplistic way, it can be argued that female labor was the necessary labor—the labor from which surplus could be derived—whereas male labor could produce the luxury items, the status items. (Saidi, 2010, p. 15)

This argument ignores, however, the greater power of African women over the fruits of their labor. Often, this power was totally in the hands of women, typically older matriarchs:

The labor of young women, and young men, for that matter, in many East-Central African societies, particularly among the Sabi-speaking group of peoples, was historically controlled by the older female matrilineal kin of the young women, not by men at all. (Saidi, 2010, p. 16)

Indeed, one could argue that African men tended to occupy a peripheral role within the family:

Poewe found in her fieldwork that the marriage institution was highly flexible and discouraged strong, intense, or lasting solidarity between husband and wife. The male in these matrilineal societies did not produce for his progeny or for himself, but usually for a matriclan with whom he might or might not reside. His role, as husband, was to sexually satisfy and impregnate his wife and to take care of her during her pregnancies, but under no circumstances should a man be the object of “exclusive emotional investment or focus of attention. Instead, women are socialized to invest their emotions and material wealth in their respective matrilineages.” (Saidi, 2010, p. 16).

This situation has changed since the colonial era, largely under the impact of Christianity and Islam. Efforts have been made to restructure the African family, specifically to make the marriage bond monogamous, exclusive, and longer-lasting and to increase paternal investment in offspring.

Interestingly, there is evidence that this trend actually began some two centuries before the colonial era. Previously, the African family had been even more matrilineal, matrilocal, and matriarchal:

Murdock proves beyond reasonable doubt that the most ancient form of unilineal descent among the Niger-Congo, and therefore among the Bantu-speaking peoples (whose languages belong to the Niger-Congo family), was matriliny. He specifically reconstructs this feature for the proto-Bantu. He demonstrates it through his mapping of the scattered, relict preservation of matriliny all across the Niger-Congo-speaking regions, even in the Kordofanian branch of the Niger-Congo languages, which is spoken far away from the rest of the family in Sudan.

[…] The oral traditions of the Kanyok, who belong to the Central Savanna Bantu subgroup, still remember their shift from matrilineal to patrilineal descent a number of centuries ago, as John Yoder reveals. Most telling of all, the founders of the clans of the Gikuyu of Kenya are female, even though today the Gikuyu are strongly patrilineal. By definition, matrilineal clans must have female founders; therefore, this is undisputed evidence for prior matriliny among the Bantu-speaking peoples of the eastern Kenya highlands.

[…] The historical priority of matriliny among the Niger-Congo peoples in general—as well as among peoples speaking languages of the Bantu subgroup of Niger-Congo—is extensively and convincingly demonstrated by the comparative ethnographic evidence. Recent work in linguistic reconstruction directly supports the view that the early Bantu communities, who established themselves successively more widely in the rainforest and then in the southern savannas and eastern Africa during the last three thousand years BCE, observed unilineal descent in the form of matrilineages and/or matriclans. (Saidi, 2010, pp. 13-14)

According to Saidi (2010, pp. 12-19), sub-Saharan Africa saw a relative shift of power from women to men from around 1500 onward—apparently as a result of warfare induced by the slave trade. On the one hand, war enhanced the prestige of men through the plundered wealth they brought to their communities. On the other, women looked to men for protection during times of war.

Was there previously, then, a golden age of non-oppression? And are African women now oppressed? In truth, words like ‘oppression’ are easily abused in a context where the goal of life is not self-maximization. Yes, African women produce far beyond their own needs, but the economic surplus goes primarily to their children. If anyone is getting something for nothing, it is surely the children.

But that’s not how Africans themselves see it—or for that matter humans in any traditional society. Children are literally seen as the ‘after-life.’ It’s not out of masochism that African mothers make sacrifices for them. It’s out of a profound belief that “no one gets out of here alive”—other than one’s children and their descendents.

Reference

Saidi, C. (2010). Women’s Authority and Society in Early East-Central Africa, University of Rochester Press.