Does paternal investment increase child IQ?

The latest issue of Evolution and Human Behavior has an article on paternal investment and IQ. Using a longitudinal dataset of children born in Britain in 1958, Nettle (2008) found a significant positive correlation between a child’s IQ at age 11 and the father’s degree of family involvement. The less a father cared for his offspring, the less intelligent they were. Nettle concluded that paternal investment affects childhood IQ.

Now, correlation is not causation. It can be shown, for instance, that Presbyterian ministers in Boston have earnings that significantly correlate over time with the price of rum in Havana. But that doesn’t mean they’ve been dabbling in the rum trade. It simply means there’s a common causal factor, in this case the North American business cycle.

Similarly, a common cause may explain the correlation between low investment by fathers and low intelligence in their children. Deadbeat dads tend to be more present-oriented and probably less intelligent. Since intelligence has a large heritable component, their children would be less intelligent on average.

In all fairness, the Nettle study did control for social class, which in turn partially controls for time preference (i.e., whether the fathers were present-oriented or future-oriented). Specifically, the fathers were coded in terms of five occupational categories: professional, managerial and technical, skilled, partly skilled, and unskilled. I doubt, though, that this factor would have accounted for most variability in time preference. Even within the British working class, there is considerable variation, notably by religion and ethnicity, in the respective weighting that people give to present impulses versus future obligations.

Reference

Nettle, D. (2008). Why do some dads get more involved than others? Evidence from a large British cohort. Evolution and Human Behavior, 29, 416-423.

Many genes and one g?

The October issue of Scientific American has an article on the search for genes that influence intelligence. Twin studies suggest that such genes exist, yet efforts to date have been disappointing for Robert Plomin, a behavioral geneticist at the Institute of Psychiatry in London.

Failing to find genes for intelligence has, in itself, been very instructive for Plomin. Twin studies continue to persuade him that the genes exist. “There is ultimately DNA variation responsible for it,” he says. But each of the variations detected so far only makes a tiny contribution to differences in intelligence. “I think nobody thought that the biggest effects would account for less than 1 percent,” Plomin points out.

That means that there must be hundreds—perhaps thousands—of genes that together produce the full range of gene-based variation in intelligence.

Variation in intelligence thus seems to be an accumulation of small effects from very many genes. For a long time, I had trouble reconciling this view with the concept of g—this single mental property, still unknown, that accounts for most variation in human intelligence. I now realize that this contradiction is more apparent than real. Whatever this single property might be, it doesn’t necessarily correspond to a single gene or even a single gene complex. It could be affected by an indefinite number of genes.

Reference

Zimmer, C. (2008). The search for intelligence. Scientific American, October, pp. 68-75.

Echoes of the Upper Paleolithic?

Among early modern humans, men faced less mate competition with increasing distance from the equator. They were proportionately fewer in number and fewer of them could afford a second wife. This was partly because hunting distances were correspondingly longer, so that more men died of hunting fatalities, and partly because longer winters made polygyny costlier. With fewer opportunities for food gathering, women were less self-reliant and depended more on men for food (Frost, 2006).

But what happened to those women who remained single? If they had no providers, wouldn’t they have died of starvation and wouldn’t these deaths have balanced out the operational sex ratio?

The short answer is that they remained with their parents and never had children. It is, above all, children who incur food-provisioning costs and make a male provider necessary. Nonetheless, it is interesting to speculate about these single women who must have been numerous among early Europeans, particularly in the continental Arctic of Ice Age Europe where opportunities for food gathering were few and far between.

If contemporary hunter-gatherers are a guide, such women become secondary caregivers by caring for younger siblings or aging parents. This spinsterhood is temporary, unless the woman suffers from a serious disability. How, then, would a hunter-gatherer society cope with large numbers of women who remain single? I addressed this question in my 1994 Human Evolution article.

Patterns of behaviour become stereotyped over time, with the result that their ritualized vestiges can persist much longer than the conditions that created them. A surplus of unattached females should be associated with a pattern of specialization in communal rather than family-oriented niches, e.g. shamanism, maintenance of base-camp dwellings, and tending of communal fires. Another pattern should be taboos that would have come to define this caste of unmarried women, e.g. virginity as a mark of caste membership, immunity from harm for fear of their shamanistic power.

Shamanism is strongly linked in early European traditions to women, especially virgins. This linkage is weaker in Siberian cultures, where female shamans predominate but are nonetheless married, and virtually unknown to the native peoples of North America, among whom most shamans are married men (Czaplicka, 1969: 243-255; Saladin d'Anglure, 1988; Hallowell, 1971: 19-22). The oldest sources from Greco-Roman, Germanic, and Slavic culture areas show an overwhelming preponderance of women among seers, witches, sibyls, oracles, and soothsayers (Baroja 1964: 24-57). Gimbutas (1982) and Dexter (1985) have argued that virgin females in early Europe were seen as "storehouses" of fertile energy and thereby possessed of extraordinary power. Thus, at the dawn of the Christian era the geographer Pomponius Mela mentioned nine virgin priestesses on an island off Brittany who knew the future and gave oracular responses to sailors who consulted them (Chadwick, 1966: 79). The first-century historian Cornelius Tacitus described a virgin prophetess among the Bructeri in present-day Germany, saying that this tribe "regards many women as endowed with prophetic powers and, as the superstition grows, attributes divinity to them" (Tacitus Histories 4:61). A similar caste of prophetesses, called dryades, existed among the Gauls (Chadwick, 1966: 80-81).

Single women also figured in what appear to be ritualized communal activities. The first-century geographer Strabo described a community of women who inhabited an island at the mouth of the Loire where "no man sets foot." (Geography 4.4.6) A sacred rite required them to unroof the temple and roof it again before sunset, a rite which Lefèvre (1900: 93) interpreted as recalling an age when women daily removed their hut's thatched roof to air the smoke-filled interior.

Another pattern links female virginity to the tending of communal fires. In both Roman and Greek mythology a virgin goddess, Vesta or Hestia, guards the communal hearth. The cult of Vesta required that the sacred fire of Rome be tended by a caste of virgin women — the Vestals. There is general agreement that this cult constituted an archaic element of Roman religion; the word Vesta, itself an archaism, appears to have come down unchanged from proto-Indo-European (6,000 B.P.), suggesting ritualization at an early date (Dumézil, 1970: 311-324). The idea that a celibate female must guard the hearth still survives in European folklore, the most familiar example being Cinderella — an unmarried woman whose name came from her having to sleep by the cinders of the fireplace.

Is there direct evidence of ‘excess’ women among Ice Age Europeans? We have a snapshot of one extended family from the Magdalenian period. The Maszycka Cave in Poland has yielded the remains of 3 men, 5 women and 8 children, all apparently from the same family and all apparently dying the same sudden death (Kozlowski & Sachse-Kozlowska, 1995).

References

Baroja, J.C. (1964). The World of Witches, trans. by N. Glendinning, Weidenfeld and Nicolson.

Chadwick, N.K. (1966). The Druids, University of Wales Press.

Czaplicka, M.A. (1969). Aboriginal Siberia. A Study in Social Anthropology, Clarendon.

Dexter, M.R. (1985). Indo-European reflection of virginity and autonomy. Mankind Quarterly, 26, 57-74.

Dumézil, G. (1970). Archaic Roman Religion, University of Chicago Press.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103

Frost, P. (1994). Geographic distribution of human skin colour: A selective compromise between natural selection and sexual selection? Human Evolution, 9, 141-153.

Gimbutas, M.A. (1982). The Goddesses and Gods of Old Europe, 6,500-3,500 BC, myths and cult images. University of California.

Hallowell, A.I. (1971). The Role of Conjuring in Saulteaux Society, Publications of the Philadelphia Anthropological Society, Vol. 2, Octagon.

Kozlowski, S.K., & Sachse‑Kozlowska, E. (1995). Magdalenian family from the Maszycka Cave. Jahrbuch der Römisch Germanischen Zentral Museums Mainz, 40, 115‑205. Jahrgang 1993, Mainz.

Lefèvre, A. (1900). Les Gaulois - origines et croyances, Librairie C. Reinwald.

Saladin d'Anglure, B. (1988). Penser le "féminin" chamanique. Recherches amérindiennes au Québec, 18(2-3), 19-50.

Strabo. (1923). The Geography of Strabo, Loeb Classical Library, William Heinemann.

Tacitus. (1969). The Histories, Loeb Classical Library, William Heinemann.

Vitamin D and skin color. Part II

Is white skin an adaptation to the cereal diet that Europeans have been consuming for the past five to seven thousand years?

When early Europeans switched from hunting and gathering to cereal agriculture, the new diet may have provided less vitamin D (i.e., from fatty fish), which the body needs to metabolize calcium and create strong bones. There would thus have been stronger selection for endogenous production of vitamin D in the skin’s tissues. Since such production requires UV-B light and since melanin blocks UV, this selection may have favored a lighter skin color (Sweet, 2002). In addition, cereals seem to increase vitamin D requirements by decreasing calcium absorption and by shortening the half-life of the main blood metabolite of vitamin D (Pettifor, 1994; see Paleodiet).

Undoubtedly, lighter skin allows more UV-B into the skin. As Robins (1991, pp. 60-61) notes, black African skin transmits three to five times less UV than does European skin. But is this a serious constraint on vitamin D production? Apparently not. Blood metabolites of vitamin D show similar increases in Asian, Caucasoid, and Negroid subjects when their skin is either artificially irradiated with UV-B or exposed to natural sunlight from March to October in Birmingham, England (Brazerol et al., 1988; Ellis et al., 1977; Lo et al., 1986; Stamp, 1975; also see discussion in Robins, 1991, pp. 204-205).

The vitamin D hypothesis also implies that European skin turned white almost at the dawn of human history. Cereal agriculture did not reach northern Europe until some 5,000 years ago and, presumably, the whitening of northern European skin would not have been complete until well into the historical period. Is this a realistic assumption, given the depictions of white-skinned Europeans in early Egyptian art?

References

Brazerol, W.F., McPhee, A.J., Mimouni, F., Specker, B.L., & Tsang, R.C. (1988). Serial ultraviolet B exposure and serum 25 hydroxyvitamin D response in young adult American blacks and whites: no racial differences. Journal of the American College of Nutrition, 7, 111-118.

Ellis, G., Woodhead, J.S., & Cooke, W.T. (1977). Serum-25-hydroxyvitamin-D concentrations in adolescent boys. Lancet, 1, 825-828.

Lo, C.W., Paris, P.W., & Holick, M.F. (1986). Indian and Pakistani immigrants have the capacity as Caucasians to produce vitamin D in response to ultraviolet radiation. American Journal of Clinical Nutrition, 44, 683-685.

Pettifor, J.M. (1994). Privational rickets: a modern perspective. Journal of the Royal Society of Medicine, 87, 723-725.

Robins, A.H. (1991). Biological perspectives on human pigmentation. Cambridge Studies in Biological Anthropology, Cambridge: Cambridge University Press.

Stamp, T.C. (1975). Factors in human vitamin D nutrition and in the production and cure of classical rickets. Proceedings of the Nutrition Society, 34, 119-130.

Sweet, F.W. (2002). The paleo-etiology of human skin tone. Backintyme Essays.

Skin color and vitamin D

Differences in human skin color are commonly explained as an adaptive response to solar UV radiation and latitude. The further away from the equator you are, the weaker will be solar UV and the less your skin will need melanin to prevent sunburn and skin cancer.

A variant of this explanation involves vitamin D, which the body needs to make strong bones and which the skin produces with the help of UV-B. The further away from the equator you are, the lighter your skin will be to let enough UV-B into its tissues for vitamin D production. Or so the explanation goes.

To test this hypothesis, Osborne et al. (2008) measured skin color and bone strength in a hundred white and Asian adolescent girls from Hawaii. Skin color was measured at the forehead and the inner arm. Bone strength was measured by section modulus (Z) and bone mineral content (BMC) at the proximal femur. A multiple regression was then performed to investigate the influences of skin color, physical activity, age, ethnicity, developmental age, calcium intake, and lean body mass on Z and BMC. Result: no significant relationship between skin color and bone strength.

Is there, in fact, any hard evidence that humans vary in skin color because they need to maintain the same level of vitamin D production in the face of varying levels of UV-B? Robins (1991, pp. 204-205) found the data to be unconvincing when he reviewed the literature. In particular, there seems to be little relationship between skin color and blood levels of 25-OHD—one of the main circulating metabolites of vitamin D:

The vulnerability of British Asians to rickets and osteomalacia has been ascribed in part to their darker skin colour, but this idea is not upheld by observations that British residents of West Indian (Afro-Caribbean) origin, who have deeper skin pigmentation than the Asians, very rarely manifest clinical rickets … Moreover, artificial irradiation of Asian, Caucasoid and Negroid subjects with UV-B produced similar increases in blood 25-OHD levels irrespective of skin pigmentation … A study under natural conditions in Birmingham, England, revealed comparable increases in 25-OHD levels after the summer sunshine from March to October in groups of Asians, West Indians and Caucasoids … This absence of a blunted 25-OHD response to sunlight in the dark-skinned West Indians at high northerly latitudes (England lies farther north than the entire United States of America except for Alaska) proves that skin colour is not a major contributor to vitamin D deficiency in northern climes.

The higher incidence of rickets in British Asians probably has less to do with their dark color than with their systematic avoidance of sunlight (to remain as light-skinned as possible).

Skin color and natural selection via solar UV

Solar UV seems to be a weak agent of natural selection, be it through sunburn, skin cancer, or vitamin D deficiency. Brace et al. (1999) studied skin color variation in Amerindians, who have inhabited their continents for 12,000-15,000 years, and in Australian Aborigines, who have inhabited theirs for some 50,000 years. Assuming that latitudinal skin-color variation in both groups tracks natural selection by solar UV, their calculations show that this selection would have taken over 100,000 years to create the skin-color difference between black Africans and northern Chinese and ~ 200,000 years to create the one between black Africans and northern Europeans (Brace et al., 1999). Yet modern humans began to spread out of Africa only about 50,000 years ago. Clearly, something other than solar UV has also influenced human variation in skin color ... and one may wonder whether lack of solar UV has played any role, via natural selection, in the extreme whitening of some human populations.

Indeed, people seem to do just fine with a light brown color from the Arctic Circle to the equator. Skeletal remains from pre-contact Amerindian sites show little evidence of rickets or other signs of vitamin D deficiency—even at latitudes where Amerindian skin is much darker than European skin (Robins, 1991, p. 206).

Why, then, are Europeans so fair-skinned when ground-level UV radiation is equally weak across Europe, northern Asia, and North America at all latitudes above 47º N? (Jablonski & Chaplin, 2000). Proponents of the vitamin D hypothesis will point to the Inuit and say that non-Europeans get enough vitamin D at high northerly latitudes from fatty fish. So they don’t need light skin. In actual fact, if we look at the indigenous peoples of northern Asia and North America above 47º N, most of them live far inland and get little vitamin D from their diet. For instance, although the Athapaskans of Canada and Alaska live as far north as the Inuit and are even somewhat darker-skinned, their diet consists largely of meat from land animals (caribou, deer, ptarmigan, etc.). The same may be said for the native peoples of Siberia.

Conversely, fish consumption is high among the coastal peoples of northwestern Europe. Skeletal remains of Danes living 6,000-7,000 years ago have the same carbon isotope profile as those of Greenland Inuit, whose diet is 70-95% of marine origin (Tauber, 1981). So why are Danes so light-skinned despite a diet that has long included fatty fish?

Skin color and sexual selection via male choice

Latitudinal variation in human skin color is largely an artefact of very dark skin in sub-Saharan agricultural peoples and very light skin in northern and eastern Europeans. Elsewhere, the correlation with latitude is much weaker. Indeed, human skin color seems to be more highly correlated with the incidence of polygyny than with latitude (Manning et al., 2004).

This second correlation is especially evident in sub-Saharan Africa, where high-polygyny agriculturalists are visibly darker than low-polygyny hunter-gatherers (i.e., Khoisans, pygmies) although both are equally indigenous. Year-round agriculture allows women to become primary food producers, thereby freeing men to take more wives. Thus, fewer women remain unmated and men are less able to translate their mate-choice criteria into actual mate choice. Such criteria include a preference, widely attested in the African ethnographic literature, for so-called 'red' or 'yellow' women — this being part of a general cross-cultural preference for lighter-skinned women (van den Berghe & Frost, 1986). Less mate choice means weaker sexual selection for light skin in women and, hence, less counterbalancing of natural selection for dark skin in either sex to protect against sunburn and skin cancer. Result: a net increase in selection for dark skin.

Just as weaker sexual selection may explain the unusually dark skin of sub-Saharan agricultural peoples, stronger sexual selection may explain the unusually light skin of northern and eastern Europeans, as well as other highly visible color traits.

Among early modern humans, sexual selection of women varied in intensity along a north-south axis. First, the incidence of polygyny decreased with distance from the equator. The longer the winter, the more it cost a man to provision a second wife and her children, since women could not gather food in winter. Second, the male death rate increased with distance from the equator. Because the land could not support as many game animals per unit of land area, hunting distance increased proportionately and hunters more often encountered mishaps (drowning, falls, cold exposure, etc.) or ran out of food, especially if other food sources were scarce.

Sexual selection of women was strongest where the ratio of unmated women to unmated men was highest. This would have been in the ‘continental Arctic’, a steppe-tundra environment where women depended the most on men for food and where hunting distances were the longest (i.e., long-distance hunting of highly mobile herds with no alternate food sources). Today, this environment is confined to the northern fringes of Eurasia and North America. As late as 10,000 years ago, it reached much further south. This was particularly so in Europe, where the Scandinavian icecap had pushed the continental Arctic down to the plains of northern and eastern Europe (Frost, 2006).

The same area now corresponds to a zone where skin is almost at the physiological limit of depigmentation and where hair and eye color have diversified into a broad palette of vivid hues. This ‘European exception’ constitutes a major deviation from geographic variation in hair, eye, and skin color (Cavalli-Sforza et al., 1994, pp. 266-267).

References

Brace, C.L., Henneberg, M., & Relethford, J.H. (1999). Skin color as an index of timing in human evolution. American Journal of Physical Anthropology, 108 (supp. 28), 95-96.

Cavalli-Sforza, L.L., Menozzi, P., & Piazza, A. (1994). The History and Geography of Human Genes. Princeton: Princeton University Press.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103

Jablonski, N.G. & G. Chaplin. (2000). The evolution of human skin coloration, Journal of Human Evolution, 39, 57-106.

Manning, J.T., Bundred, P.E., & Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior, 25, 38-50.

Osborne, D.L., C.M. Weaver, L.D. McCabe, G.M. McCabe, R. Novotony, C. Boushey, & D.A. Savaiano. (2008). Assessing the relationship between skin pigmentation and measures of bone strength in adolescent females living in Hawaii. American Journal of Physical Anthropology, 135(S46), 167.

Robins, A.H. (1991). Biological perspectives on human pigmentation. Cambridge Studies in Biological Anthropology, Cambridge: Cambridge University Press.

Tauber, H. (1981). 13C evidence for dietary habits of prehistoric man in Denmark. Nature, 292, 332-333.

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

More on gene-culture co-evolution

According to the online magazine Seed, “a growing number of scientists argue that human culture itself has become the foremost agent of biological change.” Much of this change has been surprisingly recent:

In the DNA of a group of 5,000-year-old skeletons from Germany, they discovered no trace of the lactase allele, even though it had originated a good 3,000 years beforehand. Similar tests done on 3,000-year-old skeletons from Ukraine showed a 30 percent frequency of the allele. In the modern populations of both locales, the frequency is around 90 percent.

I thought I was on top of the literature, but this was new to me. It’s even more proof that human evolution did not stop with the advent of Homo sapiens. It has continued … even after the transition from prehistory to history!

The same article also has some thoughts from Bruce Lahn, the evolutionary geneticist who has mapped human variation at two genes, ASPM and microcephalin, that seem to regulate the growth of brain tissue.

Even if Lahn could prove to everyone's satisfaction that ASPM and microcephalin are under selection, whether intelligence is the trait being selected for would be far from a settled question. It could be, as Lahn suggested, that some other mental trait is being selected, or that the activity of ASPM and microcephalin in other parts of the body is what is under selection. More work will certainly be done. But one can speculate with far more confidence about what drove the dramatic increase in intelligence attested by the fossil record: the advent of human culture.

"Intelligence builds on top of intelligence," says Lahn. "[Culture] creates a stringent selection regime for enhanced intelligence. This is a positive feedback loop, I would think." Increasing intelligence increases the complexity of culture, which pressures intelligence levels to rise, which creates a more complex culture, and so on. Culture is not an escape from conditioning environments. It is an environment of a different kind.

Reference

Phelan, B. (2008). How we evolve. A growing number of scientists argue that human culture itself has become the foremost agent of biological change. Seed Posted October 7, 2008

Polygyny or patrilocality?

Have all humans been more or less equally polygynous? The answer seems to be yes if we believe a team of researchers from the University of Arizona. They found that genetic diversity is higher on the maternally inherited X chromosome than on chromosomes inherited by both sexes (autosomes) in samples from five different populations: Biaka (Central African Republic), Mandenka (Senegal), San (Namibia), Basques (France), Han (China), and Melanesians (Papua New Guinea). Their conclusion: “our results point to a systematic difference between the sexes in the variance in reproductive success; namely, the widespread effects of polygyny in human populations.” In other words, proportionately more women than men have been contributing to the gene pool (Hammer et al., 2008).

It’s no surprise that polygyny has existed in the five populations under study. Almost all human populations are polygynous to some degree. The surprise is the relative lack of difference between the European and African subjects. Indeed, according to this study, the Basques have been more polygynous than the Mandenka have been. This is truly counterintuitive. Among the Basques, polygyny is normally limited to its serial form (marriage to a second wife upon the death of the first), as well as occasional cuckoldry. Among the Mandenka, polygyny is the preferred marriage type.

For some people in the blogosphere, this is simply scientific truth and we just have to accept it, however counterintuitive it may seem. There is nonetheless an alternate explanation: patrilocality. In many societies, a wife goes to live in her husband’s community after marriage. This has the effect of inflating the genetic diversity of women in any one community.

These two confounding levels of explanation, polygyny and patrilocality, bedeviled the previous methodology of comparing maternally inherited mtDNA with the paternally inherited Y chromosome. With the new methodology, patrilocality biases the results even more because the Y chromosome is no longer a point of reference.

The University of Arizona researchers do not mention patrilocality in their paper although they do discuss ‘sex-biased forces.’ Under this heading, they tested a model where only females migrate between communities (‘demes’) and at such a rate that panmixia eventually results. They concluded that this factor could not be significant. To my mind, the model is unrealistic, partly because the assumed migration rate is far too high and partly because two demes are used to represent a real world where brides are exchanged among many communities separated by varying genetic distances. To be specific, the more genetically different a bride is from her host community, the further away will be her community of origin, and the lower will be the probability of panmixia between the two.

To the extent that the methodology is biased toward patrilocality effects, any polygyny effects will be less apparent. If this new methodology primarily tracks differences in patrilocality, no major differences would be observable among the different population samples.

In addition, there may be a weak inverse relationship between patrilocality and polygyny. Patrilocality correlates with patriarchy, which correlates with high paternal investment, which inversely correlates with polygyny. If so, the two effects – polygyny and patrilocality – would tend to cancel each other out in the data.

Finally, the burden of proof is on those who propose new methodologies, especially one that produces inconsistent results. The University of Arizona researchers themselves say as much: “Our findings of high levels of diversity on the X chromosome relative to the autosomes are in marked contrast to results of previous studies in a wide range of species including humans.” More importantly, their findings run counter to the comparative literature on human mating systems. To cite only one authority, Pebley and Mbugua (1989) note:

In non-African societies in which polygyny is, or was, socially permissible, only a relatively small fraction of the population is in polygynous marriages. Chamie's (1986) analysis of data for Arab Muslim countries between the 1950s and 1980s shows that only 5 to 12 percent of men in these countries have more than one wife. … Smith and Kunz (1976) report that less than 10 percent of nineteenth-century American Mormon husbands were polygynists. By contrast, throughout most of southern West Africa and western Central Africa, as many as 20 to 50 percent of married men have more than one wife … 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.

References

Hammer, M.F., Mendez, F.L., Cox, M.P., Woerner, A.E., & Wall, J.D. (2008). Sex-biased evolutionary forces shape genomic patterns of human diversity. PLoS Genet, 4(9), e1000202. doi:10.1371/journal.pgen.1000202

Pebley, A. R., & Mbugua, W. (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.