Thursday, November 27, 2008

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.


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.

Wednesday, November 19, 2008

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.


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

Tuesday, November 11, 2008

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).


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.

Tuesday, November 4, 2008

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?


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.