Female brains and male skin tone

It is well known that women feel attracted to certain visual, auditory, and olfactory characteristics of men. This attraction seems to be hormonally regulated, as suggested by psychosexual studies of women at different phases of the menstrual cycle. In general, women are more strongly attracted to male characteristics during the estrogen-dominant phase of their cycle (first two-thirds) than during the progesterone-dominant phase (last third).

This cyclical change has been most recently shown by Rupp et al. (2009). MRI scans were used to measure how female brains process pictures of male faces that morphing software had either masculinized or feminized. The subjects were tested on days 10-12 and days 19-23 of the menstrual cycle. Measurements were taken of their levels of estradiol, progesterone, free testosterone, and total testosterone. The subjects also filled out questionnaires about their psychosexual profile (propensity for short-term sexual encounters, for sexual excitation, for sexual inhibition, etc.).

During the first time window (days 10-12), five brain regions showed a stronger neural response to masculinized faces than to feminized faces. No brain region showed the reverse pattern. During the second time window (days 19-23), only one region responded more to masculinized faces than to feminized faces. For both windows, some regions showed significant correlations between neural activation and hormone level. Estradiol correlated positively with neural activation whereas progesterone correlated negatively. The correlations were negative or positive for free and total testosterone. In some brain regions, neural activation also correlated with psychosexual variables.

What was driving these neural responses? What facial feature was turning these women on? In response to an e-mail, the lead author, Heather Rupp, told me that the morphing software had varied the shape of the male faces and their skin tone. The masculinized faces were darker-skinned and the feminized faces lighter-skinned. Her results may thus dovetail with my own findings on female preferences with regard to male skin tone, i.e., my subjects more strongly preferred darker male faces during the estrogen-dominant phase of their menstrual cycle than during the progesterone-dominant phase (Frost 1994).

References

Frost P. (1994). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship, Perceptual and Motor Skills, 79, 507-514.

Rupp, H.A., T.W. James, E.D. Ketterson, D.R. Sengelaub, E. Janssen, and J.R. Heiman. (2009). Neural activation in women in response to masculinized male faces: mediation by hormones and psychosexual factors. Evolution and Human Behavior, 30, 1-10.

Blond Inuit?

One of the mysteries of anthropology is the reported presence of ‘blond’ Inuit in the western Canadian Arctic, specifically on and around Victoria Island. They were first noticed by the explorer Sir John Franklin and by Alaskan whalers.

The travellers who have been most surprised by the appearance of the Victoria Island Eskimos are those who are most used to Eskimos of the regular or “pure” type. Such a party were Klinkenberg’s white and (Alaskan) Eskimo crew of the American whaling ship Olga … and it was through her captain and crew that I first half-realized that the people of western Victoria Island were conspicuously different from other Eskimos. … What half-convinced me that he [the captain] was right was the emphatic corroboration of the Alaska Eskimo members of Klinkenberg’s crew, who said that the Victoria Islanders were in appearance like a group of half-castes, although they were wholly Eskimo in language and customs. (Stefansson, 1908, pp. 375-376)

These impressions were confirmed by the anthropologist Vilhjalmur Stefansson for Victoria Island and by the anthropologist Knud Rasmussen for adjacent islands:

In reality these blond types are not peculiar to Victoria Island. In King William Island and on Back River, as well as on Kent Peninsula, I found types which had exactly the same outward characteristics, the same light complexion, the same reddish or brownish hair, the gray and even nearly blue eyes, and remarkably abundant beards—something which is elsewhere uncommon among Eskimos.
(Stefansson, 1908, p. 379)

Stefansson advanced four possible explanations: a) recent European intermixture with whalers or fur traders; b) ancient European intermixture with the Greenland Norse; c) ancient migration of a fair-haired Eurasian people from across the Bering Strait; and d) independent mutation. He was skeptical about the first two explanations:

… no whaler or other person familiar with it has ever suggested that any whaler came in contact with the Victoria Islanders before Captain Klinkenberg in 1905.

The only tenable hypothesis in connection with whaling is that the European blood may have come from the east side of the continent through the Americans, Scotch, and others who have engaged in Hudson Bay and Baffin Bay whaling for centuries. Here we are dealing with no impossibility any more than we were in the case of the earlier and more numerous Greenland Norsemen. But if the mixing of races is so recent, it would appear that it should be most conspicuous farther east where the whalers had their headquarters, fading away as one goes westward. The opposite is the case.
(Stefansson, 1908, pp. 377-378)

Stefansson opted for the last explanation as the most probable: “It is possible that for some so-called accidental reason blond individuals may have been born from time to time in the past from parents of pure Eskimo blood, and that these may have perpetuated themselves” (Stefansson, 1908, p. 381).

Some new light has been shed by a team of researchers headed by Palsson (2008). They collected genetic data from 299 Inuit on Victoria Island and at adjacent locations, as well as from other Inuit or Inuit-related groups (Greenland Inuit, Chukchi, Siberian Yupiit, and Alaskan Aleut). No evidence of European admixture is apparent in the Victoria Island Inuit with respect to either maternally or paternally inherited lineages. But there is evidence of maternal lineages from a pre-Inuit source, possibly the Dorset people who inhabited the Canadian Arctic a thousand years ago.

What conclusion should we draw? Palsson (2008) concluded that the existence of blondism among the Victoria Island Inuit had been blown out of all proportion. Alternately, it may be that new alleles for hair and eye color arose independently through mutation, with some kind of selection pressure favoring these color traits over the original black hair and brown eyes. Finally, it may be that a pre-Inuit population of Eurasian origin, perhaps the Dorset or even the earlier Paleoeskimos, had a significant incidence of fair hair and fair eyes. Such traits might then have persisted through admixture in this Inuit group. The last possibility might not be so far-fetched, since fair hair has been reported among the Yukaghir of eastern Siberia (von Hellwald, 1882).

References

Palsson, G. (2008). Genomic anthropology. Coming in from the cold? Current Anthropology, 49, 545-568.

Stefansson, V. (1927). My Life with the Eskimos. New York: The MacMillan Co.

von Hellwald, F. (1882). Völkerkunde, Nurnberg.

Polygyny and X-chromosome diversity

Last year, a team of University of Arizona researchers found evidence of widespread polygyny in five different human populations: Biaka (Central African Republic), Mandenka (Senegal), San (Namibia), Basques (France), Han (China), and Melanesians (Papua New Guinea). In short, the maternally inherited X chromosome was genetically more diverse than the chromosomes inherited by both sexes (autosomes) (Hammer et al., 2008). So more women than men seem to have contributed to the gene pool. Surprisingly, there was little difference in this respect between the Mandenka (known to be highly polygynous) and the Basques and the Han (among whom the incidence of polygyny is much lower).

I was frankly skeptical. For one thing, maternally inherited genetic diversity reflects not only the number of women who contribute to the gene pool but also their own genetic diversity. If these women are drawn from a larger geographic area than the men are, the female gene pool will be more genetically diverse than the male gene pool. This is often the case. In a patriarchal society, land ownership is vested in the man’s lineage, so women are usually the ones who move to their mate’s community when they get married. We see this ‘patrilocality’ even in societies where land ownership is matrilineal. Among the Iroquois, wives were often abducted from other tribes through warfare.

In any case, the above findings have now been challenged. Another study has found much less maternally inherited genetic diversity in East Asians and Europeans than in West Africans (Keinan et al., 2008).

So what gives? The methodology is similar in both studies. John Hawks points out that the second study scales X-chromosome diversity to the human-macaque divergence whereas the first study uses the human-orangutan divergence. While this might explain differences in calculation of mutation rate and hence X-chromosome diversity, I don’t see how it could explain why one study found geographic differences (i.e., African versus non-African) and the other did not.

I suspect that the key difference is that the first study just did not have enough resolution to pick out these geographic differences, i.e., its dataset was too small. The second study used 130,000 loci (SNPs) whereas the first one used 40.


Please check out my latest article: “Sexual selection and human geographic variation” in The Journal of Social, Evolutionary & Cultural Psychology.

References

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.
http://www.jsecjournal.com/articles/volume2/issue4/NEEPSfrost.pdf

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

Keinan, A., Mullikin, J.C., Patterson, N., & Reich, D. (2008). Accelerated genetic drift on chromosome X during the human dispersal out of Africa. Nature Genetics, early view December 2008; doi:10.1038/ng.303

Mental traits and human variation

An interesting article has come out in Edge by Jonathan Haidt, a University of Virginia psychologist and author of The Happiness Hypothesis. Essentially, he argues that science will soon embrace the idea that human nature evolved not only in the hunter-gatherer environments of the Pleistocene but also in the diverse social environments of the past 10,000 years.

Russian scientists showed in the 1990s that a strong selection pressure (picking out and breeding only the tamest fox pups in each generation) created what was — in behavior as well as body — essentially a new species in just 30 generations. That would correspond to about 750 years for humans. Humans may never have experienced such a strong selection pressure for such a long period, but they surely experienced many weaker selection pressures that lasted far longer, and for which some heritable personality traits were more adaptive than others. It stands to reason that local populations (not continent-wide "races") adapted to local circumstances by a process known as "co-evolution" in which genes and cultural elements change over time and mutually influence each other.

… No new mental modules can be created from scratch in a few millennia, but slight tweaks to existing mechanisms can happen quickly, and small genetic changes can have big behavioral effects, as with those Russian foxes. We must therefore begin looking beyond the Pleistocene and turn our attention to the Holocene era as well - the last 10,000 years. This was the period after the spread of agriculture during which the pace of genetic change sped up in response to the enormous increase in the variety of ways that humans earned their living, formed larger coalitions, fought wars, and competed for resources and mates.

… traits that led to Darwinian success in one of the many new niches and occupations of Holocene life — traits such as collectivism, clannishness, aggressiveness, docility, or the ability to delay gratification — are often seen as virtues or vices. Virtues are acquired slowly, by practice within a cultural context, but the discovery that there might be ethnically-linked genetic variations in the ease with which people can acquire specific virtues is — and this is my prediction — going to be a "game changing" scientific event.

Haidt predicts that the paradigm shift will occur as genome research uncovers evidence that many mental traits differ among human populations. These findings will be unexpected and disturbing to some. “Expectations, after all, are not based purely on current evidence; they are biased, even if only slightly, by the gut feelings of the researchers, and those gut feelings include disgust toward racism.” In making this prediction, Haidt feels some foreboding:

I believe that the "Bell Curve" wars of the 1990s, over race differences in intelligence, will seem genteel and short-lived compared to the coming arguments over ethnic differences in moralized traits. I predict that this "war" will break out between 2012 and 2017.