December 22, 2005

Group differences, pathogens, & evolutionary psychology

A new paper (pdf) by Wang et al. shows that selection in homo sapiens has continued even after the Out-of-Africa expansions which resulted in racial diversification. The researchers have found regional selection for about 1,800 genes, the bulk of which fall into the following categories: those having to do with pathogen response, cell cycle, neuronal function, reproduction, DNA metabolism, and protein metabolism. See the entries and comments at GNXP, Dienekes, John Hawks, and iSteve for more detail.

I'll emphasize a point Steve Sailer raised, which is that this is contra the assumption in Evolutionary Psychology (TM) that selection worked its stuff while our species was located in Africa, and more or less froze us in that state before various groups left, since natural selection hasn't had "enough time" to work on our brain within the ~50,000 years since. Most cultural differences are assumed to reflect differences in the local ecology -- that is, we're all born with mostly the same cognitive architecture, but depending on how this interacts during our lifetime with salient features of the environment, some groups could show a psychological or behavioral difference from others. For example, many in the modernized West take our personal safety largely for granted because we have laws, police forces, and courts. Were we to be thrown into a hunter-gatherer society, we would take nothing for granted about our safety and would show roughly the same level of violence as the members of our adoptive society, as we learned what worked to keep us alive. In sum, the same human mind shared by all humans reacts in different ways depending on the environment it finds itself in.

However, as is the problem with Jared Diamond's Guns, Germs, and Steel, the more one emphasizes long-lasting differences in the local ecology, the more they are implicitly arguing that the populations occupying these areas over time face different selection pressures, which would tend to introduce a greater genetic component into explaining why different populations differ, as they became adapted to their region. This is uncontroversial when it comes to the most obvious racial difference: skin color. Whatever the particular genes turn out to be, it's clear that variation in skin color across populations has a large genetic component -- a sub-Saharan African baby born and raised in Finland will still have brownish skin, while a Finn raised in Ughanda will still have whitish skin. While exposure to lots of sun may make a white person a bit more tan than they would have been otherwise, this trait seems to not respond strongly to the here-and-now environment in which the individual finds himself (i.e., strongly enough to make a person dark brown vs pale white). No one alternatively proposes a Super Suntanning Model -- SSM -- whereby our innate endowment has been naturally selected to come in some common default color until it interacts with some environmental feature such as sun exposure (or lack thereof). Note that this SSM is not the blank slate theory -- according to the SSM, the endowment is rich and innate (contra blank slate) but common to all normal humans.

When it comes to the brain, however, the psychological / behavioral equivalent of the SSM is assumed to account for population differences. However, way back in 1993 Steven Gangestad and David Buss published an article, "Pathogen prevalence and human mate preferences" (Ethology and Sociobiology, 14, 89-96), wherein they described how the degree to which nasty pathogens were present in the local ecology strongly correlated with the degree to which the people of that region placed an emphasis on physical attractiveness when selecting a mate, as documented by Buss' seminal 1989 cross-cultural survey of human mate preferences (Ctrl+F BBS). I read both articles on Buss' website as recently as June of 2005, but since then the pathogen pdf has been removed for some reason, and I cannot locate it anywhere else on the web. I have no access to online journals, nor am I about to lay down $30 to read it at sciencedirect. Here's the gist, though. Buss had already examined how much emphasis people in various cultures place on good looks when choosing a mate; each subject rated this from 0 to 3, with 3 being most important, and he took the mean of all subjects in a given culture. G&B then examined the correlation of this average with the prevalence of 6 or so nasty pathogens in the geographical areas studied in mate preferences study -- by nasty I mean something like malaria, whose post-Out-of-Africa pressure has resulted in vast group differences in sickle-cell anemia. The hypothesis was that those in more pathogen-ridden areas would place more emphasis on good looks since they are a reliable cue for good health & immune system, which are more crucial for survival in an area teeming with microscopic bugs ready to eat you up. Here's Buss' summary from another paper which is available online (under 2001, "Human Nature and Culture..."):

G&B "found that cultural variation in the prevalence of pathogens was correlated +.71 with the average cultural importance placed on physical attractiveness in a potential mate, accounting for a virtually unprecedented 50% of the cultural variation (Gangestad & Buss, 1993). Assuming further tests confirm this hypothesis, cultural variation in a psychological variable, in this example, can be traced, in part, to variation in an important hazard of the local ecology" (p. 969; my emphasis).

Aha! So despite the wording here about local hazards and the abstract's claim that "the [universal] human mind contains many complex psychological mechanisms that are selectively activated, depending on cultural contexts" (my emphasis) -- this is actually an instance of the Baldwin Effect, whereby learned behaviors become progressively more "hard-wired," even if not completely so, in order to increase fitness. But as anticipated by Darwin the Second (aka William Hamilton), and as confirmed now by the Wang et al paper, nasty pathogens play a central role in the directions natural selection takes in human beings. Combine this with the great variation in pathogen prevalence between geographical regions plus the Baldwin Effect, and you get psychological group differences with substantial genetic components. As I recall from the G&B paper, the Finns and Zulus face a milder pathogen army and on average don't care much about how their partner looks, whereas the Nigerians and Bulgarians are the reverse. A future test of how hard-wired these psychological differences are would be to survey members of an ethnic group whose ancestral region is high (or low) in pathogens but who were raised in a region with a low (or high) level of pathogens. Would their preferences cluster more closely around the population average of their ancestral or "adoptive" region? Also, what other psychological group differences might trace back partly to the selective pressure of pathogens?

Typo: Buss' 1989 survey used a scale from 0 to 3, not 0 to 4 as I had at first. No change in meaning, though.


  1. Kanazawa and Kozar published in Intelligence 7-04 an account of "Why Beautiful People are More Intelligent". They report a significant positive correlation, from several studies. If the beauty preference runs systematically higher towards the equatorial strongholds of pathogen intensity, and IQ rises on average with distance therefrom, in terms of ancestral adaptations; these effects run contrary to each other.

  2. the buss paper is on site again....

  3. Gangestad/Buss:

    related: googling I found this:
    Pathogen prevalence predicts human cross-cultural variability in individualism/collectivism
    Corey L Fincher,1* Randy Thornhill,1 Damian R Murray,2 and Mark Schaller2


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