How Heritable is Intelligence?

The good old Sarrazin debate taking place in my room this morning made me search for some scientific publications on the issue myself. I found one highly cited paper that seems to lay out the situation quite clearly:

Studies of correlations among twins or adoptees and their biological and adoptive parents typically yield large genetic effects and relatively smaller effects of family environment, whereas studies that compare the mean IQs of children rescued from poverty with the IQs of their parents or impoverished siblings often find large differences that are attributed to the environment (Turkheimer, 1991).

So what Sarrazin quotes in terms of high heritability is only half of the truth. How can these disparate findings be unified? Here is the hypothesis that was confirmed in the study:

One possible resolution of this paradox is that the effect of family environment on cognitive ability could be nonlinear (Jensen, 1981; Scarr, 1981). If differences between impoverished environments and adequate ones have large consequences for cognitive ability, but differences between adequate and enriched environments do not, one would expect amelioration of impoverished status to show a substantial effect, whereas correlational findings based on middle-class family members in typical twin and adoption studies would not.

In a way (as has been rightfully pointed out for the debate as a whole) this is just elaborating on what has been said on differences in IQ between groups before: A certain degree of heritability within a group says close to nothing about the genetic differences between groups. The quoted article just shows how this applies to socioeconomic groups as well es ethnic groups, which had been a central issue in the similar debate in the US some 15 years ago.

Which makes Sarrazins point (Germany is threatened by an increase in proportion of genetically less intelligent individuals due to different rates of reproduction between socio-economic groups) scientifically implausible on top of what it has been from the beginning: politically and morally tasteless and neglecting of the historic dimension and practical implications of trying to change the genetic composition of society.

Apart from that, I think it is useful to point out an old misunderstanding with the theory of evolution: One naively tends to conclude that evolution must favor intelligence, and that the „natural“ way of things should be for intelligent people to procreate more. This would make the (assumed) opposite direction of „evolutionary shift“ due to the way our society is structured „unnatural“ and imply that it is wrong (which is itself a moral-philosophic misunderstanding called the Naturalistic Fallacy). However, the fact that there still are genetically more and less intelligent (one should say: predisposed to developing intelligence) people around proves that „evolution“ has nothing against them. If one wanted to speculate wildly and freely one could say that „evolution likes them“ precisely for the fact that they procreate more… A remotely similar point has recently been made about sheep with stronger and weaker immune systems and, some years ago, about learning ability in fruit flies.

I have strayed from the topic, but I’m glad I had an opportunity to link to that sheep article in a useful way after all ;)

Datum: Sonntag, 31. Oktober 2010 15:38
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