The Heritability and Malleability of IQ

Cosma returns to the topic (definitely worth reading):

People seem to be experiencing more than the usual difficulty grasping what I was getting at in my posts on accent and intelligence. This is my fault, for trying to be cute rather than trying to be clear. (I realize I'm too murky even when I am trying to be clear.) I am already heartily sick of the subject, which is turning into the huge time-suck I was afraid it would be, and which presents a depressing prospect from every point of view, not least those which make it clear how rare it is for anyone to change their mind on any aspect of it for any cause at all. (I do wonder if I should've stuck with the original title of "Duet for Leo and Razib.") My aim here is to lay everything out cleanly and explicitly, and be done with this matter.

I was originally going to do just one post, explaining why I called the general factor of intelligence a "statistical myth", why I don't put any real faith in what I regard as even the best of the current estimates of IQ's heritability, and the evidence for IQ's malleability. But the thing grew unwieldy, and the only thing which I find more dreary, right now, than discussing heritability and malleability is explaining why factor analysis can't do what people want it to, so I'll save that for later, and stick to the heritability and plasticity of IQ here. Whether IQ means anything or not, it is, unlike general intelligence, unquestionably something we can measure, so we can consider how heritable and malleable it is. I am going to assume that you know what "variance" and "correlation" are, but not too much else.

To summarize: Heritability is a technical measure of how much of the variance in a quantitative trait (such as IQ) is associated with genetic differences, in a population with a certain distribution of genotypes and environments. Under some very strong simplifying assumptions, quantitative geneticists use it to calculate the changes to be expected from artificial or natural selection in a statistically steady environment. It says nothing about how much the over-all level of the trait is under genetic control, and it says nothing about how much the trait can change under environmental interventions. If, despite this, one does want to find out the heritability of IQ for some human population, the fact that the simplifying assumptions I mentioned are clearly false in this case means that existing estimates are unreliable, and probably too high, maybe much too high.