Kevin Mitchell wrote an essay this summer (“The genetics of stupidity”) that got some attention. The idea is that most or all of the genetic influence affecting intelligence is genetic load. Although each deleterious mutation is very rare, the average individual carries many of them – and some people carry more than others. People with more messed-up genes than average would be less smart than average.
This has to be part of the truth, and it might be most of the truth, particularly within a population. People have had no success looking for single alleles that explain much of the variation in IQ – but as far as I am aware, all of those studies have been of Europeans. In the next population you look at, the genetic architecture might be different. For example, height is highly polygenic in Europeans. No single allele explains much of the variance, although a few have large-enough influence to be detectable (unlike IQ, thus far). But the situation is different in Pygmies. There, we see loci that influence height, as well as a general tendency of height increasing with the fraction of the Bantu ancestry.
So you don’t know until you look. It could well be that there are IQ QTLs among the Pygmies: if nothing else, being bigger, with a bigger brain, might well increase IQ. If I had to guess, I would suggest that IQ in Pygmies might also increase with the fraction of Bantu ancestry, which, if true, would certainly mystify many people.
I doubt if genetic load is the entire story. Selection happens. But load surely matters, and may be part of the explanation both within and between populations.
Years ago, I don’t think it was obvious that you wouldn’t find a few fair-sized IQ QTLs in Europeans. It was obvious that genetic load would be part of the story – I’ve been thinking about it for a long time. There I go again, using that mysterious and undefinable word again – ‘obvious’. Have to be careful about that.