Rushton thought that the explanation of geographical variation in IQ was adaptation to a world with winters. I didn’t think that was crazy, but I was pretty sure I could come up with ten other comparably plausible explanations. In some, the key events might have happened tens of thousands of years ago, while others might have been recent enough to be documented in the historical record.

Increasingly, I suspect that there is no single explanation. Maybe several of my notions are partly true. The Ashkenazi Jews look like a case of recent selection for white-collarism in a reproductively closed merchant caste – and maybe there are similar explanations for the Parsees and some of the high Indian castes. Cousin marriage explains some of what we see in places like Iraq or Uttar Pradesh. Increased genetic load resulting from high average paternal age among polygamists probably plays a role in sub-Saharan Africa and Australia. Adaptive introgression from Neanderthals? Could be. Increased population size among agriculturalists, resulting in Fisherian acceleration? Possible: certainly civilized peoples generally beat out hunter-gatherers. Iodine shortages? Surely. Brain damage from capsaicin? You know it.

Pygmies apparently score very poorly on cognitive tests. I wonder if that’s a side effect of being small: small brains must have lower performance, all else equal. Something about rain forest environments must select for pygmyism, since that phenotype has apparently evolved independently a number of times. That may be yet another path leading to low IQ.