Is The Blood Thicker Near The Tropics? Trade-Offs Of Living In The Cold
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There's been some development in recent years (as well as some push-back) for a more specific hypothesis of how climate and disease affect brain development to produce national IQ differences that I think is pretty interesting. An analysis in 2010 by Chris Eppig (doi:10.1098/rspb.2010.0973) found that the intensity of parasite stress was a strong predictor of national average IQ (roughly r = 0.8), and remained a significant predictor after controlling for things like average temperature (despite the fact that latitude is a pretty good predictor of disease prevalence), GDP, education measures, and distance from Africa. (Parasite stress was measured in that paper with DALY, a measure of life years lost due to infectious disease, although I've seen papers that used measures like parasite biodiversity and it tends to predict the same outcomes as DALY does.)
The paper suggests that the effect is happening over developmental time, with energy required for immune functions trading off against optimal brain development. So this wouldn't predict anything about adults moving from one region to another being more or less effective, but it could provide an explanation for stubborn seemingly-innate national IQ differences, and doesn't even require one to contemplate uncomfortable hypotheses about genetic differences!