Infectious disease and national IQs
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Christopher Eppig is back, this time in Scientific American, with his study showing a high correlation between average national IQs around the world and infectious disease burden. I would hardly be surprised if this were partly true, but I can't say I've found the evidence highly persuasive yet that the arrow of causality doesn't mostly run in the opposite direction: e.g.,  that high average IQ Singapore has used its smarts to cut infectious disease more than low average IQ Lagos has managed to do, despite both being at similar latitudes and altitudes. Eppig argues
Despite the strength of our findings, our study was not without its limitations. We did our best to control for the effects of education. But what we really needed was to repeat our analysis across regions within a single nation, preferably one with standardized, compulsory education. The nation we chose was the United States. Average IQ varies in the states. (Massachusetts, New Hampshire, and Vermont are at the high end, for example; California, Louisiana, and Mississippi are near the low end.) Again, infectious disease was an excellent predictor of average state IQ. 
The fever swamps of California? The swarming hordes of malarial mosquitoes in Compton? I don't really get this idea that Californians have their infectious disease burdens to blame for their stupidity. The climate in California is famously healthy: dry, sunny, few mosquitoes or other insects, and low humidity. Health nuts have been moving to California for 125 years. For example, my grandfather was a health nut and he moved to Altadena in 1929.
A reader points out:
It is hard for me to believe that infectious disease is by far the most important cause of IQ variation.  If this were so, I would expect to see significant numbers of very high IQs even in low-average-IQ regions (asymmetrically long high-end tails of IQ bell curves for these regions), because even in high-infectious-disease areas (I assume) the number of uninfected people is significant.  In reality, the IQ bell curve is shifted to the left for low-IQ regions, but retains its symmetrical shape.  Only if everyone in low-average-IQ regions had been infected with IQ-lowering disease does the infectious disease hypothesis predict that normal distribution curves will be shifted to the left without the shape being affected.

For example, Kenya has high rates of infectious disease, infant mortality, crippling accidents, and so forth. Yet, the Kenyans who avoid all that tend to do extraordinarily well in Olympic running events.

Also, the hypothesis doesn't square with the fact that people have lower IQs in any region when their family origins and genetic pedigrees originate in low-IQ areas.
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