Why do some nations have lower IQ scores?

Using data on national “disease burdens” (life years lost due to infectious diseases) and average intelligence scores, the authors found a striking inverse correlation—around 67 percent. The countries with the lowest average IQ scores—Equatorial Guinea, Cameroon, Mozambique, Gabon—have among the highest disease burdens. In contrast, nations with low disease burdens top the IQ list, with Singapore, South Korea, China, Japan, and Italy in the lead.

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The study controlled for other potential causes of the IQ gap, such as the aforementioned education, agricultural labor, and income levels, as well as climate (colder lands tend to have higher IQ scores, and some theorists have proposed that lower temperatures may evolutionarily select for higher intelligence) and distance from humanity’s African cradle, which is the notion that unfamiliar lands might have forced migrating humans to become smarter. However, with the exception of this last theory—which has in any case been challenged—it turns out that “infectious disease remains the most powerful predictor of average national IQ.” The study’s findings may also help explain the Flynn effect, which can’t be accounted for by evolution (the IQ gains occur over time spans too short for natural selection). So what’s going on instead? As nations develop, they improve their population’s access to safe drinking water and to vaccines and medicine—all of which lower parasitic infection rates.

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