Education, economic growth and nutrition aren’t the only explanations for differences in global intelligence. Two new studies have suggested that the prevalence of infectious disease is also one of the best predictors of a country’s IQ.


Christopher Hassall and Thomas Sherratt, biologists from Ottawa’s Carleton University, found a strong link between the level of parasitic infections and average national IQ, as noted this month in Scientific American. They also posited that people living in warmer climates tend to have lower average IQ precisely because there are more parasitic infections in places with higher temperatures.

Their findings confirm a similar 2010 study by University of New Mexico researchers. “Controlling for the effects of education, national wealth, temperature, and distance from sub-Saharan Africa, infectious disease emerged as the best predictor of the bunch,” writes Christopher Eppig, one of the study’s authors. (Some researchers have suggested that average IQ is higher, the farther one travels from the evolutionary origin of humans in Africa, positing that migrating humans needed higher intelligence to survive.)

Why does there seem to be such a strong link? Eppig explains:

One study found that newborn humans spend close to 90 percent of their calories on building and running their brains...If, during childhood, when the brain is being built, some unexpected energy cost comes along, the brain will suffer. Infectious disease is a factor that may rob large amounts of energy away from a developing brain.

Eppig and his team replicated their analysis in the United States, finding that infectious disease rates were very good at predicting a state’s average IQ. Massachusetts, New Hampshire and Vermont — states with high average IQ — had low infectious disease rates, while California, Louisiana and Mississippi had low average IQ and high infectious disease rates. That being said, there seems to be some glaring weaknesses in the state-level study, at least: Eppig assumed that education was controlled for across the country because there’s standardized mandatory education in the United States, when there’s arguably considerable variation in the quality of education across different states, the proportion of new immigrant students, and so forth.