Why Is Average IQ Higher in Some Places?

{snip} One study found that newborn humans spend close to 90 percent of their calories on building and running their brains. (Even as adults, our brains consume as much as a quarter of our energy.) 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. This was our hypothesis, anyway, when my colleagues, Corey Fincher and Randy Thornhill, and I published a paper on the global diversity of human intelligence.

A great deal of research has shown that average IQ varies around the world, both across nations and within them. {snip} At the heart of this debate is whether these differences are due to genetics, environment or both.

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{snip} Satoshi Kanazawa suggested that evolution favors higher IQ in areas that are farther from the evolutionary origin of humans: sub-Saharan Africa. Evolution, the hypothesis goes, equipped us to survive in our ancestral home without thinking about it too hard. As we migrated away, though, the environment became more challenging, requiring the evolution of higher intelligence to survive.

We tested all these ideas. In our 2010 study, we not only found a very strong relationship between levels of infectious disease and IQ, but 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. A recent study by Christopher Hassall and Thomas Sherratt repeated our analysis using more sophisticated statistical methods, and concluded that infectious disease may be the only really important predictor of average national IQ.

Support for this hypothesis comes not only from cross-national studies, but from studies of individuals. There have been many studies, for example, showing that children infected with intestinal worms have lower IQ later in life. {snip} In practical terms, however, this means that human intelligence is mutable. If differences in IQ across the world are largely due to exposure to infectious disease during childhood, then reducing exposure to disease should increase IQ.

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 states with the five lowest average IQ all have higher levels of infectious disease than the states with the five highest average IQ, and the relationship was good across all of the states in between.

So far, the evidence suggests that infectious disease is a primary cause of the global variation in human intelligence. {snip}

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