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. Since this is a developmental cause, rather than a genetic one, it’s good news for anyone who is interested in reducing global inequality associated with IQ. If the primary factors were genetic, as some have suggested, IQ would be very difficult to change.
As our research continues, we hope to discover the periods of development that are most sensitive to disruption by infectious disease, and determine which diseases are most harmful to brain development. If the evidence continues to come out in favor of our hypothesis, it will allow people interested in using this information to raise the IQ of people around the world to target their efforts most effectively and efficiently.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.