What can you do to make your kids smarter? Keeping them healthy might help. A new study suggests that worldwide differences in intelligence can be explained by disparities in infectious disease. The researchers found that countries most heavily affected by infectious diseases generally had the lowest average IQs. They propose that these illnesses hinder children's brain development, though their conclusion is gathering mixed reviews. The new research relies on data first published in 2002 in a controversial book called IQ and the Wealth of Nations. In the book, psychologist Richard Lynn of the University of Ulster in the United Kingdom and political scientist Tatu Vanhanen of the University of Tampere in Finland searched the published literature to come up with measures of average IQ for 81 countries. They also estimated IQ for another 104 countries by averaging the IQs of nearby nations. Hong Kong topped the list, with an average IQ of 107. The authors argued that national differences in IQ at least partly explained differences in national wealth. In 2006, they expanded the data to include IQ measurements from 113 countries and new estimates for 79 more.
Several groups have attempted to explain the pattern. In the new study, Christopher Eppig, a Ph.D. candidate in biology at the University of New Mexico, Albuquerque, and his colleagues propose that low IQ is tied to the toll of infectious diseases. Their idea, which the researchers call the “parasite-stress hypothesis,” is that children who contract “parasites,” which they define to include everything from intestinal worms to bacteria and viruses, devote more energy to fighting off infection. As a result, they have less energy available for brain development. Countries where infectious diseases are prevalent, Eppig and colleagues argue, will have lower intelligence.
Picture: In this map, countries shaded purple have the highest average IQ. Those shaded dark red have the lowest IQ—and, typically, the highest incidence of infectious disease.