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.
To test this idea, the researchers statistically analyzed the relationship between Lynn and Vanhanen's 2006 data and 2004 data on infectious disease burden from the World Health Organization, which measures potential years of healthy life lost to premature death and illness as a result of 28 infectious diseases, including malaria, hepatitis, and tetanus. The researchers also reexamined factors that other research groups had linked to IQ, such as nutrition, literacy, education, gross domestic product, and temperature.
The numbers seem to support the hypothesis, the team will report  online tomorrow in the Proceedings of the Royal Society B. When the researchers analyzed each factor independently, they found that infectious disease burden was more closely correlated to average IQ than the other variables. "Parasites alone account for 67% of the worldwide variation in intelligence," Eppig says. To further assess the relationship, the researchers built a statistical model that allowed them to test the predictive power of infectious disease burden against other variables previously associated with IQ, such as education, temperature, distance from sub-Saharan Africa, and wealth. Infectious disease burden again came out on top, although temperature and distance from sub-Saharan Africa explained some of the variation as well.
Eppig points out that their study can't rule out any of the other factors. "I would never say that parasites are the only thing affecting the global diversity of intelligence."
Maureen Black, a pediatric psychologist at the University of Maryland School of Medicine in Baltimore, is skeptical. She argues that health by itself isn't enough for full brain development. "For children to develop intellectual skills, they need not only strong bodies and the absence of infections, they also need opportunities to explore and opportunities for enrichment." Those opportunities might be lacking in countries with low average IQ.
But Richard Guerrant, a physician and infectious disease expert at the University of Virginia School of Medicine in Charlottesville, says the researchers are on the right track. His work suggests a link between diarrheal diseases, malnutrition, stunted growth, and lower IQs. The next challenge, he says, will be to uncover the exact mechanisms.