Infection plagues IQ?
As a nation’s economy develops and its standard of living rises, the average intelligence of its inhabitants also increases. But why? In what Faculty of 1000 Member and University of Münster evolutionary biologist Joachim Kurtz calls a “thought-provoking” study, Christopher Eppig suggests that healthier inhabitants could be the answer: Infectious diseases are rarer in developed nations than they are in the developing world, leaving their citizens with more metabolic energy available for cognitive development (Proc R Soc B, doi:10.1098/rspb.2010.0973, 2010).
The intelligence quotient (IQ) test has been used around the world as a measure of cognitive ability. Although controversial, results from these tests have been used to obtain average IQ measures for a nation or geographical region. The average IQ of a country correlates with many factors: temperature, gross domestic product, secondary education, nutrition. And in a nation on its way to economic prosperity, changes in average intelligence can be very rapid, in some cases increasing by 10–15 points in a single generation—much too fast to be explained by evolutionary processes alone.
By examining and comparing published datasets, Eppig, a PhD student at the University of New Mexico, Albuquerque, found that countries with high levels of infectious disease consistently had lower average IQ scores. This correlation was stronger than with any other measure previously correlated with IQ, including education. He found that the average citizens of some disease-riddled countries scored up to 30 points lower on IQ tests than people living in healthier nations.
But the suitability of IQ tests as a measure of cognitive ability is a contentious matter. “It is difficult to compare IQ estimates for different cultures,” Kurtz says, adding that this limitation also applies to correlations of intelligence with other factors such as nutrition, GDP and education. Eppig disputes the assertion that IQ tests are biased towards Westerners. “That’s empirically false,” he says, as Westerners do not always score the highest on them, and they have been validated in non-Western societies. But he is now writing up an extended study, in which he examined the correlation between disease and intelligence across the United States. This, he says, should eliminate cultural variation and neutralize criticisms of the use of IQ tests for analyses of this kind.
But why should disease affect intelligence? The answer to this question lies in the fact that the brain is metabolically expensive to produce and maintain. Newborn babies spend nearly 90% of their “metabolic budget” on developing the brain, Eppig says. “That was really the key to the rest of the study.”
Not only does the immune system require more metabolic resources when it has to combat an infection, but individuals burdened with parasites will suffer more severely from nutritional defects. Resources spent fighting disease are not available for feeding the brain, so if a child gets sick, cognitive development is likely to suffer, a problem compounded by widespread malnutrition in poorer parts of the world.
Eppig also suggests that exposure to parasites and infectious microbes at critical stages in development might stimulate a metabolic pathway that permanently invests more energy in the immune system. Even when these unfortunate individuals are healthy, their brains might not get all the nourishment they need because the body is wired to direct resources to the immune system.
According to Kurtz, it’s “astonishing that parasites seem to have so far been ignored” as a potential influence on cognitive ability. He says that if the results hold up to scrutiny, Eppig’s study opens up many new perspectives on intelligence research. They may also have profound implications for the distribution of aid by both government and nongovernment agencies. Eppig thinks that until now, we haven’t fully understood all the social ramifications of disease: IQ is an extremely powerful predictor of economic success in a country, so malaria, for example, does not simply have a direct human and economic cost, but also cripples the intellectual and fiscal development of a country. For those trying to turn a nonindustrialized nation into an economic power, “probably the first thing you want to do is eliminate parasites,” he says.