According to recent work by Corey Fincher and Randy Thornhill, where you live can affect everything from brainpower to family ties. A 2010 article in the Proceedings of the Royal Society B reports that states with the lowest prevalence of infectious disease also have the highest average IQ. Clocking in with the highest IQs were Massachusetts, New Hampshire, Vermont and North Dakota. On the lower end were Mississippi, Louisiana, California and Hawaii. Although many factors could explain the relationship between disease rates and IQ, the authors controlled for the most likely factors, particularly the average wealth and education level in each state.

The reasoning behind the authors’ IQ and parasite-stress hypothesis is fairly simple. The body can be compared to an economy with finite amount of resources. An newborn’s brain requires 87% of the baby’s metabolic energy. When the body’s resources are being used to fight off disease, particularly during the crucial developmental years of childhood, energy that normally would be used in brain building is diverted to the immune system. Chronic childhood disease also sets up a pattern where energy is continually diverted to the immune system, and the brain is not able to return to full development and functioning even after the threat has passed.

The study, led by Christopher Eppig, a researcher at the University of New Mexico, replicates a previous finding using nation-level data (Eppig's study has since been successfully replicated using even more complex statistical methods) including GDP, education, climate, and distance from sub-Saharan Africa. Eppig’s team found that infectious diseases, particularly diarrhea—the scourge of the developing world—played the most significant role in predicting average national IQ. While clean drinking water and diarrhea are not serious problems in the United States, California has been hit with an outbreak of pertussis, and diseases like meningitis, rubella, tetanus, and tuberculosis are still problems elsewhere in the U.S.

The role of parasites doesn’t stop with our IQs—last year the authors' applied their 'parasite stress theory of human values' to the issue of homicide and child maltreatment finding that greater parasite stress increased the chances of child maltreatment and homicide; a study by the team out last April shows that parasite stress is associated with greater in-group sociality resulting in stronger family ties and religiosity. Significantly, in areas with high levels of non-zoonotic parasite stress (infectious disease transferred by people rather than animals) in-group sociality was stronger than in areas with high levels of zoonotic parasite stress.

According to Thornhill, we may even have low parasite stress to thank for democratic ideals and free societies. In a 2008 study, researchers found that areas with low evolutionary levels of parasite stress and infectious disease were less likely to be highly ethnocentric. The parasite stress theory of human values It makes good evolutionary sense. People co-evolve with parasites, each evolving adaptations and co-adaptations in a kind of ecological arms race, with people eventually becoming immune until the next microbe evolution outdoes them. In areas of high parasite stress, an encounter with an outside group was likely to result in an outbreak of a disease for which no immunity had evolved. This was less of a problem in the colder climes, which may explain why trade and liberal democracies were quicker to develop there.

There's a highly edifying umbrella quality to the theory--it seems to explain so much with one relatively simple and elegant theory. But is the parasite stress theory of human values too good to be true? Such findings tend to be incendiary (Thornhill is perhaps most broadly known for the controversy surrounding his 2000 book, A Natural History of Rape) as they can often be used to imply that certain countries, races or regions are just ‘dumber’ than others, resulting in poor development and poverty. A penchant for controversial headlines is not a de facto indication of sloppy or skewed science, but in the wake of numerous scandals in the field of social psychology and evolutionary biology, it's sure to evoke an added layer of scrutiny.  In 2002, the controversial book IQ and the Wealth of Nations argued that national differences in IQ could partially explain underdevelopment. But this study’s findings take another tack on the chicken-egg conundrum, arguing that IQ is lower because disease is more prevalent in underdeveloped areas, particularly on the international scale where the disparities in health and wealth are much greater. While the findings may stir controversy, ferretting out the root cause of poverty can help guide governments and NGOs, redirecting aid investment to maximize gains and minimize the loss of human capital over time.

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