The threat of disease is not uniform around the world. In general, higher, colder, and drier regions have fewer infectious diseases than warmer, wetter climates. To survive, people in this latter sort of terrain must withstand a higher degree of “pathogen stress.” Thornhill and his colleagues theorize that, over time, the pathogen stress endemic to a place tends to steer a culture in distinct ways. Research has long shown that people in tropical climates with high pathogen loads, for example, are more likely to develop a taste for spicy food, because certain compounds in these foods have antimicrobial properties. They are also prone to value physical attractiveness — a signal of health and “immunocompetence,” according to evolutionary theorists — more highly in mates than people living in cooler latitudes do. But the implications don’t stop there. According to the “pathogen stress theory of values,” the evolutionary case that Thornhill and his colleagues have put forward, our behavioral immune systems — our group responses to local disease threats — play a decisive role in shaping our various political systems, religions, and shared moral views.
If they are right, Thornhill and his colleagues may be on their way to unlocking some of the most stubborn mysteries of human behavior. Their theory may help explain why authoritarian governments tend to persist in certain latitudes while democracies rise in others; why some cultures are xenophobic and others are relatively open to strangers; why certain peoples value equality and individuality while others prize hierarchical structures and strict adherence to tradition. What’s more, their work may offer a clear insight into how societies change. According to Thornhill’s findings, striking at the root of infectious disease threats is by far the most effective form of social engineering available to any would-be reformer.
...
Over the last few years, an increasing number of papers from other social scientists have backed the theory. While many of these researchers work with the same large data sets and long timescales that Fincher and Thornhill study, others have figured out ways to tease out the behavioral immune response in real time, on a smaller scale. Schaller and his colleagues, for example, set up a test to see if disease cues could influence laboratory subjects’ opinions of foreigners. Schaller’s team had one group of subjects watch a slideshow about germs and disease while another group watched a show about everyday accidents and dangers. The researchers then told the subjects that the Canadian government was going to spend money to attract immigrants to the country. As Schaller predicted, the test subjects who had been cued with the disease presentation were less inclined to spend money to attract people from unfamiliar countries.
...
Thornhill grew up in Alabama in the 1940s and ’50s. He says he witnessed firsthand the rank sexism, racism, and xenophobia that was rampant in the South during that period. And he is well acquainted with the region’s strong family ties and firm religious beliefs. But he is also aware of a somewhat lesser known set of facts about his native soil. Around the time of his childhood, Southern states were finally getting a hold on a pair of diseases that had long plagued the region: malaria and hookworm. These diseases, writes Peter Hotez, the founding dean of the
National School of Tropical Medicine at Baylor University, had turned generations of Southerners into “anemic, weak, and unproductive children and adults.” Not surprisingly, Thornhill believes that the collectivism of the old South — the adherence to tradition, ethnocentrism, and suspicion of outsiders that marked his childhood — stemmed from its historically high pathogen load.
