Much of what we like to think of as politics, morality, and culture is really disease avoidance behavior:
Some species of primate, Thornhill told me, will ostracize sick members of the group to avoid the spread of disease. Cows and other ungulates are known to rotate their movements among pastures in such a way as to avoid the larvae of intestinal worms that hatch in their waste. And in ant societies, only a small number of workers are given the task of hauling away the dead, while sick ants will sometimes leave the nest to die apart from the group.
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.
Fincher suspected that many behaviors in collectivist cultures might be masks for behavioral immune responses. To take one key example, collectivist cultures tend to be both more xenophobic and more ethnocentric than individualist cultures. Keeping strangers away might be a valuable defense against foreign pathogens, Fincher thought. And a strong preference for in-group mating might help maintain a community’s hereditary immunities to local disease strains. To test his hypothesis, Fincher set out to see whether places with heavier disease loads also tended toward these sorts of collectivist values.
Working with Damian Murray and Mark Schaller, two psychologists from the University of British Columbia, and Thornhill, Fincher compared existing databases that rated cultural groups on the individualist-collectivist spectrum with data collected from the Global Infectious Diseases and Epidemiology Network and other sources. The team paid special attention to nine pathogens (including malaria, leprosy, dengue, typhus, and tuberculosis) that are detrimental to human reproductive fitness. What the team found was a strong correlation between collectivist values and places with high pathogen stress. In 2008, Fincher, Thornhill, Schaller, and Murray published a major paper in Proceedings of the Royal Society B that laid out the connection.
Thornhill and Fincher found further evidence for the pathogen stress theory by looking at geographical regions that had not only severe disease stress but also a highly diverse patchwork of local pathogen populations. The critters that make us ill — not only the viruses and bacteria, but also the ticks, flies, and mosquitoes that spread them — are tiny and lack the ability to regulate their own heat as larger organisms do. They often flourish only in very narrow climatic zones, where they are adapted to certain temperature and moisture levels. As a result, pathogen threats can be highly localized. One study, for instance, found at least 124 genetically distinct strains of the parasite Leishmania braziliensis across Peru and Bolivia.
If you were to live in such a pathogenically diverse place, you and your family would likely develop a resistance or immunity to your local parasites. But that defense might be useless if you were to move in with a group just a short distance away — or if a stranger, carrying a foreign pathogen load, were to insinuate himself into your clan. In such places, then, it would be important for neighboring groups to be able to tell the difference between “us” and “them.” With that thought in mind, Thornhill and his colleagues made a prediction: that regions with a balkanized landscape of localized parasites would in turn display a balkanized landscape of localized customs and conspicuous cultural differences among human populations — dialects, unique religious displays, distinctive art and music, and the like. While there is much more research to be done, early findings suggest that — particularly when it comes to the development of local languages and religions — pathogen stress does appear to spawn cultural diversity.