This theory fits with how the Hindu caste system in India developed. As Yuval Noah Harari writes in his book Sapiens,171 about three thousand years ago Indo-Aryan invaders stratified the Indian population into castes in hopes of permanently subjugating them. The Indo-Aryans placed themselves at the top of the heap. Mixing with the lowest castes, which were kept from having access to good sanitation, was considered “polluting” oneself. The very lowest caste became known as the “Untouchables,” and other people, accordingly, avoided even touching them. It behooved the Indo-Aryans to stoke the behavioral immune systems of their subjects, so they did.

This being psychology, some researchers think the entire behavioral immune system concept is, basically, bunk. Some scholars I reached out to told me the effect sizes of these studies tend to be small, which in science-speak means they don’t really explain all that much. Favoring your own kind, some argue, has more to do with government effectiveness than with diseases. In places where governments don’t take care of the roads, schools, and clinics properly, people seem to invest more in their immediate families and groups as a form of social security172—not because they’re disgusted, but because they have no other choice.

Douglas Fry, the anthropologist, told me the idea of the behavioral immune system “sounds crazy.” Germ theory, after all, is only a few hundred years old. People in our ancestral past would have had no idea they were avoiding invisible particles that caused sickness. And, he points out, in modern hunter-gatherer societies, people still have all sorts of supernatural explanations for illnesses that don’t correlate with outsiders coming in or out. Returning to the Nobel study, another anthropologist, Agustín Fuentes, told me you can’t compare modern-day countries as different as Denmark and Singapore, since they have very different histories.

One of the major studies in this field, the one about family ties and religiosity, was met with a slew of critical commentaries.173 The critics claimed there are other things, like a country’s wealth, that might explain why countries that have more parasites are more religious. One person offered up a study of fifty-six Mayans to show that religious people tend to get sick more, not less, because they’re always out proselytizing and helping the poor. So if religious people want to avoid germs, they have a funny way of showing it. Other critics said it’s not always the case that foreign pathogens are any worse for you than local ones, so it wouldn’t necessarily benefit our ancestors to avoid foreigners for health reasons. The study authors had responses to all of these critiques, but I’m going to spare us all and just say this field of research is, well, controversial.