According to a newish theory in psychology, this happens because in addition to our regular immune systems, which vanquish germs and keep us healthy, we are equipped with so-called behavioral immune systems. Our behavioral immune systems predispose us to avoid people who break our social norms because we, subconsciously, fear they might harbor illnesses we are not equipped to fight off. There are three ways this instinct might work. First, people who don’t follow local customs are likely to be from faraway lands, and they could bring in unfamiliar pathogens that could wipe out your tribe. Second, people who break certain norms, like the one against having lots of sexual partners, might be more likely to spread diseases even if they are from your tribe. Finally, an unusual appearance or strange behavior might signal an infectious disease—think leprosy, which disfigures the face. All three kinds of people, our gut reaction tells us, should be avoided. Thus, we may shun people who break our local norms, even if there’s nothing actually wrong with them.

As I described in a 2016 Atlantic article,158 our fear of getting sick appears to make us less open to outsiders in several ways. Several studies have now shown that when people are more worried about disease, they react more negatively toward foreigners and norm violators. (Is it any wonder that immigrants are often referred to as “animals” or “parasites” or other terms for disease vectors?) People in more disease-ridden countries are less likely159 to be extroverted or open to new experiences, suggesting that they want to stay away from the unfamiliar and avoid exposing themselves to even more sicknesses. Pregnant women in their first trimester are more prone to infections, and as it happens, during that trimester women exhibit more ethnocentric and xenophobic attitudes than when they’re further along in their pregnancies.160 One study found that states and countries that are more plagued by infectious diseases tend to have stronger family ties and greater levels of religiosity161; the authors interpreted these measures as indicating a preference for sticking with your own kind of people.