Though not all studies have supported this connection,162 a substantial body of research has found that people who are more easily disgusted are more likely to be tough on crime, anti-gay,163 and even politically conservative.164 People who are more sensitive to the smell of sweat and urine are more likely to support authoritarian political leaders—perhaps because these leaders would be more likely to keep disease-bearing outside groups out.165 People who searched for more news about the 2014 Ebola outbreak were more likely to vote Republican, which suggests, to some researchers, that they were worried about the virus and wanted someone in office whose immigration policy might keep supposedly Ebola-infected outsiders away.166 (Sometimes pundits make this connection explicit: in one 2005 broadcast, Lou Dobbs’s show claimed illegal immigrants167 were spreading leprosy in the United States. Later, his correspondent was found to have confused a thirty-year figure with a three-year one.) The behavioral immune system operates like a strong gag reflex. It instinctively keeps away anyone who might bring something weird to the table, all with the goal of keeping us healthy.

Gelfand, the researcher who examined tight and loose cultures, also found that a high level of threats leads to cultural tightness—the cultures with stricter norms. In her 2011 study, she found that tight nations had fewer natural resources, including less farmland and food reserves. They were more densely populated. They had worse air and water quality, experienced more natural disasters, like floods and droughts, and had a greater risk of invasions. Tight countries had more deaths from communicable diseases and had higher child mortality rates. “When you have a collective threat, you can’t solve it on your own,” Gelfand told me. You need to stick—firm, close, tight—with your fellow man. Perhaps because of how many threats they’ve faced, people in tight societies are less willing to live near people unlike themselves, one study168 found.

Some researchers think the behavioral immune system is not a psychological impulse so much as a cultural one. It’s not necessarily any healthier to avoid outsiders, this line of thinking goes, but groups have historically grown and prospered when they were closed off to outside influences. (For example, the Mormons, who proselytize aggressively and mandate everything down to the style of underwear adherents must wear, are fast-growing,169 while membership in the Unitarian Universalists, whose beliefs are much more flexible, is basically stagnant.)170 Group leaders, then, might be manipulating our natural sense of disgust toward unfamiliar diseases in order to stoke xenophobic feelings whenever it’s convenient. If a politician wants people to start turning against immigrants, he might just start talking about Ebola-ridden immigrants sneaking across the border.