Recent incidents like the San Bernardino shootings and the Cologne sexual assaults have certainly contributed to these anxieties. But on a broader scale, the human desire to keep out “the other” might be partly explained by a vestigial psychological quirk that was once meant to help protect us from infection. The mechanism, called the behavioral immune system, tells us to avoid things that are unfamiliar because they might harbor harmful pathogens.

The phenomenon helps explain a whole host of subconscious reactions, like prejudice against people with facial birthmarks, an attraction to symmetrical faces, and, yes, strong anti-immigration views. In some cases, the behavioral immune system might have a point: The sight of pus and blood makes some people queasy because touching another person’s infected wound without gloves is dangerous. But the behavioral immune system, like an allergy, is so sensitive that it’s often wrong. It sometimes hurts us more than it helps.

In a new review paper published this week in Advances in Experimental Social Psychology, the psychologists Damian R. Murray, from Tulane University, and Mark Schaller, from the University of British Columbia, explain where these behavioral defenses against infection came from—and how they can deceive.