Tajfel, who by this point had become a professor at the University of Bristol in the U.K., thought explanations like these were too simplistic. He believed the mere existence of groups was enough to stir intergroup conflict. In a telling set of experiments in the early 1970s, he divided British schoolboys up into two arbitrary groups—sometimes as basic as “group X” and “group W.”84 He then had the boys distribute money among all their classmates, but leave themselves out of the deal. No matter what, the boys gave their own group members more money—even though this would not benefit them in any way and they had no expectation of interacting with this group in the future. They preferred their own tribe, even if they had only been assigned to it moments before. Seeing our group win matters, even if the group itself doesn’t matter at all.

The question of how ordinary people could so easily turn on one another seemed to be on Tajfel’s mind as he did this work. Studies like these helped affirm Tajfel’s belief that prejudice was not an inborn personality trait, specific to one nationality or another. Instead, it’s a reflection of the dynamics between groups. Prejudice, to him, was more about group-think than gut-feel. “It is therefore important and useful,” Tajfel wrote in his 1981 book, Human Groups & Social Categories,85 that we consider prejudice to be “in the minds, rather than in the guts of people…”

As it would turn out, it’s in both.

The Shiwiar, a remote Amazonian people in eastern Ecuador and northeastern Peru, live somewhat like humans lived before modernity. The men hunt and fish, and the women tend two to four gardens of manioc, plantains, and yams. They live according to a value called shiir waras, which means to do what’s expected of your age and gender. For the most part, each Shiwiar household grows and obtains its own food; they don’t pool their resources. One of the main situations in which they do give each other resources for prolonged periods is when they’re sick.

And they do get sick. The Shiwiar have hard lives. They are stung by bees, bitten by bats and snakes, and come down with malaria and measles. They fall off of things and get struck by machetes. Lawrence Sugiyama,86 an anthropologist at the University of Oregon who interviewed some Shiwiar in the 1990s, found that almost everyone in his sample was disabled for a week or more, and over 60 percent were on bedrest for a month or longer. To make matters worse, they have little access to Western medicine.