The 1990s saw a pivot toward trainings that were targeted toward everyone, not just minorities. But the result was not necessarily better. “Many interpreted the key learning point as having to walk on egg shells around women and minorities,” Anand and Winters write. “Some surmised that it meant White men were villains, still others assumed that they would lose their jobs to minorities and women, while others concluded that women and minorities were simply too sensitive.” As a result, by the turn of the millennium underrepresented groups were still not seamlessly embraced by their coworkers.

Then again, perhaps we shouldn’t blame past generations for being so wary of outsiders. After all, their brains made them do it.

When we see people who belong to other races, ethnicities, or groups, our brains snap into action, scanning the intruder and branding them in red warning letters: DIFFERENT.

The amygdala, the threat center of the brain, becomes activated when we see a face belonging to someone from another tribe. Several studies122 reveal that the amygdala responds more strongly to pictures of people of other races, but this can also happen with any group our brain perceives as different or threatening. The amygdala’s activity causes us to stare at or distance ourselves from the person. Our heart rates rise and blood flow surges in preparation for a fight.123 (On a more adorable note, in a paper called “The Cooties Effect,” researchers showed that the amygdalae of kids tend to react less strongly—and thus less negatively—toward the opposite sex as they get older.)124

The brain also exhibits less empathy125-like activity when it comes to people who are from groups outside our own,126 be they supporters of rival sports teams127 or people from other races. In experiments, people who watched others get poked with needles reacted as though they were feeling the pain themselves, but only when the people being prodded were of their same race.128 When outsiders are in pain, there’s activation in the brain that looks more like schadenfreude.129

David Amodio, a professor of psychology and neural science at New York University, has found that white people even process black people’s faces in a way that dehumanizes them. Rather than seeing them as having human faces, white people process black faces more like they would an upside-down human face or the face of an animal. Then, because they don’t see the black faces as very face-like, the white participants are less willing to allocate money to the black faces. Meanwhile, viewing pictures of social outcasts—people like the homeless and drug addicts—elicits in many people brain patterns consistent with disgust, similar to looking at a picture of an overflowing toilet.130