A few excerpts:
As a kid, I saw the 1968 version of Planet of the Apes. As a future primatologist, I was mesmerized. Years later I discovered an anecdote about its filming: At lunchtime, the people playing chimps and those playing gorillas ate in separate groups.
It’s been said, “There are two kinds of people in the world: those who divide the world into two kinds of people and those who don’t.” In reality, there’s lots more of the former. And it can be vastly consequential when people are divided into Us and Them, ingroup and outgroup, “the people” (i.e., our kind) and the Others.
Considerable evidence suggests that dividing the world into Us and Them is deeply hard-wired in our brains, with an ancient evolutionary legacy. For starters, we detect Us/Them differences with stunning speed. Stick someone in a “functional MRI”—a brain scanner that indicates activity in various brain regions under particular circumstances. Flash up pictures of faces for 50 milliseconds—a 20th of a second—barely at the level of detection. And remarkably, with even such minimal exposure, the brain processes faces of Thems differently than Us-es.
This has been studied extensively with the inflammatory Us/Them of race. Briefly flash up the face of someone of a different race (compared with a same-race face) and, on average, there is preferential activation of the amygdala, a brain region associated with fear, anxiety, and aggression. Moreover, other-race faces cause less activation than do same-race faces in the fusiform cortex, a region specializing in facial recognition; along with that comes less accuracy at remembering other-race faces. Watching a film of a hand being poked with a needle causes an “isomorphic reflex,” where the part of the motor cortex corresponding to your own hand activates, and your hand clenches—unless the hand is of another race, in which case less of this effect is produced.