The problem with such an otherwise adaptive tendency is that once we draw those lines, we can’t help falling in love with us and growing cool to them. It’s not enough for your home group to be the one you know best; it also must actually be the best. The language of your tribe is more lyrical than the gibberish other tribes speak; your music is sweeter than that noise you hear coming from across the valley. Similarly, your men are stronger, your women lovelier, your clothes prettier, your food tastier.
“You want some psychological mechanism to make your group cohesive,” says developmental psychologist Yarrow Dunham of the University of California at Merced. “The features that define groups vary a lot, but once they are defined, you prefer your own. And when someone belongs to your group, you like that person better too.” So inclined are humans to divide themselves into clans that even when groups don’t naturally exist, we invent them. We identify ourselves by our clubs, our communities, our political parties. Army duels Navy for supremacy in football. The Crips kill the Bloods for supremacy in the streets. Baseball lovers in Boston describe themselves not as just fans but as nothing short of a Red Sox nation. “There is something very basic about in group and outgroup distinctions,” says social psychologist John Bargh of Yale University. “Groups succeed or fail together.”
Sometimes, affiliations can be created by something as inconsequential as an article of clothing. Dunham recently conducted a study in which he divided preschoolers into two groups according to the color of T shirt he gave them to wear, then read them stories about each group. Later on, the kids were much likelier to remember the good things that had been said about their own group and the less good things said about the other. “Kids show these preferences right away, in the lab, on the spot,” Dunham says.
Preschoolers, of course, can always change the color of their T shirt, but it’s a lot harder to change the color of your skin. Before the 1500s, most humans were unable to travel far enough from their homeland to encounter people of other races. When we did, our brains went slightly nuts. The human mind is what research psychologist Joshua Correll of the University of Chicago calls a “meaning-making machine.” Constantly pelted by information, it needs to make order of the storm, and one of the best ways to do that is by sorting things. There are big things and small things, loud things and soft things, hot things and cold things. “The categories may not all be real,” says Moses, ” but they’re a way of trying to understand things.”