Whether they are proposing to build a wall or to exit an international coalition, populist politicians like to pitch themselves as keeping “outsiders” at bay, and it clearly strikes a chord with their home crowd. To understand this phenomenon, evolutionary and social psychologists have offered a simple explanation. Humans, we’re told, have a deep-rooted inclination to mistrust “the other”—people who do not belong to our community or ingroup.
Classic work published in 1970 by the Polish-born psychologist Henri Tajfel showed how rapidly and arbitrarily teenage schoolboys form a sense of loyalty to their own group, and a bias against the out-group, even when group membership was based on nothing more than preference for one abstract artist or another. More recently, research shows that even preschoolers have a preference for playing with children of their own ethnicity or those who speak the same language.
One evolutionary hypothesis for our tendency toward ingroup loyalty is that it would have been advantageous to our tribal hunter-gatherer ancestors in their competition with rival tribes (as groups with more loyal and devoted members would have been more likely to survive and reproduce). The warring behavior seen in our chimpanzee cousins, who form coalitions to steal the territory of rival groups, is cited as evidence that supports this theory.