HOMOPHILY
Since our preferences are strongly influenced by whom we associate with, social divides are particularly costly because there is very little mixing across these divides; people tend to associate with others like themselves.
In US schools, black teenagers mostly associate with blacks, and whites with whites.55 This is what sociologists call homophily. For obvious reasons, this is especially true of those from the largest social group in the school. Those who are a part of a small minority have no choice but to have relatively more friends outside their group.56This does not have to be evidence of intense prejudice. That students in the biggest group do not reach out to outsiders can easily be explained by the fact it is easy for them to meet others like them, and therefore as long as they have a mild preference for their own group, they have no reason to reach out beyond it.
The source of the mild preference does not have to be a negative view of anyone else; it could just be that it is easier to be with people who speak the same language, who share the same gestures and the same sense of humor, who watch the same TV shows and enjoy the same music, or who make the same unstated assumptions about what is appropriate or not. Abhijit, who is from India, is always struck by how easy it is for him to talk to people from Pakistan, notwithstanding the past seventy years of animus between India and that country. The sense of what is funny or what is private (hint: South Asians are nosy), what creates intimacy and what distracts from it, is something, he says, instinctive in all of us South Asians, something partition did not manage to destroy.
The downside to this very natural behavioral pattern becomes evident when we meet people from other groups. We hold back; we walk on eggshells, rationing our human warmth because we worry we might be misunderstood. Or we blunder forward, giving offense when none was intended. Either way, something important gets lost, with the result that we are less likely to communicate smoothly with people from other groups.
This is partly why people mostly marry people like themselves. A little over fifty years after the landmark decision Loving v. Virginia, which in 1967 struck down prohibition of interracial marriage in the United States, only about one in six American newly married couples was biracial.57 In India, 74 percent of families say they believe marriages should be made within castes. Our research suggests this is in part because the men in each caste are looking for women who are the equivalents of their sisters (in other words, the familiar) and likewise for women, and the best place to find such a match is naturally within the group they belong to.58