But homophily, researchers said, is also the basis of tribalism, xenophobia and racism, the urge to “otherize” those who differ from you and your beloved friends in one or more ways.
The impulse can yield absurd results. One recent study from the University of Michigan had subjects stand outside on a cold winter day and read a brief story about a hiker who was described as either a “left-wing, pro-gay-rights Democrat” or a “right-wing, anti-gay-rights Republican.”
When asked whether the hypothetical hiker might feel chilly as well, participants were far more likely to say yes if the protagonist’s political affiliation agreed with their own. But a political adversary — does that person even have skin, let alone a working set of thermal sensors?
“Why must it be the case that we love our own and hate the other?” Nicholas Christakis of Yale University said. “I have struggled with this, and read and studied a tremendous amount, and I have mostly dispiriting news. It’s awful. Xenophobia and in-group bias go hand-in-hand.”
Game theory models predict it, real-life examples confirm it. “In order to band together, we need a common enemy,” Dr. Christakis said.
Fortunately, he added, no model insists that the out-group must be exterminated or otherwise eliminated from the scene. “It’s possible to treat the out-group with mild dislike or even grudging respect,” he said. “Cultivating in-group distinctiveness does not require that the other must be killed.”
Nevertheless, even the ordinary business of making friends is an exclusionary act, a judgment call, and therefore threaded with the potential for pain.
“A friendship is always a little bit of a conspiracy,” said Alexander Nehamas, a professor of philosophy at Princeton. “We two are here, they are over there, and we’re going to do our thing whether they want us to or not.”
And if they try to join us, we can say, no, sorry, that seat is taken. We’re saving it for a friend.
Who may not return the favor. Abdullah Almaatouq of the Massachusetts Institute of Technology and his colleagues recently showed that people are poor judges of who their friends are.
When the researchers asked 84 college students to identify which of their classmates qualified as friends, the researchers found that in half the cases, those labeled friends failed to reciprocate the designation.
Other studies have shown similar discordances or worse, with one survey revealing that 66 percent of supposed friendships were cases of unrequited like.
Friendships are also surprisingly fragile. Based on a detailed survey of 540 participants, researchers at Oxford University determined that people had a falling out with a member of their social circle about once every 7.2 months, or nearly two times annually, and that a year later 40 percent of those ruptures remained unhealed.
The overall rates of friendship conflict did not differ between men and women, but women were more likely to clash with close friends, to express feelings of anguish over the breakup, and to be more demanding of evidence of remorse before reconciling.
Sure, love may mean never having to say you’re sorry. But friendship is a stricter taskmaster, and sorry may not be enough.