Researchers at Yale and UC San Diego found that among a sample of almost two thousand subjects, none of them related, pairs of friends were significantly more likely to share gene variants than pairs of strangers; on average, close friends were the genetic equivalent of fourth cousins, making them “functional kin.” “Not only do we form ties with people superficially like ourselves,” said sociologist Nicholas Christakis, one of the study’s authors, “we form ties with people who are like us on a deep genetic level.”