We all know that having friends is important, but why do we form friendshipswith some people and not others? In some ways, the answer is simple: you becomefriends with someone because you have things in common. Maybe you both like thesame soccer team. Or perhaps you both love to play video games. Or maybe yourpersonalities are similar: you’re a bit shy and the other person is, too.But two American scientists, Dr. Peter DeScioli and Robert Kurzban, found that weform friendships with certain people for another reason: because these relationshipsprotect us in some way. Their research also showed that we rank our friends on howlikely they are to “have our back”—that is, to support us when there is trouble. Themore likely a person is to help you, the closer a friend he or she is. For this reason,it’s possible to be friends with someone who is different from you. You get alongbecause the person can help you in some way, the researchers say, and that’s evenmore important than your differences.