Professor Richerson and his longtime collaborator, Robert Boyd, an anthropologist at U.S.C.'s hated enemy, U.C.L.A., argue that we will sign up for membership in tribelike groups for the same reason birds sing: It feels right because we evolved to do it. ''We want to live in tribes,'' Professor Richerson says. Humans are ''looking to be told what group they belong to, and then once they do that, they want to know, 'What are the rules?'''The tricky part, says Professor Sapolsky of Stanford, Cal-Berkeley's bitter rival, is that humans alone among animals can think about what a tribe is and who belongs. ''Humans actually think about who is an 'us' and who is a 'them' rather than just knowing it,'' he says. ''The second it becomes a cognitive process, it is immensely subject to manipulation.''And, of course, studying the phenomenon won't make you immune. ''I'm true blue,'' says Professor Banaji, who taught at Yale from 1986 until 2002, when she joined the Harvard faculty. ''I was physically unable to sit through a women's basketball game between Harvard and Yale on the Harvard side.''