Biographical note: GintisHerbert: Herbert Gintis holds faculty positions at the Santa Fe Institute and Central European University. He is the author of "Game Theory Evolving" (Princeton), coauthor of "A Cooperative Species: Human Reciprocity and Its Evolution" with Samuel Bowles (Princeton), and...

Human groups maintain a high level of sociality despite a low level of relatedness among group members. The behavioral basis of this sociality remains in doubt. This paper reviews the evidence for an empirically identifiable form of prosocial behavior in humans, which we call "strong...