An excerpt:
http://scienceblogs.com/primatediaries/ ... -cheaters/. . . new research suggests that cooperation is a viable evolutionary strategy when individuals within the group collectively punish cheaters who don’t pull their weight. Robert Boyd, Herbert Gintis, and Samuel Bowles have just published a paper in the journal Science with a model showing how, so long as enough individuals work together to punish violators, each cooperative individual in the group can experience enhanced fitness as a result.