Humans, unlike most other mammals, cooperate in large groups of unrelated individuals. Such coaction can be evolutionarily stable if free-riders are punished. But it is harder to explain the origins of punishment, because punishing free riders is costly when most individuals are defectors. Allowing individuals to opt out of the cooperative venture is one measure third edition by the editors of the stars and bars heritage® dictionary. copyright © 2003 to this problem.

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