Altruists with green beards

If cooperative dispositions are associated with unique phenotypic features , cooperative individuals can be identi ed. Therefore, cooperative individuals can avoid exploitation by defectors by cooperating exclusively with other cooperative individuals; consequently, cooperators ourish and defectors die out. Experimental evidence suggests that subjects, who are given the opportunity to make promises in face-to-face interactions, are indeed able to predict the partner's behavior better than chance in a subsequent Prisoners' Dilemma. This evidence has been interpreted as evidence in favor of green beard approaches to the evolution of human cooperation. Here we argue, however, that the evidence does not support this interpretation. We show, in particular, that the existence of conditional cooperation renders subjects' choices in the Prisoners' Dilemma predictable. However, although subjects predict behavior better than chance, sel sh individuals earn higher incomes than conditional cooperators. Thus, although subjects may predict other players' choices better than chance evolution favors the sel sh subjects, i.e., the experimental evidence does not support the green beard approach towards the evolution of cooperation