Altruists with Green Beards

Abstract

If cooperative dispositions are associated with unique phenotypic features (’green beards’), cooperative individuals can be identified. Therefore, cooperative individuals can avoid exploitation by defectors by cooperating exclusively with other cooperative individuals; consequently, cooperators flourish 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, selfish individuals earn higher incomes than conditional cooperators. Thus, although subjects may predict other players’ choices better than chance evolution favors the selfish subjects, i.e., the experimental evidence does not support the green beard approach towards the evolution of cooperation.