THE GREATER GENEROSITY OF THE SPATIALIZED PRISONERS-DILEMMA

The iterated Prisoner's Dilemma is the standard model for the evolutio n of cooperative behavior in a community of egoistic agents. Within th at model, a strategy of ''tit-for-tat'' has established a reputation f or being particularly robust. Nowak and Sigmund have shown that in a w orld of imperfect information it is not tit-for-tat that finally trium phs, however, but ''generous tit-for-tat''-a variant which ''forgives' ' with a probability of 1/3. In the present study, a spatial dimension is added to the standard model. The surprising result is that generos ity is favored even more: the optimal strategy in a spatialized Prison er's Dilemma carries a forgiveness probability of 2/3-twice as generou s as ''generous tit-for-tat''.