WORKING-MEMORY CONSTRAINS HUMAN COOPERATION IN THE PRISONERS-DILEMMA

Many problems in human society reflect the inability of selfish partie s to cooperate, The 'Iterated Prisoner's Dilemma'' has been used widel y as a model for the evolution of cooperation in societies, Axelrod's computer tournaments and the extensive simulations of evolution by Now ak and Sigmund and others have shown that natural selection can favor cooperative strategies in the Prisoner's Dilemma. Rigorous empirical t ests, however, lag behind the progress made by theorists, Clear predic tions differ depending on the players' capacity to remember previous r ounds of the game, To test whether humans use the kind of cooperative strategies predicted, ne asked students to play the iterated Prisoner' s Dilemma game either continuously or interrupted after each round by a secondary memory task (i,e,, playing the game ''Memory'') that const rained the students' working-memory capacity, When playing without int erruption, most students used ''Pavlovian'' strategies, as predicted, for greater memory capacity, and the rest used ''generous tit-for-tat' ' strategies. The proportion of generous tit-for-tat strategies increa sed when games of Memory interfered with the subjects' working memory, as predicted, Students who continued to use complex Pavlovian strateg ies mere less successful in the Memory game, but more successful in th e Prisoner's Dilemma, which indicates a trade-off in memory capacity f or the two tasks, Our results suggest that the set of strategies predi cted by game theorists approximates human reality.