Female college students ยฎrst played a pseudo-prisoner's dilemma (PPD) game with the experimenter, who followed a ยฎxed strategy. In the ยฎrst experiment the experimenter's strategies for dierent groups of subjects were: (a) play tit-for-tat; (b) play randomly; (c) always cooperate; (d) always defect (cooperation' and defection', deยฎned as in an actual prisoner's dilemma game). Only the tit-for-tat group increased cooperation over trials; other groups decreased cooperation. After playing the PPD with the experimenter, subjects played an actual prisoner's dilemma (PD) game with each other. In the PD game, subjects began cooperating moderately but cooperation deteriorated regardless of what the experimenter's strategy had been in the earlier (PPD) game. In a second experiment, subjects again played a PPD game with the experimenter and then played a PD game with each other. Half played one trial at a time as in the ยฎrst experiment while half played in patterns of four trials at a time. In the PD game, patterning of trials retarded the development of mutual defection regardless of previous experience. The cooperation-preserving eect of patterning of trials in this social task is compared with similar eects on individual tasks involving self-control and riskaversion.