Using a 2-person, Prisoner's Dilemma game, 2 related experiments were conducted to test the hypothesis that individuals would reciprocate cooperative behavior by a simulated partner. The 1st study systematically varied the conditional probabilities of cooperative and competitive responses by the simulated partner, given cooperative and competitive responses by S, respectively. The 2nd study tested the effects of a simulated partner who matched the previous response of S. In general, most Ss did not reciprocate cooperative choice. However, a marked sex difference was obtained. Males reciprocated cooperative choice more than females when such behavior maximized gain, but reciprocated less when such behavior was nonoptimal. (PsycINFO Database Record (c) 2016 APA, all rights reserved)