The authors previously developed multiple identification theory (MIT) as a system of simulation game design intended to promote attitude change. The present study further tests MIT's effectiveness. The authors created a game (CULTURE & CREED) via MIT as a complex simulation of Middle Eastern conflict resolution, designed to change attitudes and behaviors from competition to cooperation. Participants completed pretest questionnaires measuring "cooperation versus competition," "truthfulness versus deception," and "viewing a situation from a social versus an individual perspective." Participants played the simulation twice and then completed posttest questionnaires. The authors recorded behavioral measures of players' cooperative or aggressive actions during the simulation. Statistical analysis found significant change in a cooperative direction for both attitudinal and behavioral measures. Robust effect sizes were observed. More than 60% of participants reported transferring the cooperative attitudes they learned in the game to life situations. Simulations designed with MIT could be applied to other conflict scenarios and used to modify attitudes. (Contains 10 tables and 1 figure.)