What motivates individual self-sacrificial behavior in intergroup conflicts? Is it the altruistic desire to help the in-group or the aggressive drive to hurt the out-group? This article introduces a new game paradigm, the intergroup prisoner’s dilemma-maximizing difference (IPD-MD) game, designed specifically to distinguish between these two motives. The game involves two groups. Each group member is given a monetary endowment and can decide how much of it to contribute. Contribution can be made to either of two pools, one that benefits the in-group at a personal cost and another that, in addition, harms the out-group. An experiment demonstrated that contributions in the IPD-MD game are made almost exclusively to the cooperative, within-group pool. Moreover, preplay intragroup communication increases intragroup cooperation, but not intergroup competition. These results are compared with those observed in the intergroup prisoner’s dilemma game, in which group members’ contributions are restricted to the competitive, between-group pool.