Null results for the steal-framing effect on out-group aggression.

Whether intergroup conflict is a necessary condition for the evolution of human prosociality has been a matter of debate. At the center of the debate is the coevolutionary model of parochial altruism-that human cooperation with in-group members has coevolved with aggression toward out-group members. Studies using the intergroup prisoner's dilemma-maximizing difference game to test the model have repeatedly shown that people do not exhibit out-group aggression, possibly because of an inappropriate operationalization and framing of out-group aggression. The coevolutionary model predicts out-group aggression when the actor understands that it will lead to the in-group's benefit. However, in the game, such an aspect of out-group aggression that benefits the in-group is typically not well communicated to participants. Thus, this study tested the hypothesis that out-group aggression in the game would be promoted by a framing that emphasizes that attacking out-group members enhances the in-group's gain. Results of two laboratory experiments with 176 Japanese university students in total showed that such a framing did not promote out-group aggression and individuals invested more money to cooperate with in-group members only, avoiding the strategy of cooperating with in-group members to harm out-group members. These results do not support the coevolutionary model.