The purpose of the present study was to examine the effect of similarity of group member and cooperative experience on aggression by the third party person who was not personally suffered. 96 participants were assigned into one of four conditions; 2 (high similarity or low similarity) x 2 (cooperative experience or non-cooperative experience). In every condition, the participants observed the victim and then, they were given a chance to retaliate against the harm-doer. The result showed that the cooperative experience increased aggressive behavior. And the perception of threat and aggressive intent mediated between them. Further, the cooperative experience directly increased the aggressive intent and the similarity prompted the aggressive response. We discussed these results from a social identity perspective.

Key words: third party aggression, group identification, cooperative experience