In intergroup conflict a third party sometimes intervenes aggressively into the interactions between the concerned parties, escalating the conflicts. We hypothesized that the third party will become aggressive if they perceive that a fellow member of their group, with whom they strongly identified themselves, is harmed by the other group and that the perception of harm is more definitely determined by unfairness than by the objective severity of the harm. Group identification was manipulated by cooperative ingroup activities. Half of the participants observed that an ingroup fellow member was harmed by an outgroup member based on unfair evaluation, while the others observed that the harm was given based on fair evaluation. They were then given a chance to retaliate against the harm-doer by creating unpleasant noises. The results indicated that both aggressive motivation and behaviors were increased by unfair harm only when participants strongly identified with the ingroup. The group identification did not affect the perception of unfairness. These results suggest that symbolic or psychological harm affects third-party aggression.
