The deindividuation theory focuses primarily on inferred intraindividual processes such as guilt, self-awareness, and the capacity for self-regulation to account for group aggression. The emergent norm theory represents a more sociological, group-level approach to the study of group aggression. The two theories were tested using the frustration-aggression theory of Brown which assumes that individuals and groups, male or female, will react with angry aggression when important social justice norms are violated. It was hypothesized that groups would be more aggressive than single individuals, males would be more aggressive than females, and more aggression would occur when people could be easily identified than when they remained anonymous to each other. The study sample included 144 students, half male and half female, at the University of Utrecht in the Netherlands. Consistent with the frustration-aggression theory, individuals and groups reacted with more angry aggression to norm violations of others than when group members adhered to conventional norms of appropriate behavior. Groups were more likely to react with intense aggression than single individuals to norm violations. Males reacted with more instrumental and angry aggression to norm violations than females. Significant gender differences in aggressive behavior between individual and group situations, however, were not observed. Generally, more support was found for the emergent norm theory than for the deindividuation theory. 68 references, 6 tables, and 1 figure