Retaliatory aggression in individuals and groups was compared. Physically and verbally attacked males in group or individual conditions were given the opportunity to retaliate against their tormentor by administering him electric shocks in the context of a bogus learning experiment. Groups retaliated much more severely than did individuals. Analysis of the group decision-making dynamics suggested the process of social modelling as a mediator of group aggression. Additional data suggested the mediating influence of diffusion of responsibility as well.