Contrary to induced subject expectations, the programmed party performed in a competitive rather than the promised cooperative way, resulting in losses to the subject. Subjects playing as individuals or as members of a group, but without group interactions, were permitted to administer a painful white noise to the other party following the norm violation. Results indicate no significant differences in aggression by subjects acting as individuals and those acting in the groups' interests, suggesting that intragroup interaction is necessary to the greater aggression usually exhibited by groups. Subjects responded more aggressively toward an individual than a group opponent, and males responded more aggressively than did females. The greater the loss experienced from the norm violation, the greater the aggressive reaction, particularly for subjects playing as members of a group. Results are discussed in terms of a behavioral interaction model. 3 tables, 1 note, and 58 references.