Cooperative and competitive interaction (interpersonal relationship) between pairs of retarded and nonretarded children of ages 6 to 7 and 11 to 12 were assessed in a situation involving a marble pull apparatus in which competitive interaction was nonadaptive in terms of reward attainment. The retarded group was significantly more cooperative than the nonretarded group and the 6 to 7 year retarded group was more cooperative than the 11 to 12 year retarded group. The results were discussed in relation to previous developmental studies of cooperation and competition and placed in the context of cognitive and reinforcement theories of social development. (Author)