The aim of the studies was to assess the effefcs of social categorization on intergroup behaviour when, in the intergroup situation, neither calculations of individual interest nor previously existing attitudes of hostility could have been said to have determined discriminative behaviour against an outgroup. These conditions were satisfied in the experimental design. In the first series of experiments, it was found that the subjects favoured their own group in the distribution of real rewards and penalities in a situation in which nothing but the variable of fairly irrelevant classification distinguished between the ingroup and the outgroup. In the second series of experiments it was found that: 1) maximum joint profit independent of group membership did not affect significantly the manner in which the subjects divided real pecuniary rewards; 2) maximum profit for own group did affect the distribution of rewards; 3) the clearest effect on the distribution of rewards was due to the subjects' attempt to achieve a maximum difference between the ingroup and the outgroup even at the price of sacrificing other ‘objective’ advantages.

The design and the results of the study are theoretically discussed within the framework of social norms and expectations and particularly in relation to a ‘generic’ norm of outgroup behaviour prevalent in some societies.