On the basis of social identity theory, we argue that the search for a positive social identity is characterized by the accentuation of perceived ingroup homogeneity relative to perceived outgroup homogeneity (the ingroup homogeneity effect). To test our specific hypotheses, we conducted an experiment in which some subjects were provided with a well-defined group membership and others were allocated to ill-defined groups. We also manipulated the information about the relative sizes of the groups. Subsequently, several measures of perceived dimensional and general group homogeneity were administered. As predicted, members of well-defined groups revealed the ingroup homogeneity effect for an attribute associated with the definition of their group. On the other hand, members of ill-defined groups displayed social creativity and selected alternative attributes to accentuate the relative ingroup homogeneity. Moreover, when intergroup comparisons did not contribute positively to the self-concept, subjects accentuated their positive personal identity and evinced the outgroup homogeneity effect. Consistent with previous research, subjects identified more strongly with a well-defined group when it was a minority rather than a majority. Minority members also perceived more general homogeneity within the ingroup than within the outgroup, whereas the majority members showed the opposite effect. Finally, the interrelationship between personal and social identity is briefly discussed.