Experimental research on intergroup discrimination in favor of one's own group is reviewed in terms of the basis of differentiati on between in-group and outgroup and in terms of the response measure on which in-group bias is assessed. Results of the research reviewed suggest that (a) factors such as intergroup competition, similarity, and status differentials affect in-group bias indirectly by influencing the salience of distinctions between in-group and out-group, (b) the degree of intergroup differentiation on a particular response dimension is a joint function of the relevance of intergroup distinctions and the favorableness of the in-group's position on that dimension, and (c) the enhancement of in-group bias is more related to increased favoritism toward in-group members than to increased hostility toward out-group members. The implications of these results for positive applications of group identification are discussed. In 1906, sociologist William Sumner articulated a functionalist approach to the nature of intergroup attitudes in his exposition of the concept of ethnocentrism . The differentiation of peoples into distinct ethnic groups originates, according to Sumner, in context of the conditions of the struggle for existence. At the individual level, the psychological consequences of this differentiation both reflect and sustain the basic state of conflict between the in-group (or we-group) and out-groups (or others-groups):