Most behavioral scientists today accept the basic premise that human beings are adapted for group living. Even a cursory review of the physical endowments of our species – weak, relatively slow-footed, extended infancy – makes it clear that we are not suited for survival as lone individuals, or even as small family units. We require groups to survive, and our psychology has been shaped by the necessity of accommodating to, cooperating with, and coordinating with others as members of social groups.

My own work as a social psychologist has focused on this group-living aspect of human nature. I have sought to understand more about our need to belong to groups and how our social group memberships influence the way we think about and act toward others. The framework for my research is the concept of social identity – the idea that individuals’ sense of self includes more than their personal, individualized identity, but also incorporates the collective identity of the groups to which they belong. Social identity involves a shift from “I” to “we,” so that the sense of self (and well-being) becomes inextricably tied to the fortunes and destiny of the group as a whole.

One consequence of our strong need to belong and our social identification with groups is ethnocentrism. The term “ethnocentrism” was coined in 1906 by William Graham Sumner in his book Folkways. Sumner made the observation that human societies are universally characterized by differentiation into “ingroups” and “outgroups” – the distinctions that demarcate boundaries of loyalty and cooperation among individuals. Attitudes and values are shaped by this ingroup–outgroup distinction in that individuals view all others from the perspective of the ingroup (hence the term, ethnocentrism).

Sumner's conceptualization of ethnocentrism contained two important assertions. First, that all human societies divide the world into “us” (ingroup) and “them” (outgroups); and second, that people favor their ingroups, evaluating them more positively than outgroups and reserving loyalty and trust for fellow ingroup members. Since the conceptualization, results from years of social psychological on intergroup relations have strongly substantiated Sumner's claims about ethnocentric attitudes. Hundreds of studies in the laboratory and the field have documented ingroup favoritism in myriad forms.