The study of the psychological basis of intergroup relations is one of the major endeavors in social psychology (see, e.g., Brewer & Brown, 1998; Brown & Gaertner, 2001; Sedikides et al., 1998; Stephan & Stephan, 1996a). This interest is not surprising in view of the fact that most social life takes place within groups. Individuals are born into a group or organize themselves into groups, and as a result most of their behavior is performed within a group framework. Also, as a consequence of being part of a group, people develop their social identity as group members, and much of their thinking, feeling, and acting is carried out in the framework of knowledge about this identity. Because group membership, as reflected in a person's social identity, is one of the most salient and important human characteristics, individuals not only consider themselves as group members but also perceive and treat others according to their group membership.
The categorization of self as a group member and others in terms of group membership is a pervasive and central human cognitive process that enables the organization of the complex social world into a meaningful structure (Tajfel, 1969, 1981b). In this process, individuals aggregate people who share particular properties into one category and view them as a separate entity. There are numerous ways to classify people into social categories because humans have many different characteristics.