People of all cultures belong to groups, and researchers have long known that individuals are more sympathetic to their own groups (ingroups) than to others (outgroups). This tendency to favour one’s own group over others exists even when the groups are arbitrarily formed, and is sometimes, but not always, accompanied by negative attitudes or hostility towards outgroups. Negative intergroup attitudes and behaviours may sometimes be adaptive, biologically or socially. Individuals may derive their sense of self from group membership, and may perceive outgroup members as less diverse than members of the ingroup. Group distinctions may form the basis for prejudice, stereotyping, and discrimination, particularly when group characteristics are salient, as in politics, religion, or ethnicity. Cultural context is an important contributing factor to group formation and individual sense of self within the group.