Us and Them
Much of who we feel we are comes not from our individual traits but from the groups we belong to. Social identity theory, developed by Henri Tajfel, holds that part of a person's self-concept — their social identity — derives from membership in social groups (nation, team, profession, faith), and that people are motivated to see the groups they belong to positively, because those groups reflect on the self.
This produces a robust pair of biases. In-group favoritism is the tendency to favor and think well of one's own group (the in-group); out-group bias is the tendency to view other groups (out-groups) less favorably and to see their members as more alike ("they're all the same"). Tajfel's minimal group experiments showed how little it takes to trigger this: even when people were divided into groups by something utterly trivial and meaningless — the flip of a coin, a preference between two painters — they promptly favored their own group in allocating rewards.
The theory explains why intergroup conflict is so easy to ignite: raising the status of the in-group relative to the out-group boosts self-esteem, so competition and comparison between groups feel personally rewarding. It also points to a remedy — building a shared, superordinate identity that includes both groups can dissolve the boundary.
Common pitfall: assuming in-group favoritism requires a real history of conflict, competition, or grievance between the groups. Tajfel's minimal group studies show the opposite: merely being categorized into a group — even a meaningless one assigned at random — is enough to trigger favoritism. Categorization itself is the spark.