In-Group Out-Group Bias

We automatically favor members of our own group and apply harsher judgment to out-group members — even when the group distinctions are trivially arbitrary.

• In Henri Tajfel's experiments, people randomly sorted into 'blue' and 'red' groups immediately allocated more resources to their own group — despite the groups being meaningless.
• Sports fans believe referees systematically favor the opposing team — a bias observed in fans of every team, simultaneously.
• Soldiers who would never harm a civilian can harm an enemy combatant with similar demographics — the in-group/out-group distinction overrides human empathy.