In-group bias, in-group favoritism, in-group–out-group bias and intergroup bias are all terms used to describe the fact that we prefer people who we perceive as belonging to the same group as ourselves over "outsiders". The bias is so strong that it starts appearing in groups that have just been constituted, even if what the group members have in common is minimal (for example, having the same eye color). This bias affects how we perceive others and can result in discrimination. Some theories about the causes of this bias include the competition that can exist between groups, and the need to maintain self-esteem.
In a study by Miller, Downs and Prentice conducted in 1998, participants were led to believe that they were playing a prisoner's dilemna game with another person, who they were sometimes told shared their birthday. If the participant was told that the other person shared his birthday, he was more likely to choose to 'cooperate', meaning that his fictional birthday-sharer would receive more money.