Fans of a sports team. Members of a church congregation. Soldiers in the same unit. An in-group is a social group to which a person feels they belong. It provides identity, loyalty, and a sense of us. The flip side is the out-group — them — which may be viewed with suspicion or hostility.

William Graham Sumner introduced the terms in 1906. He argued that in-group favoritism is universal. People favor their own group in resource allocation, even when group membership is arbitrary. The Robbers Cave experiment in 1954 showed how quickly boys at a summer camp formed rival groups and hostility.

How in-groups work

• Shared symbols: flags, slang, dress
• Boundary maintenance: who is in, who is out
• Loyalty and sacrifice for members
• Bias against outsiders

In-groups can be a source of support and meaning. They can also fuel prejudice, discrimination, and conflict. The same mechanism that builds solidarity within can build hostility without.