Exogamy is the rule that marriage must take place outside one's own social group. The group might be a family, a lineage, a clan, or a village. The rule forces alliances between groups and prevents the kind of close interbreeding that can concentrate genetic disorders. It is one of the most widespread marriage regulations in human societies.

In many cultures, exogamy works alongside a ban on incest. You must marry outside your lineage, but also within a broader category that is considered appropriate. A classic example is cross-cousin marriage, where a man marries his mother's brother's daughter or his father's sister's daughter. This keeps alliances circulating between two lineages over generations.

Why it exists

• Creates political and economic ties between groups.
• Reduces the risk of inbreeding.
• Prevents property from concentrating too tightly.
• Reinforces the boundary of the group you must marry outside of.

Exogamy and endogamy often operate at different levels. You marry outside your family but inside your religion, caste, or class. Together they shape who is a possible partner and who is not.