Endogamy is the practice of marrying within a social group because of social norms that encourage or require it. An example of endogamy is that practiced by the tribes of Iraq. Because tribal membership is determined by patrilineal lines, a woman who marries outside a tribe leaves that tribe. As a result, marriage between cousins or second cousins is frequent and encouraged.

The opposite of endogamy is exogamy.

See also: incest.