The incest taboo is a universal or near-universal prohibition of sexual relationships between certain culturally specified relatives, most commonly between parents and their children, and between siblings. While the specific definition of which relatives are included in the taboo can vary across cultures, the prohibition against sexual relations within the immediate family is widespread. The incest taboo serves several important social functions, including preventing genetic defects that can result from inbreeding, maintaining clarity in family roles and boundaries, promoting exogamy (marriage outside the family) and the formation of broader social alliances, and reducing sexual competition and conflict within the family unit. The strength and enforcement of the incest taboo vary across societies, but it remains a fundamental norm in virtually all cultures.