Cousin marriage
Cousin marriage is important in several anthropological theories by prominent authors such as Claude Lévi-Strauss, Sir Edward Tylor, and Lewis Henry Morgan. Lévi-Strauss viewed cross-cousin marriage as a form of exogamy in the context of a unilineal descent group, meaning either matrilineal or patrilineal descent. Matrilateral cross-cousin marriage in societies with matrilineal descent meant that a male married into the family his mother's brother, building an alliance between the two families. However, marriage to a mother's sister daughter (a parallel cousin) would be endogamous, here meaning inside the same descent group, and would therefore fail to build alliances between different groups. Correspondingly, in societies like China with patrilineal descent, marriage to a father's brother's daughter would fail at alliance building. And in societies with both types of descent, where a person belongs to the group of his mother's mother and father's father but not mother's father or father's mother, only cross-cousin marriages would successfully build alliances.