Exogamy
A requirement, found in some form in the overwhelming majority of documented societies, that a person marry outside a defined group — most universally, outside the immediate family. Claude Lévi-Strauss read exogamy and the incest taboo underlying it as a rule of reciprocity in disguise: by prohibiting marriage within the group, it forces kin groups to exchange spouses with their neighbors, manufacturing alliance and obligation as a byproduct of the marriage rule itself.