Exogamy is the social norm of marrying outside one's social group. The group defines the scope and extent of exogamy, and the rules and enforcement mechanisms that ensure its continuity. One form of exogamy is dual exogamy, in which two groups engage in continual wife exchange. In social science, exogamy is viewed as a combination of two related aspects: biological and cultural. Biological exogamy is marriage of nonblood-related beings, regulated by forms of incest law. Cultural exogamy is marrying outside a specific cultural group; the opposite being endogamy, marriage within a social group.