Exogamy is a social arrangement where marriage is allowed only outside a social group. The social groups define the scope and extent of exogamy, and the rules and enforcement mechanisms that ensure its continuity. In social studies exogamy is viewed as a combination of two related aspects: biological and cultural. Biological exogamy is marriage of non blood-related beings, regulated by forms of incest law. A form of exogamy is dual exogamy, in which two groups engage in continual wife exchange. Cultural exogamy is marrying outside a specific cultural group; the opposite being endogamy, marriage within a social group.