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.
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In biology, exogamy more generally refers to the mating of individuals who are relatively less related genetically: that is, outbreeding as opposed to inbreeding. This benefits the offspring as it reduces the risk of the offspring inheriting two copies of a defective gene. Increasing the genetic diversity of the offspring is thought to improve their chances of surviving to reproduce themselves.
Scientists surmise that the drive in humans, as in many animals, to engage in exogamy (outbreeding) is evolutionarily adaptive, as it reduces the risk of children having genetic defects caused by inbreeding, as a result of inheriting two copies of a recessive gene. The genetic principles involved apply to all species, not just humans.
Individuals who breed with more 'exotic' (or distant) partners and avoid incestuous relationships tend to have healthier offspring, due to the benefits of heterosis. Maladapative genetic conditions are more likely to be inherited where inbreeding takes place, or within relatively closed populations over long periods of time. An example is cystic fibrosis, which has developed as a genetic disease inherited chiefly by people of European descent. Another genetic disease specific to certain populations is sickle-cell anemia, for which people of African descent are more at risk; it developed among Africans together with higher immunity to malaria, which is endemic on the continent. Offspring may not always inherit such adaptations that evolved in specific geographic areas. Genetic concerns are not the only cause for exogamy; many social and political aspects support this system of marriage, throughout societies and species.
Cultural exogamy is the custom of marrying outside a specified group of people to which a person belongs. Thus, persons may be expected to marry outside their totem clan(s) or other groups, in addition to outside closer blood relatives.
Researchers have proposed different theories to account for the origin of exogamy. Edvard Westermarck said an aversion to marriage between blood relatives or near kin emerged with a parental deterrence of incest. From a genetic point of view, aversion to breeding with close relatives results in fewer congenital diseases. If one person has a faulty gene, breeding outside his group increases the chances that his partner will have another functional type gene and their child may not suffer the defect. Outbreeding favours the condition of heterozygosity, that is having two nonidentical copies of a given gene. J. F. McLennan holds that exogamy was due originally to a scarcity of women among small bands. Men were obliged to seek wives from other groups, including marriage by capture, and exogamy developed as a cultural custom.
Émile Durkheim derives exogamy from totemism. He said that a people had religious respect for the blood of a totemic clan, for the clan totem is a god and is present especially in the blood, a sacred substance.
Morgan maintains that exogamy was introduced to prevent marriage between blood relations, especially between brother and sister, which had been common in an earlier state of promiscuity. Frazer says that exogamy was begun to maintain the survival of family groups, especially when single families became larger political groups. Lang in 1905 argued against Howitt's claim of group marriage and claims that so-called group marriage is only tribe-regulated licence.
Claude Lévi-Strauss introduced the "Alliance Theory" of exogamy, that is, that small groups must force their members to marry outside so as to build alliances with other groups. According to this theory, groups that engaged in exogamy would flourish, while those that did not would all die, either literally or because they lacked sufficient ties for cultural and economic exchange, leaving them at a disadvantage. The exchange of men or women served as a uniting force between groups.
Dual exogamy is a traditional form of arranging marriages in numerous modern societies and in many societies described in classical literature. It can be matrilineal or patrilineal. It is practiced by some Australian tribes, historically widespread in the Turkic societies, Taï societies (Ivory Coast),Eskimo, among Ugrians and others. In tribal societies, the dual exogamy union lasted for many generations, ultimately uniting the groups initially unrelated by blood or language into a single tribe or nation.
Linguistic exogamy is a form of cultural exogamy in which marriage occurs only between speakers of different languages. The custom is common among indigenous groups in the northwest Amazon, such as the Tucano tribes. It is also used to describe families in Atlantic Canada with a Francophone and an Anglophone parent.