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Exogamy is a social arrangement where marriage is allowed only outside of 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 a dual exogamy, in which two groups engage in continual wife exchange. Cultural exogamy is the marrying outside of a specific cultural group. The opposite of exogamy is endogamy, a marriage within a social group.
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 by avoiding the chance of the offspring inheriting two copies of a defective gene, and also by increasing the genetic diversity of the offspring, improving the chances that more of the offspring will have the required adaptations to survive.
Exogamy in humans
There may be a drive in humans as in many animals to engage in exogamy (outbreeding) because procreating with individuals who are more closely related means any children will be more likely to suffer from genetic defects caused by inbreeding. The genetic principles involved apply to all species, not just humans. Individuals who breed with more exotic partners and thereby avoid incestuous relationships tend to have healthier offspring due to the benefits of Heterosis. There are many conditions that are more likely where inbreeding takes place, one example being cystic fibrosis when a couple of primarily European genetics have children, another being sickle-cell anemia when a couple of primarily African genetics have children. However, the offspring are also susceptible to losing specific geographic adaptations. Genetic concerns are not the only cause for exogamy though. There are also many social and political aspects that affect this system of marriage, throughout different societies and species.
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Cultural Exogamy is the custom of marrying outside a specified group of people to which a person belongs. In addition to blood relatives, marriage to members of a specific totem, clan(s) or other groups may be forbidden. Different theories are proposed to account for the origin of exogamy. Edvard Westermarck said it arose in the aversion to marriage between blood relatives or near kin: that is, in horror of incest. From a genetic point of view, aversion to breeding with close relatives results in fewer congenital diseases, because, where one gene is faulty, there is a greater chance that the other - being from a different line - is of another functional type and can take over. Outbreeding thus favours the condition of heterozygosity, that is having two non-identical copies of a given gene. J. F. McLennan holds that exogamy was due originally to scarcity of women, which obliged men to seek wives from other groups, including marriage by capture, and this in time grew into a custom. Émile Durkheim  derives exogamy from totemism, and says it arose from a religious respect for the blood of a totemic clan, for the clan totem is a god and is especially in the blood. Morgan maintains that exogamy was introduced to prevent marriage between blood relations, especially between brother and sister, which had been common in a previous state of promiscuity. Frazer says that exogamy began to maintain the survival of family groups, especially when single families became larger political groups. Lang, however, argues 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 ties for cultural and economic exchange, leaving them at a disadvantage. The exchange of men or women therefore 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, Tai societies  Eskimo, among Finnic people 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. Also used to describe families in Atlantic Canada with a Francophone and an Anglophone parent.
- Interfaith marriage
- Interracial marriage
- Emmanuel Todd, author of several demographic-history textbooks on the impact of exogamy on political-religious ideology of the civ in question
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