Exogamy has two related definitions, both biological and cultural.
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.
There may be a drive in humans as well as animals to engage in exogamy (outbreeding); this is because procreating with individuals who are more closely related means any children will be more likely to suffer from genetic defects caused by inbreeding. Individuals who breed with more exotic partners and thereby avoid incestuous relationships will have healthier offspring due to the benefits of outbreeding. 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. Therefore, the drive to reproduce with individuals genetically different from oneself may derive from an innate drive to seek the healthiest combination of DNA possible for one's offspring by outbreeding.
Conversely, exogamy can cause serious difficulties in modern medicine; for example, exogamous patients who require organ transplants may find it difficult to find matching donors.
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 group(s) may be forbidden.
The opposite of exogamy is endogamy. Different theories have been proposed to account for the origin of exogamy. Some are detailed here.
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 and Howitt maintain 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 this is the true solution, that it really introduced group marriage, which is an advance to monogamy, and that the most complete record of this is the classificatory system of relationship. Lang, however, denies there is any group marriage, and says that so-called group marriage is only tribe-regulated licence. Hill-Tout writes that exogamous rules arose for political reasons by marriage treaties between groups. Darwin denies primitive promiscuous intercourse, and says exogamy arose from the strongest male driving the other males out of the group. This is also the opinion of Lang, Atkinson, and Letourneau .
Claude Levi-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 didn't would die out, either literally or because they lacked ties for cultural and economic exchange, leaving them at a disadvantage. The exchange of men and/or women therefore served as a uniting force between groups.
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