Human societies throughout history have frequently enforced either exogamous or endogamous rules concerning the selection of marriage partners. In the case of exogamy, even the exact group into which a person must marry can be specified, such as in the case of the Haida and Tlingit that are divided into two moieties and marriage to a member of the other moiety was required. Endogamous societies may have equally strong prohibitions on marrying outside one's own group. The royal and noble families of Europe became severely inbred, although the original intention was intermarriage in order to strengthen political alliances with the royal families of other nations.
As human history has progressed, the appropriateness of exogamy and endogamy has fluctuated. In cases where a minority group has been threatened, endogamy has solidified and maintained their identity. On the other hand, when alliances between otherwise warring factions has been the goal, exogamy in the form of intermarriage between the two has maintained harmony and prosperity and the health of the lineages.
Exogamy is marriage outside of one's tribe or social unit. The word comes from the Greek, from exo- + gamos, literally "outside marriage." The opposite of exogamy is endogamy—marriage to someone inside the same group.
Different theories have been proposed to account for the origin of exogamy.
John Ferguson McLennan introduced the terms "exogamy" (marriage outside the group, as in bride capture between warring tribes) and "endogamy" (marriage within a specific group, leading to monogamy and determination of kinship through males, rather than females). He argued that exogamy was originally due 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.
McLennan also connected exogamy with totemism, suggesting that the worship of plants and animals by primitive cultures was the first religion. Functioning as a "totem," an animal, plant, or other object provided symbolic meaning for early social groups. McLennan argued that the entire human race had passed through a totemic stage at some point in the distant past and such totemism cemented the clan as the unit of society, particularly in the context of exogamy and matrilineal descent.
Émile Durkheim suggested that the origin of exogamy is religious. In his argument regarding the taboo against incest, Durkheim stated that its root lay the law of exogamy, which is defined as prohibition to marry inside the same clan, where a clan is defined by people who have the same totem.
Examples of this type include the Haida and Tlingit of the Pacific Northwest, whose societies are based on moiety lineages. Haida society is divided into two groupings, one called "Raven" and the other "Eagle." The moieties and their subgroups of clans, or matrilineal lineages, own unique combinations of crests and other intellectual properties such as songs and names. Marriages had to take place between Eagles and Ravens, rather than those who belonged to the same moiety, and children became members of the same moiety as their mother. Similarly the Tlingit society is wholly divided into two distinct moieties, termed Raven and Eagle or Wolf. Members of one moiety traditionally may only marry a person of the opposite moiety.
Sigmund Freud, in his, Totem and Taboo, discussed various ways in which the exogamy of the totem system prevented incest not only among the nuclear family, but among the extended family, and the whole totem clan. He explained that the existence of marriage restrictions between the members of the same tribes derived from a time when group marriages were allowed (but not "incest" within a group family).
James Frazer's Totemism and Exogamy (1910) rejected the contention that totemism was the earliest form of religion, considered totemic practices rather to be a form of magic, historically prior to the emergence of religion. Frazer also disagreed with the idea that totemism was necessarily associated with exogamy, noting that there were numerous cultures where totemism existed without exogamy, or vice versa.
Lewis Henry Morgan, in his work on kinship, maintained that exogamy was introduced to prevent marriage between blood relatives, especially between brother and sister, which had been common in a previous state of promiscuity.
Charles Darwin said that exogamy arose from the strongest male driving the other males out of the group. Edvard Westermarck suggested that exogamy arose from the instinctive aversion to marriage between blood relatives or near kin, that is, a 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 favors the condition of heterozygosity, that is having two non-identical copies of a given gene.
It has also been suggested that exogamous rules arose for political reasons, promoting marriage treaties between groups. 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 did not 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.
In biology, exogamy more generally refers to the mating of individuals who are less related genetically, that is outbreeding as opposed to inbreeding. This benefits the offspring by avoiding their chance of inheriting two copies of a defective gene. It also increases 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 human beings 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 genetics defects caused by inbreeding.
There are many conditions that are more likely where inbreeding takes place. One example is cystic fibrosis when a couple of European origin have children; another is sickle-cell anemia when a couple of African origin 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.
Endogamy is the practice of marrying within one's social group. Cultures who practice endogamy require marriage between specified social groups, classes, or ethnicities. Just about any accepted social grouping may provide a boundary for endogamy. Thus, nationality may suffice, such that a Danish endogamist would require marriage only to other Danes. Despite the fact that many people tend to marry members of their own social group, there are some groups that practice endogamy very strictly as an inherent part of their moral values, traditions, or religious beliefs. For example, the caste-system of India is based on an order of (mostly) endogamous groups. In endogamous groups marriage outside one's group may be forbidden, with penalties ranging from mild disapproval to exile, disowning, or even death.
Endogamy encourages group affiliation and bonding. It is a common practice among displanted cultures attempting to make roots in new countries as it encourages group solidarity and ensures greater control over group resources (which may be important to preserve where a group is attempting to establish itself within an alien culture). It helps minorities to survive over a long time in societies with other practices and beliefs. Famous examples of strictly endogamous religious groups are the Yazidi in Northern Iraq (under Islamic majority), the Armenian-Iranians, Orthodox Jews, Old Order Amish, Jehovah's Witnesses, and the Parsi minority in India. Many religions require both parties to be of the faith, or converts, in order to participate in a religious marriage ceremony.
Ironically, endogamy can also lead to a group's extinction rather than its survival. While long serving to preserve their religion, the Samaritans' practice of endogamy now threatens this community. Refusal to intermarry as well as to accept converts has meant that the population of this ethnic group has dwindled to less than a thousand, and the small gene pool has contributed to disease within the community.
The taboo of incest has been discussed by many social scientists. Anthropologists attest that it exists in most cultures. As inbreeding within the first generation would produce expression of recessive traits, the prohibition has been discussed as a possible functional response to the requirement of culling those born deformed, or with undesirable traits. The eugenicists used breeding techniques to promulgate their ideas of human perfection and "illness" on all humans.
The royal and noble families of Europe have close blood ties which have been strengthened by royal intermarriage. Examples abound in every royal family; in particular, the ruling dynasties of Spain and Portugal were very inbred. Several Habsburgs, Bourbons, and Wittelsbachs married aunts, uncles, nieces, and nephews. Even in the British royal family, which is very moderate in comparison, there has scarcely been a monarch in 300 years who has not married a (near or distant) relative. Indeed, Queen Elizabeth II and her husband Prince Philip, Duke of Edinburgh are second cousins once removed, both being descended from King Christian IX of Denmark. They are also third cousins as great-great-grandchildren of Queen Victoria of the United Kingdom. European monarchies did avoid brother-sister marriages, though Jean V of Armagnac was an exception.
Other examples of royal family intermarriage include:
Intermarriage in European royal families is no longer practiced as often as in the past. This is likely due to changes in the importance of marriage as a method of forming political alliances through kinship ties between nobility. These ties were often sealed only upon the birth of progeny within the arranged marriage. Marriage was seen as a union of lines of nobility, not as a contract between individuals as it is often seen today. During the tumult of the removal, sometimes by revolution, of most lines of nobility from state government, it became less important to marry for the good of the respective monarchies and the states they governed. More marry for "love," well illustrated by the second marriage of Prince Charles of the United Kingdom.
It is not necessarily the case that there was a greater amount of inbreeding within royalty than there is in the population as a whole: It may simply be better documented. Among genetic populations that are isolated, opportunities for exogamy are reduced. Isolation may be geographical, leading to inbreeding among peasants in remote mountain valleys. Or isolation may be social, induced by the lack of appropriate partners, such as Protestant princesses for Protestant royal heirs. Since the late Middle Ages, it is the urban middle class that has had the widest opportunity for outbreeding.
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