Endogamy

Endogamy is the cultural practice of mating within a specific social group, religious denomination, caste, or ethnic group, rejecting any from outside of the group or belief structure as unsuitable for marriage or other close personal relationships. Its opposite, exogamy, describes the social norm of marriage outside of the group.

Endogamy is common in many cultures and ethnic groups. Several religious and ethnic religious groups are traditionally more endogamous, although sometimes mating outside of the group occurs with the added dimension of requiring marital religious conversion. This permits an exogamous marriage, as the convert, by accepting the partner's religion, becomes accepted within the endogamous group. Endogamy may result in a higher rate of recessive gene-linked genetic disorders.

Adherence
Endogamy can encourage sectarianism and serves as a form of self-segregation. For instance, a community resists integration or completely merging with the surrounding population. Minorities can use it to stay ethnically homogeneous over a long time as distinct communities within societies that have other practices and beliefs.

The isolationist practices of endogamy may lead to a group's extinction, as genetic diseases may develop that can affect an increasing percentage of the population. However, this disease effect would tend to be small unless there is a high degree of close inbreeding, or if the endogamous population becomes very small in size.

Social dynamics
The Urapmin, a small tribe in Papua New Guinea, practice strict endogamy. The Urapmin also have a system of kinship classes known as tanum miit. Since the classes are inherited cognatically, most Urapmin belong to all of the major classes, creating great fluidity and doing little to differentiate individuals.

The small community on the South Atlantic island of Tristan da Cunha are, because of their geographical isolation, an almost endogamic society. There are instances of health problems attributed to endogamy on the island, including glaucoma and asthma as research by the University of Toronto has demonstrated.

Genealogy
Endogamic marriage patterns may increase the frequency of various levels of cousin marriage in a population, and may cause high probability of children of first, second, third cousins, etcetera.

If a cousin marriage has accrued in a known ancestral tree of a person, in historical time, it is referred to as pedigree collapse. This may cause relations along multiple paths between a person's autosomal-DNA matches. It creates stronger DNA matches between the DNA matches than expected from the nearest path.

Cousin marriage should not be confused with double cousins, which do not cause a pedigree collapse. Certain levels of sibling marriage and cousin marriage is prevented by law in some countries, and referred to as consanguinity.

A long term pattern of endogamy in a region may increase the risk of repeated cousin marriage during a long period of time, referred to as inbreeding. It may cause additional noise in the DNA autosomal data, giving the impressions that DNA matches with roots in that region are more closely related than they are.

Examples
Examples of ethnic and religious groups that have typically practiced endogamy include:
 * Aari
 * Alawites
 * The Amish of North America.
 * Various Arab tribes
 * Assyrians, indigenous Christian people of upper Mesopotamia.
 * Armenians have a history of endogamy due to being almost entirely surrounded by Islamic neighbours while being a strongly Christian nation.
 * Coptic Christians
 * Daylamites, an ethnic group living south of the Caspian Sea in ancient and medieval Persia.
 * Druze
 * Gitanos typically practice endogamy within their raza, or patrigroup.
 * Greek Cypriots usually practice endogamy in order to maintain their status as the majority ethnic group on the island of Cyprus.
 * Iranian Turkmens
 * Lepcha, an ethnic group in India, Nepal, and Bhutan
 * Judaism traditionally mandates religious endogamy, requiring that both marriage partners be Jewish, while allowing for marriage to converts. Orthodox Judaism maintains the traditional requirement for endogamy in Judaism as a binding, inherent part of Judaism's religious beliefs and traditions.
 * The Knanaya, an endogamous group within the St. Thomas Christian Community of India. The community claims to have arrived to India in the fourth century and have been noted for their historical practice of endogamy.
 * Members of the Church of Jesus Christ of Latter-day Saints or other religious and/or cultural groups relating to Mormonism
 * Sayyids
 * Syrian Christians of Kerala, India - but Marital conversion is allowed.
 * Parsis.
 * Rajputs
 * The Vaqueiros de alzada of Spain.
 * Yazidis
 * Mandaeans
 * Most Hindu jatis on the Indian subcontinent, as well as many endogamous baradaris among South Asian Muslims
 * The Surti and the Kaanam (Bharuchis) of Gujarat. In Gujarat on either side of the Narmada River lies two major cities, Surat and Bharuch. Those lying south of Narmada are generally termed as Surtis and those north of it are termed as Kaanam. Endogamy was historically practiced amongst these two groups and it continued as they migrated to other regions of the world. Surtis would only marry Surtis and Kaanams would only marry Kaanams. It is only recently in the past 50 years that this practice has been discontinued. Before that, if a girl's family could not find a suitable partner amongst their group in the migrated land, they would prefer getting her married to a boy from back home (Gujarat) even though boys of marriageable age would be available in their land of migration.