Endogamy refers to the practice of marriage within a social group. There are a number of different forms, based on convenience, religious beliefs, cultural values, or a desire to consolidate power, among other things. The practice is widespread throughout the world, especially among small social groups which are concerned about the possibility of dying out. When one marries outside of a social group, it is called exogamy.
Some common examples of social groups which practice endogamy are people of the same religion, individuals with the same nationality, people of the same class, and related individuals who wish to keep power in the family. Essentially any social group can practice endogamy, although the larger the group, the more successful it will be in the long term. Small groups may actually cause themselves to die out as a result of the practice, by concentrating deleterious genes which lead to sterility, serious birth defects, and other issues.
Often, social groups simply tend towards marriage within the group, with people preferring partners with similar life and cultural experiences. Sometimes, endogamy is actually heavily enforced through centuries of custom, laws, or cultural pressure. In some regions of the world, certain types — such as lineage endogamy — are actually outlawed, due to the risk of birth defects and developmental disabilities.
There are four rough categories of endogamy: caste, village, class, and lineage. In the first sense, people within a rigid caste system prefer to marry people of the same caste. This is particularly common in India, a nation with an extensive caste system. Village endogamy occurs when people in the same village or town marry each other; sometimes this is done by convenience, since other partners may be difficult to find, especially in highly rural areas.
Class endogamy is one of the more widespread forms. It refers to marrying within a particular class; for examples, explore the wedding announcements in a large metropolitan newspaper, which typically document unions of wealthy and powerful individuals. Cultural values often enforce this form, with practitioners seeking out partners with similar beliefs and life histories. Finally, lineage endogamy refers to keeping marriages within an extended family; the Egyptian royal family, for example, followed this practice.
Refugees and migrant communities often practice endogamy in an attempt to keep their culture intact. This is especially common with small communities which would otherwise be swamped by a larger population.