For the societies that practice marriage there are rules about whom one can marry and cannot marry (note: not all groups marry; traditionally the Na in Southwest China do not marry). All societies have some form of an incest taboo that forbids sexual relationships with certain people. This is variable from culture to culture. Several explanations have been preferred to explain the origins of incest taboos. One cites biological reasons. Non-human primates seem to have an instinctual aversion to having sex with near relatives, so perhaps the same happens for humans. Another biological reason is that the incest taboo was established to maintain biological diversity. This suggests that people understood the consequences of breeding with relatives.
Another theory suggests that familiarity breeds contempt, while yet another suggests that incest taboos were developed to ensure that alliances were made outside of the family. Whatever the case may be, there have been culturally approved violations of the incest taboo usually in royal families such as those in pre-contact Hawaii, ancient Peru and Egypt (Bonvillain 2010).
Exogamy stipulates that an individual must marry outside of a kin, residential, or other specified group. For instance, the Yanomami must marry outside of their residential village. Endogamy, on the other hand, stipulates that an individual must marry within a specified kinship categories or social group. The classic example of endogamy is the Indian caste system. Arranged marriages are quite common among human societies. With arranged marriages, family elders, usually the parents, choose spouses for their children. Arranged marriages promote political, social, and economic ties.
Sometimes within the practices outlined above, other rules that single out certain kin as ideal marriage partners are adhered to. Cross-cousin marriage unites cousins linked by parents of opposite sex (brother/sister) while parallel-cousin marriage unites the children of siblings of the same sex. The benefits of these types of marriages is that it helps to maintain the family lineage.
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