First-cousin marriage

Cousin marriage is marriage between cousins (i.e. people with common grandparents or people who share other fairly recent ancestors). Opinions and practice vary widely across the world. In some cultures and communities, cousin marriage is considered ideal and actively encouraged; in others, it is subject to social stigma. Cousin marriage is common in the Middle East, for instance, where it accounts for over half of all marriages in some countries. In some countries outside that region, it is uncommon but still legal. In others, it is seen as incestuous and is legally prohibited: it is banned in China and Taiwan, North Korea, South Korea, and in fewer than half of the United States. Supporters of cousin marriage where it is banned may view the prohibition as discrimination, while opponents may appeal to moral or other arguments. Worldwide, more than 10% of marriages are between first or second cousins.

In the past, cousin marriage was practiced within indigenous cultures in Australia, North America, South America, and Polynesia. Various religions have ranged from prohibiting sixth cousins or closer from marrying, to freely allowing first-cousin marriage. Cousin marriage is an important topic in anthropology and alliance theory.

Children of first-cousin marriages may have an increased risk of genetic disorders, particularly if their parents both carry a harmful recessive mutation, but this can only be estimated empirically, and those estimates are likely to be specific to particular populations in specific environments. Children of more distantly related cousins have less risk of harmful genetic mutations. In fact, a study of Icelandic records indicated that marriages between third or fourth cousins (people with common great-great- or great-great-great-grandparents) may be optimal, at least from the perspective of producing the most children and grandchildren.

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