In the Roman Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for a declaration of nullity, but during the 11th and 12th centuries, dispensations were granted with increasing frequency due to the thousands of persons encompassed in the prohibition at seven degrees and the hardships this posed for finding potential spouses.It began to fall out of favor in the 19th century as women became socially mobile.The National Conference of Commissioners on Uniform State Laws unanimously recommended in 1970 that all such laws should be repealed, but no state has dropped its prohibition.
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.
Cousin marriage has often been chosen to keep cultural values intact, preserve family wealth, maintain geographic proximity, keep tradition, strengthen family ties, and maintain family structure or a closer relationship between the wife and her in-laws.
Many such marriages are arranged (see also pages on arranged marriage in the Indian subcontinent, arranged marriages in Pakistan, and arranged marriages in Japan).
Within the next two decades, numerous reports (e.g., one from the Kentucky Deaf and Dumb Asylum) appeared with similar conclusions: that cousin marriage sometimes resulted in deafness, blindness, and idiocy.
Perhaps most important was the report of physician Samuel Merrifield Bemiss for the American Medical Association, which concluded cousin inbreeding does lead to the physical and mental depravation of the offspring".
Despite being contradicted by other studies like those of George Darwin and Alan Huth in England and Robert Newman in New York, the report's conclusions were widely accepted.
These developments led to 13 states and territories passing cousin marriage prohibitions by the 1880s.
In the past, cousin marriage was practised 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.
When a question about cousin marriage was eventually considered in 1871 for the census, according to George Darwin, it was rejected on the grounds that the idle curiosity of philosophers was not to be satisfied.