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By the 1870s, Lewis Henry Morgan (1818–1881) was writing about "the advantages of marriages between unrelated persons" and the necessity of avoiding "the evils of consanguine marriage", avoidance of which would "increase the vigor of the stock".
To many, Morgan included, cousin marriage, and more specifically parallel-cousin marriage, was a remnant of a more primitive stage of human social organization. Briggs appointed a commission to study "idiots" in the state, and this study implicated cousin marriage as responsible for idiocy.
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 24 of the 50 United States.
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.
Cousin and sibling marriage were legal in ancient Rome from the Second Punic War (218–201 BC), until it was banned by the Christian emperor Theodosius I in 381 in the West, and until after the death of Justinian (565) in the East, but the proportion of such marriages is not clear.
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.
Eventually, the nobility became too interrelated to marry easily as the local pool of unrelated prospective spouses became smaller; increasingly, large payments to the church were required for exemptions ("dispensations"), or retrospective legitimizations of children, in what amounted to a 'protection racket' by the church.
Later studies by George Darwin found results that resemble those estimated today.