United States
Cousin marriage was legal in all States before the Civil War. However, according to Kansas anthropology professor Martin Ottenheimer, after the Civil War the main purpose of marriage prohibitions was increasingly seen less as maintaining the social order and upholding religious morality and more as safeguarding the creation of fit offspring. Indeed, writers such as Noah Webster and ministers like Philip Milledoler and Joshua McIlvaine helped lay the groundwork for such viewpoints well before 1860. This led to a gradual shift in concern from affinal unions, like those between a man and his deceased wife's sister, to consanguineous unions. By the 1870s, Lewis Henry Morgan 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 Morgan, cousin marriage, and more specifically parallel-cousin marriage, was a remnant of a more primitive stage of human social organization.[19] Morgan himself had married his maternal uncle's daughter in 1851.[20]
In 1846 the Governor of Massachusetts appointed a commission to study "idiots" in the state, and this implicated cousin marriage as responsible for idiocy. 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 S.M. Bemiss for the American Medical Association, which concluded "that multiplication of the same blood by in-and-in marrying does incontestably lead in the aggregate 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.[21]
These developments led to thirteen states and territories passing cousin marriage prohibitions by the 1880s. Though contemporaneous, the eugenics movement did not play much of a direct role in the bans, and indeed George Louis Arner in 1908 considered the ban a clumsy and ineffective method of eugenics, which he thought would eventually be replaced by more refined techniques. Ottenheimer considers both the bans and eugenics to be "one of several reactions to the fear that American society might degenerate."[22] In any case, by the period up until the mid-1920s the number of bans had more than doubled.[7] Since that time, the only three states to add this prohibition have been Kentucky in 1943, Maine in 1985, and Texas in 2005. The NCCUSL unanimously recommended in 1970 that all such laws should be repealed, but no state has dropped its prohibition since the mid-1920s.[1][9][23]
While there may (or may not) be increased risk of genetic defects with children of cousins, we don't (any more) prohibit people with severe genetic diseases from marrying. And, I argued that marriage isn't only about procreation, so adults should be able to make their own decisions about that.
I've got no problem with doing away with that prohibition. I shouldn't have included it. Thanks for the ping.
Cheers,
Scott.