いとこ婚
Cousin marriage was legal in all states before the Civil War. Anthropologist Martin Ottenheimer argues that marriage prohibitions were introduced to maintain the social order, uphold religious morality, and safeguard the creation of fit offspring. Writers such as Noah Webster (1758–1843) and ministers like Philip Milledoler (1775–1852) 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 (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. Morgan himself had married his cousin in 1853.