Second cousins dating wrong
people with common grandparents or people who share other fairly recent ancestors).
Opinions and practice vary widely across the world.
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.
Writers such as Noah Webster (1758–1843) and ministers like Philip Milledoler (1775–1852) and Joshua Mc Ilvaine 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.
Children of more distantly related cousins have less risk of genetic disorders.
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 produce the most children and grandchildren.
Only Austria, Hungary, and Spain banned cousin marriage throughout the 19th century, with dispensations being available from the government in the last two countries.