How common were related person marriages? We don't know.
To discover the frequency of first cousin marriages, we would
probably need a reasonably complete database of births and marriages
covering four generations. For second cousin marriages, we would
probably need such a database for five generations.
Anecdotes are a different matter. My mother's paternal
grandparents had 10 children, 9 of whom survived to adulthood. They
were born 1870-1894, 8 of them in Nezhin Ukraine, then part of Russia.
One of them married her first cousin. When she died in childbirth,
her husband and the baby came to live with her family because there
was an unmarried sister who could care for the infant. Eventually,
the unmarried sister married the widower, a second first-cousin
marriage. Another of the siblings married a woman who was probably a
second cousin or a third cousin. So three of the 11 marriages (two
brothers married twice) in that generation were cousin marriages, or
27%.
I do not think that was typical. The plural of anecdote is not data.
On the other sides of my family, I know of only one cousin
marriages. There may be more, but I doubt that we have enough data to
determine in any precise way how frequent cousin marriages were in any
place in any generation.
Bert
--
Herbert Lazerow
Professor of Law, University of San Diego