[Verginius Rufus]

FDR and Eleanor were very distant cousins--5th cousins, once removed, I believed. That means that a great-great-great-grandfather of Franklin's was the same person as a great-great-great-great-grandfather of Eleanor's. A person has 32 great-great-great-grandparents and 64 great-great-great-great-grandparents, so the possibility of getting a double dose of a problematic gene is very small.

There have been societies where marriages between second cousins were customary.

According to Genesis, chapter 29, Jacob's wives Leah and Rachel were his first cousins (their father Laban was the brother of Jacob's mother).

[BartMan1]

I think you're off base.

Franklin and Eleanor were first cousins in my recollection, but I am not where I can find a reference to prove it.

[BartMan1]

Beg pardon. You were right.

"In the fall of 1902, while FDR was still at Harvard, he began to see more and more of his distant cousin, Anna Eleanor Roosevelt. A year later, FDR asked for her hand, and the two of them were married on March 17, 1905. The daughter of Elliot Roosevelt and Anna Hall, Eleanor was a member of the Oyster Bay branch of the Roosevelt family. She was also the niece of a man FDR much admired, Theodore Roosevelt, who was President at the time of their marriage and gave Eleanor away in the absence of her deceased father. The couple had six children, five of whom survived infancy. In the first years of their marriage, Eleanor's attention remained primarily focused on her family, but as the years passed, she would become more and more involved in issues of public policy and social justice."

They were in fact fifth cousins.