In the USA, 26 states out of 50 allow a person to marry first cousins.
In Afghanistan, first cousin marriage is the preferred marriage.
My neighbor in Detroit was a Chaldean (Christian from Iraq) who attributed her rare Friedreich ataxia to multiple cousin marriages.
My wife and I just got through reading L M Montgomery’s Chronicles of Avonlea, in which one of the stories talked about a long time thwarted love relationship between first cousins which was eventually successful. The grandfather of both of them attended the wedding, and there was no surname change in the wedding. In fact, they met at a family reunion.
I live in a small town. The joke is that the family tree of Fremont looks like a trunk; and the parents have to carefully check over the dates their kids bring home to avoid cousins.
but–as you said, Iceland has an interesting review of 3rd and 4th cousins being more fertile
I sometimes have to remind folks that while they explore their ancestry to see if an Amerindian or Nigerian is somewhere in their family tree… that they are STILL AFRICAN on BOTH sides of their family!
Good thinking, George