Being from the South, there are cases of 1st cousins who married. It makes it difficult when they lied on official documents to hide that fact. I have a death certificate that was filled out about a wife that died. It says the husband didn't know the maiden name of his wife's family.
That would be a pain to sort out! I wouldn't discount first cousin marriage in the north either. It's legal in about half the states today. In something like six of those it's only legal with proof that one or both is unable to reproduce. I do know some first cousins that married in modern times (in a northern state).