That said, some recessive traits are not harmful and some could even be deemed desirable, so it's not entirely the genetic train wreck across the board that it's depicted as being. If you know anything about animal husbandry and breeding, desirable traits are “fixed” in a line by doing just that. Problem is, undesirable ones get “fixed” as well but these are culled. Brutal but that's how it's done. Remember that next time you walk into somebody’s house and are startled at the resemblance, between the person standing in front of you and the portrait of great, great granddad over the mantle, lol.
Almost anyone with an old family line remaining in one place for centuries is going to find an example or two of first cousin marriage, the family tree branching back in upon itself, particularly in rural or frontier areas. There just weren't that many potential mates and people did what they do, with marriage being the honorable way to deal with the inevitable outcome. A few instances has little to no impact upon the intelligence or genetic health of the line as a whole, and what effects there are are lost in a generation with marriage to an unrelated line, “new blood.”
Royals did it, wealthy families here did it to keep money and/or land from being broken up outside the family. It wasn't always a sign of ignorance, it was done for practical reasons.
That might explain a lot of things.
“Royals did it, wealthy families here did it to keep money and/or land from being broken up outside the family. It wasn’t always a sign of ignorance, it was done for practical reasons.”
Sometimes there were problems. That Habsburg lower jaw, for example. Then there was Charles the Bewitched.
Plenty of articles online about Muslim inbreeding. Keeping the bride-price in the clan is one motivation.