I live in Utah, where the polygamy a couple generations ago has created some interesting population dynamics. You had these huge families where everyone was related to at least one person, and that's ended up inescapably propagating down the line because most people are still living and marrying in the same part of the state their ancestors were. It's also why the LDS church keeps a huge geneology database - back when it was established they were trying to figure out who was related to who.    Anecdotally my brother married a distant cousin without realizing it (their kids are fine fortunately), and I've got some other cousins who are having a lot of children with cleft palates and hip dysplasia problems, and I suspect they're probably related on some level as well. 2-3% increase in birth defects doesn't seem like it's so bad until you see it happening to people you know and you see the pain and  the costs.    That said, they have every right to keep having kids as is the mormon wont, and they'll just keep paying for the surgeries on infants. I suppose it's similar for other people who find they're both at an increased risk for deformity even without the ancestral relations.