IIRC European Jewish tradition sanctioned first cousin and even uncle-niece marriage, the logical product of both being shunned and of viewing yourself as a superior race apart, which was a particular bone of contention in a lot of the States whose domestic relations law was written by Anglo-Saxon Protestants. Of course this also has a lot to do with why traditional European Jewish populations, including those transplanted to the US, had an abnormally high concentration of genetic diseases.
On the other hand, despite a lot of propaganda based on made-up and/or anecdotal evidence about two families known as the Kallikaks and Jukes in the Appalachians and the dangers of inbreeding, more rigorous scientific study by later generations of sociologist using statistical analysis found that the whole inbreeding thing was pretty much a matter of garbage-in, garbage-out. They found that while there were some pretty messed-up inbred groups, the founders of them had been pretty messed up in the beginning anyway, and there were also fairly inbred communities that were basically indistinguishable from the regular population.
