If I could ask a point of clarification, what exactly are you referring to? When I think of regional inbreeding pre-WW2 in the US, I tend to think of Appalachia, though that covers a lot of ground (NY to Alabama). I wouldn't really consider that the south, per se, though it contains southern states. In terms of post WW2, no one speaks too much about Appalachia, but rather Alabama "roll tide" date-your-cousin variety.

So, what specifically did you have in mind. Have any examples?