Comment by clarkm

This is really evident when you look at the genetic data from places like Ancestry.com and 23andMe. The states with highest percentage British/Irish are all in the South:

http://www.sciencedirect.com/science/article/pii/S0002929714...

http://blogs.ancestry.com/ancestry/2014/04/04/a-genetic-cens...

http://blog.23andme.com/wp-content/uploads/2014/10/Bryc_ASHG...