A week ago I offered my own theory as to why 3rd and 4th cousin pairings in Iceland have been so fertile historically. Now, this from John Hawks, For maximum fertility, marry more than 20 km from your birthplace. John says:
This study isn't orthogonal to the Iceland cousin study, but it adds another element. These people are about as close to the genetics of Icelanders as we can hope to get. The authors suggest that greater mobility in the last 50 years has removed a significant inbreeding depression. Yet, the inbreeding between people born within 15 km of each other is mostly at the level of third cousins or further -- the sample that had the highest fertility in Iceland. Curious. It would be interesting to see whether this result holds over longer distances. levels out, or even reverses.