I would be enormously surprised if genetic factors were the primary driver in this. I would think cultural norms and socio-economic factors would be the greate

I would be enormously surprised if genetic factors were the primary driver in this. I would think cultural norms and socio-economic factors would be the greatest predictors of the number of offsprings. I could easily envision that those same factors could contribute to a rise in intermarriages between 3rd and 4th cousins. For example, in small communities like "hill people" in the Appalachians, or the Hasidic community in NY.