[em3rgent0rdr]

This is not a big deal. In fact likely better fertility for them: a 2008 deCODE study results show that couples related at the level of third cousins have the greatest number of offspring, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. [1]

[1] https://www.ncbi.nlm.nih.gov/pubmed/18258915?ordinalpos=1&it...

[mrunkel]

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.

[loseyourslinky]

The study is specifically of native Icelander's, between 1800 and 1965.