Some aspects of homogamy are well understood. People of the same race, social status, and religion, tend to assort together, and eventually marry. A new paper in PNAS reports results which suggest that there is an elevated relatedness between spouses even among non-Hispanic whites. The paper is Genetic and educational assortative mating among US adults. One of the key findings is that within its demographic of non-Hispanic whites educational assortment dwarfs genetic assortment.* That shouldn’t be surprising, and educational attainment is a major filter in terms of ‘suitable’ spouse material.
At this point I’m assuming most readers are wondering what the surprise is: non-Hispanic whites are not ethnically or socially homogeneous. So the genetic relatedness might simply be due to regional and cultural variation which exhibits genetic correlation. The authors attempted to ‘control’ for this by only looking at people from a particular region, filtering out population structure by looking at PCA plots to generate homogeneous samples, and looking at SNPs which were not found to be informative in terms of population structure. I’m not sure I’m totally convinced by these measures, though they are stabs at the problem. I think the ultimate issue here is that social and cultural structure among white Americans has been understudied, so we don’t have much of a sociological scaffold for the extent to which lineage networks in the United States may be relatively exclusive.
* The authors used the KING software package to assess kinship. I’ve found it to be a decent package myself when I’ve used it, if a little bare bones.