Were they careful about ethnic groupings? I mean, many African-Americans marry someone whose coloring might match their opposite-sex parent -- but that's pretty meaningless, right? How about Jews marrying Jews? Or Norwegian-Americans marrying someone from their hometown (which has a high percentage of Norwegian-Americans). Do the Irish marry a lot of redheads? Because the opposite sex parent was a redhead? Or because there are a lot of redheads in Ireland?
This report could be noteworthy, but I wonder if they missed the obvious.