It has been pointed out that this was common amongst British only a generation ago. If not an immediate cousin, they married “the boy next-door,” which had the same effects because of the many intertwined links.
There are a couple of towns north of here with particularly scruffy populations of violent losers, slatterns and their simpleton children. These are old towns tracing their histories back to before the Revolution. All of the family names go back to that era too. One of my best friends commented that these towns represented the shallow end of the Founders’ gene pool.