Endogamy features heavily in my family tree. From my Quaker and Jewish ancestors, to the big enslavers who formed the American South's elite, to my ancestors of more modest means who lived in rural areas...cousins married cousins for centuries. My Pamunkey ancestors also weren't averse to marrying cousins to help support and maintain peace.

Continue this practice of cousin marriages for long enough, and if you remain in the same county as your ancestors, it doesn't take long - 2 to 3 generations - for most, if not all, of a county to be related. Let's take a look at a purely illustrative example from the Old Ninety-Six region of South Carolina. And let's say each of the 4 topline endogamous groups depicted married cousins within the same family group for 300 to 400 years (this isn't as much of an exaggeration as you might think!).

This episode is also available as a blog post: https://genealogyadventures.net/2017/12/07/endogamy-or-how-an-entire-county-can-be-related/