I’d assume this has a biological basis in insuring proper genetic separation. By the time you reach third-cousin, the odds of birth defects are no greater than breeding with someone totally unrelated to you.
He was sitting on the levee with four or five of his classmates one fine afternoon, and they realized after some quick calculations that they were ALL cousins!
Mostly third and fourth cousins.
I know who all my third cousins are because I'm from a small Southern town and I'm a genealogy fan. I even know who most of my fourth and fifth cousins are. My great-aunt and her husband were fourth cousins, descended from the same Revolutionary Ancestor (D.A.R. members take note!)
Years ago, I went back to my dad's home town to do a little genealogical research and was referred to a retired physician who was the local authority. We did a little chatting and discovered quite quickly that we were third cousins, both descended from a lady who rejoiced in the name of Victoria Britannia Bale (her daddy was an emigre Englishman and she was born the year Queen Victoria married Prince Albert)!
I thought it meant you share a great grandparent. People that share a grandparent are first cousins. Their children would be third cousins. One of the first cousins and the other's child would be second cousins. I'm not trying to be picky, just trying to verify.
My sister and I share great-great grantparents. Are we third cousins?
Ask Rudy about tha,lol.