In fact even those adults who do play chess more often than not don't really know the rules beyond a passing familiarity ;)

There was this funny piece at chess.com quoting some of the complaints they receive from angry users about capturing en passant: https://www.chess.com/article/view/his-pawn-cheated-and-kill... :)

I did say passing familiarity, which is not a very high bar to clear. It's been my experience, and I don't travel much in rarefied intellectual circles.