But In The Past ...
Our modern apprehension about Billy Head-Tentacles showing up at our next family reunion is little more than a blip on the historical timeline. For most of human history, hooking up with your aunt or uncle's offspring was not just acceptable; it was the norm. One historian has estimated that 80 percent of all marriages in human history were between second or first cousins, which probably explains how raisins in coleslaw became a thing in the first place.
Look at this abomination.
Things changed during medieval times, when the Catholic church introduced restrictions on marriages between closer branches of one's family tree. But those rules didn't last: After some branches of Christianity decided they didn't like the pope telling them what to do, cousin shenanigans sprung up again all over. Among the more recent cousin-lovers was Charles Darwin, who hooked up with his first cousin Emma in 1839, thus making things awkward for eugenicists for centuries afterward. In the U.S., bans on cousin marriage popped up only after the Civil War, heralding a new era of slightly less confusing Hallmark cards.