Marriages between cousins, also known as consanguineous marriages, have been pretty common throughout history — especially in small communities where the pickings are slim as far as potential marriage partners go. But it happens these days, too: As of 2022, more than 10 percent of marriages worldwide were between first or second cousins.
“In some situations, especially in insular communities, marriages between distant and not-so-distant cousins have taken place many times over many generations,” says Bakkala. “In these cases, their descendants often have more than one relationship to each other. It’s possible, and in fact not uncommon, for two people to be — for instance — fourth cousins and sixth cousins once removed at the same time. Tracing the relationship in two different ways brings about two different results.”
Although it’s not that rare, marrying your cousin is extremely taboo in some places. In some countries — China, Taiwan and the Philippines among them — cousin marriage is entirely outlawed. In the U.S. some states outlaw sexual relations, cohabitation or marriage between cousins, and some prohibit all three. Interestingly, some states like Arizona and Indiana outlaw cousin marriage in those under the age of 65.
Although consanguineous offspring have a reputation for high mortality rates, mortality in first-cousin progeny is around 3.5 percent higher than in children whose parents aren’t cousins. And even if the children of cousins survive, there are other genetic considerations to account for, like an increased chance that recessive genetic traits will be expressed in their offspring.