Marrying a cousin is usually considered a bad idea, because inbreeding can lead to harmful genetic conditions. But paradoxically, in some societies, marrying a related spouse is linked to having more surviving children, research suggests. …

Pairing with close kin is known to reduce genetic diversity, which can result in the accumulation of severe and sometimes lethal mutations. For example, groups that commonly practiced intermarriage between relatives, like the British monarchy, had children with all sorts of nasty genetic problems.

Yet some research suggests an alternative perspective. “There’s this counterintuitive finding that higher spousal relatedness is related to higher reproductive success in several humans societies,” said Drew Bailey, a psychologist at Carnegie Mellon University in Pittsburgh, and co-author of the study detailed May 21 in the journal Biology Letters.