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.

Why cant you date your cousin?

To let cousins marry, they argue, is “to play Russian roulette with genetics.” Many genetic diseases are caused by recessive genes. To get the disease, you have to get the bad gene from both parents. Six states ban marriage between first cousins once removed, i.e., marrying the son or daughter of your first cousin.