Monogamy is a longstanding social norm around the world, but it wasn't always that way.

Around 10,000 years ago, approximately 85 percent of human societies were composed of relationships where men had multiple partners. And anthropologists aren't sure why that changed over the centuries.

Nor do they fully understand why some people prefer polyamorous relationships while most others either stick with couple-hood or profess monogamy while cheating on their partners.

But scientists do have a lot of theories on the nature of human relationships, and you can catch up with their latest observations and studies by watching this video from AsapSCIENCE.