Most of us think that the reason we're monogamous (well, most of us) is related to religion or culture. But according to a new study, it could be linked to Sexually Transmitted Infections (STIs).
Published in Nature Communications, a study by the University of Waterloo in Canada found evidence that prehistoric humans may have been encouraged into the social norm of monogamy - over polygamy - because of the spread of STIs.
Back in the hunter-gathering communities, it was the norm for men to have multiple sexual partners as this meant they were more likely to create offspring.
However, just like today, with unprotected sex comes the increased risk of infections that can lead to infertility if not treated, such as chlamydia, syphilis and gonorrhea.
The study found that in small societies where there was a maximum of 30 sexually mature individuals, STI outbreaks were "short-lived" and didn't have a huge impact on recreating.
But as agriculture grew, so did society sizes, and there was more opportunity for sex with an increased number of people.
Without modern day treatments however, there were higher rates of STIs, which then lead to a change in mating behaviour in larger groups - societies adapted to what was more beneficial to creating a wide pool of offspring.
In other words, they sought to find 'The One.'
The study also claims men who didn't comply with monogamy were punished by other males, but it didn't specify how exactly.
"This research shows how events in natural systems, such as the spread of contagious diseases, can strongly influence the development of social norms and in particular our group-oriented judgements," said researcher Chris Bauch.
"Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past."
"Our social norms did not develop in complete isolation from what was happening in our natural environment. On the contrary, we can't understand social norms without understanding their origins in our natural environment," Professor Bauch said.
"Our social norms were shaped by our natural environment. In turn, the environment is shaped by our social norms, as we are increasingly recognising."
The researchers noted that STIs may have been one factor, but also acknowledged female choice and technological impacts, could have influenced monogamy over polygamy.
