When compared with our closest relatives, such as chimpanzees and gorillas, the human mating strategy is a bit of an oddity. In fact, within the entire animal kingdom, creatures that practice monogamy are exceedingly rare, which raises the question of why humans have made it the social norm. A new study, published this week in Nature Communications, reports that it might not have so much to do with love and loyalty, but with syphilis and chlamydia instead.

When anthropologists and biologists pick apart our own species, Homo sapiens, many have come to conclude that all the evidence – from males being on average larger than females to girls reaching sexual maturity earlier than boys – seems to point to the fact that our natural mating system should be polygyny, in which one male mates with many females. So why, then, do we see monogamy being socially imposed across a multitude of different cultures?

The researchers of the new study suggest it could all come down to the impact of sexually transmitted infections, coupled with peer pressure, as larger communities of people settled in one place during the advent of agriculture. Using computer models, the researchers ran simulations of different mating behaviors, and then assessed how well they did when bacterial sexual infections, such as chlamydia, syphilis, and gonorrhoea, were introduced. In addition to that, they then added another dimension of societal pressure.

“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 judgments,” explains Chris Bauch of the University of Waterloo, Canada, who co-authored the study, in a statement. “Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past.”

They found that in smaller polygynous communities, which is what many think early hunter-gatherers formed, outbreaks of STIs were quickly resolved. This meant that they produced more offspring than those who formed monogamous relationships. But the researchers then found that there was a shift when communities grew in size. Here they found that STIs became endemic if the society was largely polygamous, reducing the male’s fertility, and therefore meaning that those who stayed monogamous became more successful. When this was paired with the monogamous couples punishing the polygamous ones, sticking with just one partner was the best strategy.

Despite this, there are still those who don’t buy into the idea that we as a species are naturally polygynous. By looking at the few surviving hunter-gatherer societies left, which are frequently used by anthropologists as a window into our past, monogamy is actually more common than you might expect. And the fact that in a species in which the sex ratio is roughly 50/50, like our own, polygyny creates a whole bunch of non-reproducing males who could become socially disruptive. Polygyny is also disadvantageous to females in such societies, who not only tend to have fewer offspring as they compete with other females, but also face an increased threat of infanticide from outside males. 

^{Main image: Vince Smith/Flickr CC BY 2.0}