Monogamy emerged thanks to STIs, study says istock

Sexually transmitted infections (STIs) likely played a role in the emergence of monogamy among early humans. Researchers have said the presence of STIs among prehistoric human populations, along with peer pressure, could have meant having one partner became the social norm, while having many was frowned upon.

Researchers from the University of Waterloo, Canada, used computer models to recreate the evolution of mating behaviours in human populations. They used demographic and disease transmission information to show how the spread of contagious diseases (like STIs) can influence the development of social norms – and "our group orientated judgements".

Published in the journal Nature Communications, the team found that when populations become large, the presence of STIs leads to decreased fertility rates among men who have multiple partners. This leads to a change in mating behaviour, with societies adapting to what is more beneficial to groups and individuals, ie monogamy.

They said that in early hunter gatherer societies (made up of around 30 sexually mature individuals), it was normal for a few males to have many female partners, therefore increasing the chances of producing offspring. In such small groups, STI outbreaks would be short lived and have little effect on population.

However, in larger groups, the role of STIs and their effect on social groups becomes more apparent. The impact of infertility from syphilis, chlamydia and gonorrhoea would have had a greater impact, so it would have become beneficial for monogamy to become the norm.

Researchers said there were likely more factors involved in the shift towards monogamy, but that STIs could well have been one of the elements involved.

Study author Chris Bauch said: "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.

"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. Our social norms were shaped by our natural environment. In turn, the environment is shaped by our social norms, as we are increasingly recognising."