STIs, peer pressure likely turned humans into monogamists

Prehistoric humans may have developed social norms that favour monogamy and punish polygamy due to the presence of sexually transmitted infections (STIs) and peer pressure, a new study suggests.
As hunter-gatherers began living in larger populations of early settled agriculturalists, the spread of STIs could explain a shift towards the emergence of social norms that favoured one sexual partner over many, researchers said.
Researchers from the University of Waterloo in Canada used computer modelling techniques to simulate the evolution of different social mating behaviours in human populations based on demographic and disease transmission parameters.
"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 Chris Bauch, professor at the University Research Chair at Waterloo.

The study, by Bauch and Richard McElreath from the Max Planck Institute for Evolutionary Anthropology in Germany, found that when population sizes become large, the presence of STIs decreases fertility rates more among males with multiple partners, therefore changing which mating behaviour proves to be most beneficial to individuals and groups.
In early hunter-gatherer populations, it was common for a few males to monopolise mating with multiple females in order to increase their number of offspring.

In these small societies where there is a maximum of 30 sexually mature individuals, STI outbreaks are short-lived and tend not to have as significant an effect on the population.

However, as societies evolved around agriculture and group sizes grew, the research predicts that prevalence of STIs increased amongst polygamist networks that overlapped.
With the absence of modern medicines, infertility from syphilis, chlamydia and gonorrhoea would likely have been high.
This made it more advantageous for males to mate monogamously, and more importantly, to punish other males who did not. Groups that enforced monogamous social norms could therefore outcompete groups lacking such norms.
"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," said Bauch.

"Our social norms were shaped by our natural environment. In turn, the environment is shaped by our social norms, as we are increasingly recognising," he said.
The researchers note that STIs may be one factor among many - including female choice, pathogen stress and technological impacts - that altered human behaviour from polygamy to monogamy.
The study was published in the journal Nature Communications.