STIs made monogamy a prehistoric social norm, study finds

Sexually transmitted infections in prehistoric humans may explain why humans tend to be in monogamous relationships today according to new research out of the University of Waterloo.

Chris Bauch, a professor of applied mathematics and a university research chair at the university, created a computer model to simulate how sexually transmitted infections would have moved through hunter-gatherer communities.

"It's almost like having a virtual world. A bit like SimCity or a video game, except we don't have the fancy graphical interface. But it's essentially unfolding very much like a video game inside the computer," Bauch said.

Historical model

The computer model used historical data involving demographic profiles of hunter–gatherers and agriculturalists, and bacterial STI epidemiology that was provided by Richard McElreath from the Max Planck Institute for Evolutionary Anthropology in Germany.

Each person in the computer model was defined by behavioural rules they would follow. For example, monogamists would have one mate, while polygamists would have more than one partner.

We're really interested in explaining the social norms about monogamy. Why do people have these social institutions to support monogamy and why do they get enforced?- Researcher Chris Bauch

"It's a model about monogamist norms and not behaviour per se," Bauch said. "We're really interested in explaining the social norms about monogamy. Why do people have these social institutions to support monogamy and why do they get enforced?"

Complex and surprising dynamics

It was not uncommon for a few males in early hunter-gatherer populations to mate with multiple females to increase their chances of having children. In early cases, communities had about 30 males that would mate with the females, and cases of sexually transmitted infections were short lived.

But with the emergence of agriculture and larger communities, sexually transmitted infections changed how societies viewed their relationships. 

"Polygyny dominates when groups are too small to sustain STIs," Bauch wrote in the study, which has been published in the journal Nature Communications. "However, in larger groups, STIs become endemic (especially in concurrent polygynist networks) and have an impact on fertility, thereby mediating multilevel selection."

Because of the effect of STIs on fertility, Bauch said, it became more advantageous for the males to mate with just one partner, and a social norm of monogamous relationships began. 

"What was surprising was the kind of complicated feedbacks that can develop in the model. We have a situation where human behaviour can influence not only other humans, but it can influence the way the disease spread, and then the disease spread can in turn shape human behaviour," Bauch said.

However, Bauch also noted that sexually transmitted infections may just be one factor that altered human behaviour. Female choice, pathogen stress and technological impacts may have also played a role.