STIs turned early humans on to monogamy

A new study from the University of Waterloo in Canada has theorized that sexually transmitted infections were the reason why early humans had switched from polygynous relationships and societies to monogamous ones.

Lead author Chris Bauch, a professor at the University of Waterloo, came up with this conclusion based on a computer model he created. This model simulated the possible impact of STIs on hunter-gatherer communities thousands of years ago, as Bauch and his colleague Richard McElreath from the Max Planck Institute of Evolutionary Anthropology took demographic profiles of early human hunter-gatherers and agriculturalists, as well as bacterial STI epidemiology, and defined each person in the model based on behavioral rules. These included monogamists having just one partner, and polygamists having multiple partners; male hunter-gatherers, in particular, tended to mate with more than one woman.

“It’s a model about monogamist norms and not behavior per se,” Bauch explained. “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?”

According to the study, STI outbreaks were common, but short-lived in hunter-gatherer communities with about 30 males, but when larger communities and agriculture both emerged, STIs had forced societies to lean more toward monogamous relationships. Due to STIs having an impact on fertility, males began looking to mate with only one partner, thus starting society in general on the road toward monogamy.

“What was surprising was the kind of complicated feedbacks that can develop in the model,” Bauch explained to the Canadian Broadcasting Corporation. We have a situation where human behavior 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 behavior.”

While the study seemed to single out STIs as the main driver behind societies becoming more monogamous than polygynous or polygamous, it also cited other factors that may have influenced this shift. These included pathogen stress, technology, and female choice of what type of relationship to be in.