The clam, the clap and the pox are rarely linked to romance. But new research suggests they may have helped drive humans to monogamy.

Based on insights from computer models, scientists argue that the shift away from polygynous societies – where men had many long-term partners, but women had only one – could be down to the impact of sexually transmitted infections on large communities that arose with the dawn of the agricultural age. Agriculture is thought to have taken hold around 10,000 years ago, although some studies put the date even earlier.

“That behaviour was more common in hunter gatherers and it seemed to fade when we became agriculturists,” said Chris Bauch of the University of Waterloo in Canada who co-authored the paper.

Writing in the journal Nature Communications, Bauch and his colleague Richard McElreath from the Max Planck Institute for Evolutionary Anthropology in Germany, describe how they built a computer model to explore how bacterial sexually transmitted infections such as chlamydia, gonorrhea and syphilis that can cause infertility, affected populations of different sizes. The authors considered both small hunter gatherer-like populations of around 30 individuals and large agricultural-like populations of up to 300 individuals, running 2,000 simulations for each that covered a period of 30,000 years.

In small polygynous communities, the researchers found that outbreaks of such STIs were short-lived, allowing the polygynous population to bounce back. With their offspring outnumbering those from monogamous individuals, polygyny remained the primary modus operandi.

But when the team looked at the impact of STIs on larger polygynous societies, they found a very different effect. Instead of clearing quickly, diseases such as chlamydia and gonorrhea became endemic. As a result, the population plummeted and monogamists, who did not have multiple partners, became top dog. The team also found that while monogamists who didn’t ‘punish’ polygyny could gain a temporary foothold, it was monogamists that ‘punished’ polygyny – often at their own expense of resources – that were the most successful. While the form of such punishments were not specified in the model, Bauch suggests fines or social ostracisation among the possible penalties. The results, they say, reveal that STIs could have played a role in the development of socially imposed monogamy that coincided with the rise of large communities that revolved around agriculture.

“It’s really quite exciting,” said evolutionary anthropologist Laura Fortunato of the University of Oxford who was not involved in the study. While there is little data to be had on the prevalence of STIs in either hunter gatherer populations or in early communities that embraced agriculture, Fortunato believes that there are opportunities to explore the idea further. “You could see if that mechanism is in operation in contemporary populations,” she said.

While the authors acknowledge that other factors might also have influenced the shift to monogamy, the research, they believe, highlights an oft-overlooked aspect of human behaviour. “A lot of the ways we behave with others, our rules for social interaction, also have origins in some kind of natural environment,” said Bauch.

But others describe the authors’ theory as “unlikely”. “I don’t think it is necessarily wrong but I think the basis for their modelling may be,” said Kit Opie of University College, London. Opie argues that early human society was not likely to be polygynous. “Looking at modern day hunter gatherers who provide some sort of model for pre-agricultural societies, ie any human society prior to about 10,000 years ago, then polygyny is very rare,” he said. “Hunter-gatherer marriage is a much looser affair than we are used to and polygyny may be allowed but very rarely is it actually practiced.”

Bauch believes the argument doesn’t detract from the authors’ conclusions. “I don’t think it affects our hypothesis because our hypothesis and mechanism concern general trends,” he said. While the authors note that further work that clearly distinguished between marriage and mating could add further insights, Bauch believes the new study shows the power of simulations. “Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past,” he said.

• Note: An earlier version of the story contained an erroneous use of the word ‘polygamy’ instead of ‘polygyny’. The error has been corrected now.



