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A University of Waterloo professor sought to determine why many human cultures seem to reinforce and demand monogamous behaviour even though, biologically speaking, humans seem to be at least mildly polygamousDennis Hinaris/Getty Images/iStockphoto

Romantics be warned: The towering edifice of marriage, with its emphasis on having, holding and forsaking all others, may be nothing more than a defence against germs. By forcing monogamy on each other, our agrarian ancestors were collectively less likely to succumb to the ravages of sexually transmitted diseases.

The idea is the latest in a long line of theories that seek to address why many human cultures seem to reinforce and demand monogamous behaviour even though, biologically speaking, humans seem to be at least mildly polygamous.

"It's an interesting puzzle because it's costly to impose anything on others," said Chris Bauch, a professor of applied mathematics at the University of Waterloo in Ontario.

Dr. Bauch, whose uses math to study questions in epidemiology and ecology, wondered if disease might play a role in social restrictions on sexual promiscuity.

Together with a colleague in Germany, he set up a complex series of computer simulations to see how societies of different sizes and social norms fare when faced with an STD epidemic. His digital societies lacked the benefits of modern medicine so the sexually transmitted diseases they encountered came with the same severe costs as they probably did for our prehistoric ancestors – namely disability, infertility and death.

To the extent possible, the calculations were based on real human behaviours and marriage patterns as documented by ethnographers. So while some of the simulations described societies with strong taboos against cheating, the individuals in those societies were not universally monogamous. The effort to impose the code of behaviour on others was also quantified and taken into account.

After running the numbers for the equivalent of thousands of years of simulated time, an interesting pattern emerged. Small groups of about 30 individuals – analogous to prehistoric hunter gatherers – did equally well whether they were monogamous or not. The reason could be that in a small population sexually transmitted epidemics cannot propagate far and they quickly peter out after an initial burst, regardless of social mores.

In contrast, enforced monogamy seemed to confer a big advantage on larger groups, numbering around 300 individuals. Those groups were generally healthier and therefore more likely to outcompete their more permissive rivals. The results held up across a broad range of parameters and for different types of STDs, including bacterial and HIV-like infections, Dr. Bauch said.

In a paper published on Tuesday in the journal Nature Communications, the researchers propose that the impact of sexually transmitted diseases may have started pushing humans toward monogamy during the agricultural revolution, when social groups began to grow in size to hundreds of individuals. The culturally imposed reinforcement could have taken hold even though the individuals involved would not have been aware of any longer-term survival benefit to their group over many generations.

Dr. Bauch added that the study did not suggest that STDs were the only factor behind monogamy. "There are many possible explanations and probably more than one is correct," he said.

Dan Kruger, a social psychologist who studies human reproductive strategies at the University of Michigan, noted that the significant time investment required for nurturing human children is another factor often cited to explain why we tend to be more monogamous than most other mammals. Even in cultures where multiple marriages are permitted, such arrangements tend to be practised only by an elite minority who command sufficient resources.

Dr. Kruger added that the new analysis, while an interesting contribution to the science of monogamy, is unlikely to provide the final word on the subject.

"Sex is sexy," he said. "People just find this question fascinating."

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