Push behind monogamy tied to STIs: study

UW research explores roots of social norm

WATERLOO — Sexually transmitted infections may have helped make monogamy widespread, research at the University of Waterloo suggests.

As early humans switched from huntergatherer to farmer, the spread of sexually transmitted infections among these larger settled groups could explain the emergence of social norms that favoured monogamy over multiple sexual partners.

“Polygamy is no longer successful in larger groups,” said Chris Bauch, a professor of applied mathematics at UW.

Bauch collaborated with Richard McElreath from the Max Planck Institute for Evolutionary Anthropology on the study, published recently in the journal Nature Communications.

Bauch wondered why socially imposed monogamy emerged, and why people impose it at all. “It was puzzling because there’s always a cost to impose behaviours on others,” Bauch said.

A computer model was developed to simulate how the spread of infection influ-

enced behaviour, based on demographic and disease transmission information.

The model showed in early huntergatherer groups, a few males would commonly mate with multiple females to increase their offspring. Among these small groups, outbreaks of sexually transmitted infections would be short-lived.

But as group sizes grew with agriculture, the research predicts the prevalence of these diseases rose among polygamist networks that overlapped. Infections like syphilis, Chlamydia and gonorrhea would persist and infertility would rise among

males with multiple partners.

As a result, it was more advantageous for males to stick with one partner instead of many.

The mechanism works at both the individual and group levels, since groups that enforced monogamy did better.

While other factors could also have the shift to monogamy, it’s important to recognize the influence of the natural environment on human behaviour, he said.

“Our norms are formed by our environment and it’s good to remember that,” Bauch said.