Sexually transmitted infections may have helped make monogamy the norm, study suggests
STIs are no big deal for small hunter-gatherer groups, but monogamy benefits agricultural societies
Why do modern human societies favour monogamy and punish men who have more than one wife, when hunter-gatherer societies encouraged successful men to have many wives? A University of Waterloo study suggests sexually transmitted infections may be to blame for the rise of monogamy as the socially accepted norm after the development of agriculture.
Chris Bauch, a professor of applied mathematics at the University of Waterloo, hypothesized that sexually transmitted infections, which spread more easily in polygynous societies and can cause infertility, may have played a role. Polygynous societies are those in which men can have multiple wives but women cannot have many partners.
He and Richard McElreath, director of the Max Planck Institute for Evolutionary Anthropology in Germany, decided to test that hypothesis using a computer simulation.
I think it's really a starting point for work that can help us better understand how our social norms are a reaction to our natural environment.- Chris Bauch , University of Waterloo
Their results, published in the journal Nature Communications, suggest that when people live in smaller groups of up to 30 people, as hunter-gatherers do, sexually transmitted infections can only have short-term outbreaks, whether people are monogamous or polygynous.
But in the larger communities of 300 or more that formed after the development of agriculture, sexually transmitted infections can persist in a population and spread more easily in polygynous societies because they can spread to many people within a marriage, causing infertility. Agricultural societies had an advantage if they enforced monogamy and curbed the spread of sexually transmitted infections.
"It basically suggests that the hypothesis is plausible," Bauch said. "I think it's really a starting point for work that can help us better understand how our social norms are a reaction to our natural environment."
That's something that's easy for us to forget, he added, especially those of us who live in cities.
Advantages of many wives
Anthropologists had noticed that in present-day hunter-gatherer societies, polygyny – where successful men can have multiple wives — is the norm. They believe this was likely the norm in most human hunter-gatherer groups in the past. Men with multiple wives would tend to leave behind more children than men with a single wife, which would have helped them gain advantages within their group.
"Therefore, it is unclear why patriarchal societies would voluntarily limit numbers of wives," Bauch and McElreath wrote.
But it appears that when people began farming, they generally started to impose rules and laws to enforce monogamy, the researchers said. Such laws are commonplace in the modern world — just this week, the 10th U.S. Circuit Court of Appeals upheld key parts of Utah's law against bigamy, or holding multiple marriage licenses. That is expected to make related offences easier to prosecute.
Besides limiting the number of children a man can have, another traditional disadvantage of such rules and laws is that someone has to do the enforcing, which can be personally costly. While one person takes on the role of punisher, everyone around them benefits.
Bauch said anthropologists have previously come up with a couple of explanations for why monogamy became so prevalent after the rise of agriculture. For example, the better nutrition that agriculture brings may allow a larger proportion of men in the population to be healthy and capable of taking a wife, leading to a more equal distribution. And there tended to be less violence in monogamous than polygynous societies, so that may offer an advantage.
While there is some evidence to support those hypotheses, Bauch said he wanted to explore one more possibility – the role of sexually transmitted infections. It's not possible to test the hypothesis experimentally, so computer models are "the next best thing," Bauch said.
The researchers included a wide variety of data into the simulation such as:
- Anthropological data about how many people generally marry within and outside a group
- The estimated prevalence of STIs such as chlamydia and gonorrhea in modern Africa, a relatively polygynous part of the world (2.9 per cent).
- The estimated length of infections and rate at which they cause infertility.
"We know from evidence in modern hunter-gatherers, that these STIs can have enormous impact on fertility, much more than they would on an urban population," Bauch said. "The effects can be permanent, so this can have a very significant impact on the number of children."
'Unfolding before your eyes'
The simulation showed that STIs didn't have a significant impact on populations of less than 30.
But in larger populations, they play a big role. The simulation shows that when polygynists enter a relatively disease-free monogamous population, they initially take over because of their higher reproductive rate. But at some point, the disease comes back, and at that point, groups that enforce monogamy and punish polygamists can dominate.
"That really illustrated why it's not enough to have to be a monogamist – you have to be a punishing monogamist," Bauch said. "I was surprised at some of the complex dynamics that can emerge in this model that would be difficult to think up off the top of your head, but in the computer simulation, you can see them unfolding before your eyes."
That said, he acknowledged that the model includes a lot of assumptions and simplifications – it assumes, for example, that STIs existed in prehistoric times, which is not known for sure. Also, it was difficult to calculate what the "cost" was of punishing polygamists for their offences, although in the end that didn't seem to make a big difference to the results.
But Bauch said the goal of the simulation was simply to test whether the hypothesis is plausible, and it seems to be. In the future, the researchers hope to test other explanations for the rise of monogamy and compare them.