Monogamy adopted to protect us from sexually transmitted infections, study says

A new theory has been offered as to why monogamy has become the dominant practice in human societies: to cut down on the impact of sexually transmitted infections (STIs).

Human monogamy is perplexing. Only about five per cent of mammals live as monogamous pairs, and anthropologists agree that the earliest human groupings were likely polygamous. Not to mention that polygamy is still practiced in some cultures. So, what caused the changeover in human societies from multiple mates to soul mates?

“STIs can impose enormous selective pressures on humans,” say Chris Bauch of the Department of Applied Mathematics at the University of Waterloo who together with Richard McElreath of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, authored a new study published in the journal Nature Communications. “In the absence of modern interventions such as antibiotics, latex condoms or contact tracing, bacterial STIs such as syphilis, chlamydia and gonorrhea can cause very high rates of infertility and thus major demographic impacts.”

Using a simulation model to look at how STIs might have affected early polygamous societies of different sizes, the researchers concluded that while the impact of STIs on small polygamous groups of 30 or fewer people would not likely be devastating, the same could not be said of large polygamous societies, where an STI would likely prove a significant hindrance to the group’s ability to carry forward their gene pool. Thus, when it came time for a good proportion of humanity to make the move from small, hunter-gatherer groupings to large agrarian societies the transition was likely accompanied by social conditioning towards monogamy, say the study’s authors, for the sake of avoiding the devastating effects of STIs on the larger population.

“Polygyny dominates when groups are too small to sustain STIs. However, in larger groups, STIs become endemic (especially in concurrent polygynist networks) and have an impact on fertility, thereby mediating multilevel selection,” say the study’s authors. “Therefore, we speculate that STI-imposed selective pressures on our ancestors could have been even larger.”

Of course, we are all aware of the corollary to this theory, namely, that because contemporary societies are much more able to control the spread of STIs through the use of condoms and modern medicine (but see the recent rise of antibiotic-resistant gonorrhea to put a damper on this thought), there is in fact no real reason for us to continue with the social convention of monogamy.

Further, Bauch and McElreath themselves opine that imposing monogamy requires laws, police and plenty of finger-waggers to punish potential transgressors, making monogamy a costly venture to uphold. Thus, is an easier, cheaper and more natural way of living just waiting for us to give it a go?

Before taking the plunge, we may want to consider the speculative nature of this particular anthropological enterprise. For one, there are a tonne of competing theories on why humans are, now, mostly monogamous. To wit, monogamy allows fathers to protect and care for their young, thus increasing the likelihood that their genes will carry on. Or, monogamy helps to ensure that females live longer, by having a dedicated male around to protect them. Or, monogamy reduces the messy, complicated and, contrary to Bauch and McElreath, socially disruptive nature of polygamy.

These are just a few of the ideas currently on offer. One suspects that we’ll see more, not in the least because the question is there to be mulled over. For, with so many glorious ice cream flavours out there to sample, why would each of us pick just one for life?

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