How STIs May Have Forced Cavemen To Be Monogamous

New study suggests fears of STIs prompted early humans to embrace monogamy and punish polygamists

Four out of five cavemen agree: monogamy is better than syphilis. — (REUTERS)
Apr 12, 2016 at 8:20 PM ET

Prepare to disappoint a generation hooked on caveman erotica—the science suggests that, at some point in our species’ ancient history, hominids became monogamous. But why?

By all accounts, more sexual relationships should lead to more progeny, one of the most important drivers of natural selection. So why did early humans insist on tying the knot, in defiance of Darwin? Because of STIs, according to a new study in Nature Communications.

Using mathematical models, researchers determined that sexually transmitted infections can cause so much damage that societies eventually shift their mating behaviors from rampant polygamy to strict monogamy, and begin punishing men who don’t keep their hands to themselves. Sexually transmitted diseases appear to, “foster the emergence of socially imposed monogamy in human mating,” the authors write. “This suggests pathways for the emergence of socially imposed monogamy, and enriches our understanding of costly punishment evolution.”

More What Can Caveman Erotica Teach Us About Human Evolution?

STIs are still pretty scary, but we’ve come to terms with some of the worst venereal diseases. Take syphilis, chlamydia and gonorrhea: three bacteria that barely stand a chance in the face of modern antibiotics. But before the advent of modern medicine, they were some of the principle causes of infertility (as well as other nasty side-effects) among promiscuous men and women.

Now, there is a general rule in evolutionary biology that any given species—from humans to those way too-sexed-up monkeys—to do whatever it takes to reproduce as often as possible. But if STIs that caused infertility and death were floating around, that could have made polygamy a less appealing option, even from an evolutionary standpoint.

Researchers wondered whether rampant STIs could play a role in shaping a society’s sexual behavior, and so they built an impressive mathematical model that can simulate the evolution of different social mating behaviors in human populations. The researchers ran several different simulations through the model. They considered small hunter gatherer populations of about 30 individuals and large agricultural populations of 300 individuals; polygamous communities alongside monogamous communities that punished polygamists. Then, they set their model to run 2,000 times for a simulated period of 30,000 years.

They found that small polygamous communities could more or less bounce back after STI outbreaks, but that larger societies ended up unable to kick the clap and the clam. As a result, the simulated populations plummeted, and monogamists became far more successful at reproducing than polygamists. They also found that, in scenarios where monogamists “punished” polygamists (through excommunication) their societies became even more successful, even if fewer polygamists earning their keep resulted in fewer resources for the entire community. In other words, in large societies with high rates of STIs, monogamy works best—and polygamists should be punished at almost any cost.

Of course, this is not necessarily the best course of action to follow in modern times. At least one recent study has shown that modern monogamous couples are just as likely to be exposed to STIs as polygamous couples, because “monogamous” couples seldom play by the rules. And nowadays, most of the STDs that really hurt us are viral, not bacteria, and less likely to decrease overall infertility.

Regardless, the study suggests that STIs may have been what finally convinced cavemen to settle down and that, since then, we’ve built religions, societal expectations and marriages around a decidedly Darwinian tradition. “Our social norms did not develop in complete isolation from what was happening in our natural environment. On the contrary, we can’t understand social norms without understanding their origins in our natural environment,” said coauthor Chris Bauch, professor of mathematics at the University of Waterloo in Canada in a press statement.

“Our social norms were shaped by our natural environment. In turn, the environment is shaped by our social norms.”