Rare evidence of early Homo sapiens. Photo: John Springer Collection/CORBIS

Anthropologists think they’ve found another reason why our early forebears embraced monogamy over polygamy, and it’s super romantic: infertility-causing sexually transmitted diseases!

For a new study in the journal Nature Communications, researchers in Germany and Canada built a computer model to see how STDs like chlamydia, gonorrhea, and syphilis — which can cause infertility when not treated by modern medicine — might have affected populations of different sizes with different mating habits. When some of our ancestors stopped hunting and gathering and chose to stay put and farm, they lived in groups of as many as 300 people (that’s versus a max of 30 adults for more nomadic types).

They ran simulations to cover 30,000 years of evolution and determined that sister-wiving worked out okay in smaller groups, even in the face of STDs, because the diseases didn’t spread all that much and they had more children than monogamous humans. But when larger, presumably agricultural groups practiced polygamy, they experienced full-on outbreaks and their population plummeted. Monogamist groups were unscathed in this department: their population remained steady.

The researchers admit that there’s little data on prehistoric STD rates but they invite others to test their hypothesis. Overall, they believe that these diseases are just one of several factors that could explain our current practice of coupling up. As the co-author said in a release, “Our social norms did not develop in complete isolation from what was happening in our natural environment.”

For now, we’re gonna go ahead and blame STDs for monogamy’s greatest scourge: celebrity couple names.