Monogamy is a key aspect of many cultures, but scientists have struggled to determine its purpose. Surely, a species has a better chance at survival if allowed as many sexual partners, and thus offspring, as possible. In addition, the fact that many people struggle to remain monogamous throughout their lives suggests that this behavior is anything but human instinct. Rather, a new study has put forward the interesting hypothesis that monogamy arose as a way to protect ourselves from contracting potentially deadly and sterilizing sexually transmitted diseases.

Monogamy, or the practice of having one mating partner for an entire lifetime, is rare in the natural world. According to Live Science, only 3-5 percent of the roughly 5,000 species of mammals on the planet are known to form lifelong monogamous bonds. The study, now published online in the journal Nature Communications, delved deep to understand why humans came to be a part of this selectively small club. The team used computer modeling techniques to simulate the evolution of different social mating behaviors in human populations based on demographics and disease parameters.

"Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past," said co-researcher Chris Bauch in a recent statement,

Monogamy was not always the norm in human societies. According to the study, in early hunter-gatherer based populations it was common for men to mate with multiple females in order to increase the number of offspring they could have. However, according to the calculations, as humans shifted from small hunter-gatherer groups to larger agriculture-based communities, so did we shift from a polygamous mating behavior to monogamous. The team suggests that the reason for this may have been to avoid the spread of sexually transmitted disease.

While STIs surely did exist in these smaller polygamous communities, they were more likely to die out quickly due to the small amount of sexually active members in the group (usually around 30). However, as communities grew bigger, so did the risk of spreading these sterilizing and potentially deadly diseases. This made it more advantageous for males to mate monogamously, and more important to punish other males who did not. Bauch told The Huffington Post that in their computer simulations, societies favored practices which resulted in the most offspring.

Because societies that practiced polygamy were likely to have higher rates of STIs and therefore higher rates of infertility, monogamous communities soon out reproduced them, passing on their practices to their offspring.

The idea is just a theory, and it will likely always remain that way since we can’t exactly go back in time and ask our ancestors why they made the shift from having many partners to just one partner for life. And of course, our switch to monogamy likely had other benefit aside from keeping us safe from disease. For example monogamous bonds between male and females helps to ensure children benefit by having parental investment from both parents. In addition, the team suggests that while STIs may have been an important factor in the switch to monogamy, it was likely not the only.

Female choice and pathogen stress, a theory that posits that humans behave less openly to strangers in the midst of a disease outbreak, may have also played a role in this transition. Still, the theory poses the question: Although we may have needed monogamy back then, in the age of modern medicine and contraceptives do we still need it now?

Source: Bauch CT, McElreath R. Disease dynamics and costly punishment can foster socially imposed monogamy. Nature Communications . 2016