The presence of sexually transmitted infections (STIs) and peer pressure may have forced pre-historic humans to become monogamists, say researchers.
As hunter-gatherers began living in larger populations of early-settled farmers, the spread of STIs could explain a shift towards the emergence of social norms that favoured one sexual partner over many, according to the researchers from University of Waterloo in Canada.
"The research shows how events in natural systems, such as the spread of contagious diseases, can strongly influence the development of social norms and in particular our group-oriented judgments," said Chris Bauch, professor of applied mathematics at Waterloo.
Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past, he added.
The findings, published in the journal Nature Communications, show that when population sizes become large, the presence of STIs decreases fertility rates more among males with multiple partners, therefore, changing which mating behaviour proves to be most beneficial to individuals and groups.
In early hunter-gatherer populations, it was common for a few males to monopolise mating with multiple females in order to increase their number of offspring.
In these small societies where there is a maximum of 30 sexually mature individuals, STI outbreaks are short-lived and tend not to have as significant an effect on the population.
However, as societies evolved around agriculture and group sizes grew, prevalence of STIs increased among polygamist networks that overlapped. With the absence of modern medicines, infertility from STIs like syphilis, chlamydia and gonorrhea would likely have been high.
"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," Bauch noted.
