STIs May Have Led To Prevalence Of Monogamy In Ancient Humans

The dawn of agriculture led to a gradual shift from polygamy to monogamy due to the impact of sexually transmitted infections in communities, according to a recent study based on computer simulations.

Researchers, Chris Bauch and Richard McElreath from Germany's Max Planck Institute for Evolutionary Anthropology created a computer model to study the impact of bacterial STIs like syphilis, gonorrhea and chlamydia on different sized populations. In their research, the scholars studied both small sized hunter‐gatherer groups, usually comprising of 30 individuals, as well as large agricultural groups, comprising of populations up to 300 individuals. 2,000 simulations were run for each, covering the duration of 30,000 years.

It was found that STI outbreaks were short lived in small polygynous communities, allowing the members to bounce back to health. Consequently, polygamy remained the approach of these small groups as polygamous associations led to more offspring than monogamous ones.

Studying the effect of STIs on more populous polygynous societies led to a totally different observation. Diseases such as gonorrhea and chlamydia became endemic instead of clearing away, as seen in hunter‐gatherer groups of 30.  The spread of the infectious diseases led to a fall in the population of agricultural societies. Individuals who didn't have multiple partners became sought after.

Furthermore, according to the research, monogamists who 'punished' polygamy may have gained a stronger hold on society. The researchers also speculated that social ostracization or fines may have been the punishments doled out on polygamists, to curb the practice. Therefore, on the basis of the study, the impact of STIs and the consequent condemnation may have played a part in the development of socially imposed monogamy that prevailed simultaneously with the rise of large agricultural communities.

The theory has been debated by many. "I don't think it is necessarily wrong but I think the basis for their modeling may be," said Kit Opie of London's University College. "Looking at modern day hunter-gatherers who provide some sort of model for pre-agricultural societies, i.e. any human society prior to about 10,000 years ago, then polygyny is very rare. Hunter-gatherer marriage is a much looser affair than we are used to and polygyny may be allowed but very rarely is it actually practiced." Bauch, however, believes that the argument diminishes the importance of their finding because it concerns general trends.