Sexually transmitted infections, peer pressure may have turned humans into monogamists

Chlamydia trachomatis
Chlamydia trachomatis inclusion bodies (brown) in a McCoy cell culture. Credit: public domain

Prehistoric humans may have developed social norms that favour monogamy and punish polygamy thanks to the presence of sexually transmitted infections (STIs) and peer pressure, according to new research from the University of Waterloo in Canada.

As hunter-gatherers began living in larger populations of early settled agriculturalists, the spread of STIs could explain a shift towards the emergence of that favoured one sexual partner over many.

The work, published today in Nature Communications, uses computer modeling techniques to simulate the evolution of different social mating behaviours in human populations based on demographic and disease transmission parameters.

"This 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, a professor of applied mathematics and a University Research Chair at Waterloo. "Our research illustrates how mathematical models are not only used to predict the future, but also to understand the past."

The study, by Professor Bauch and Richard McElreath from the Max Planck Institute for Evolutionary Anthropology, found 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 monopolize 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, the research predicts that prevalence of STIs increased amongst polygamist networks that overlapped. With the absence of modern medicines, infertility from syphilis, chlamydia and gonorrhea would likely have been high. This made it more advantageous for males to mate monogamously, and more importantly, to punish other males who did not. Groups that enforced monogamous social norms could therefore outcompete groups lacking such norms.

"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," Professor Bauch. "Our social norms were shaped by our natural environment. In turn, the environment is shaped by our social norms, as we are increasingly recognizing."

The researchers note that STIs may be one factor among many - including female choice, pathogen stress and technological impacts - that altered human behavior from polygamy to monogamy.


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More information: Nature Communications, dx.doi.org/10.1038/10.1038/NCOMMS11219
Journal information: Nature Communications

Citation: Sexually transmitted infections, peer pressure may have turned humans into monogamists (2016, April 12) retrieved 24 August 2019 from https://phys.org/news/2016-04-sexually-transmitted-infections-peer-pressure.html
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Apr 12, 2016
While I'm sure this kind of thing had some influence, there are many other significant advantages to monogamy. Increased survival chance per individual offspring, for example. Humans are social, and form families. The social structure of families increases survival chances for young individuals.

Apr 12, 2016
The social structure of families increases survival chances for young individuals.

Well, they do say children are a sexually transmitted disease, so the study is still valid.

Apr 12, 2016
"The social structure of families increases survival chances for young individuals."

Is there actual evidence for this? One could argue outside of social norms, there is benefit for the male with the best genes to create stronger offspring. This is seen in nature, in other primate species and with species like lions. Strong offspring also help for the offspring's survival. I think the article was pointing to evidence that in more complex social structures and larger communities of humans, there are factors pushing us towards monogamy that have benefitted our species due to the advent of cities and agriculture.

Also, in human societies across recorded history even through present day there is evidence of non-family individuals taking care of the young of the affluent, so the need for direct family is necessary to care for the young is kind of nullified in that sense too. The need for monogamy in the presence of disease and physical pain is a pretty good deterrent for polygamy.

Apr 12, 2016
How does one justify moslems, predominantly arab or south asian, within humanity inasmuch as to a man if not woman they prefer polygamy and care nothing about risk of venereal disease by 'outside sex'. Maybe muslims are a basically different species. Every Arab I have know prefers multiple wives. Iranians are not Arabs and marry differently, preferring single wives even though the Arabcentric muslime religion prefers up to four wives to maximize population growth at the expense of those limited by misplaced 'morality' to one wife. That is the key to the success of the Arab strategy of 'dawa....the invasion and outprocreation ... for taking over target nations from within.

Apr 13, 2016
How does one justify moslems, predominantly arab or south asian, within humanity inasmuch as to a man if not woman they prefer polygamy and care nothing about risk of venereal disease by 'outside sex'. Maybe muslims are a basically different species. Every Arab I have know prefers multiple wives. ...

I'm not muslim or christian or anything, really. But I wouldn't mind havin' an extra one or two around....
Mainly cuz what Mr' Crowley said...:-)

Apr 18, 2016
Harems create social instability unless the newfound bachelors are killed or made into eunuchs

Apr 18, 2016
Sorry O, but polygamy doesn't necessarily 'maximize population growth'. It allows certain males to maximize their genetic line at the expense of other males.

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