The emergence of socially imposed monogamy in human societies may have been influenced by the prevalence of sexually transmitted infections (STIs), finds a study in Nature Communications. Increases in the transmission of STIs within larger groups of people highlight a potential mechanism for the emergence of monogamy among early settled agriculturalists.
Historically, most human societies have been polygynous, a form of polygamy where males are allowed to mate with multiple female partners. This is especially true of small hunter-gatherer societies, but less so among the larger societies that emerged with the advent of settled agriculture, in which monogamy is more likely to have been socially imposed on populations by means of threat of punishment.
Chris Bauch and colleagues investigate what factors most influenced this transition by simulating the evolution of different social mating norms in human societies, based on realistic demographic and disease transmission parameters. They find that when the size of a society is large (with a maximum of 300 individuals), the prevalence of STIs becomes endemic in the population, reducing fertility rates and favouring the emergence of monogamists who punish other group members that do not conform to the same social norm. In contrast, STIs in smaller groups (with a maximum of 30 individuals) are characterized by only short-lived disease outbreaks that do not become endemic in the population. Thus, the greater fertility rates of polygynists allow polygyny to become the dominant social norm over time.
These simulations may help to explain the emergence of contrasting social norms in human societies in the absence of first-hand data on disease prevalence in prehistoric populations.