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Disease dynamics and costly punishment can foster socially imposed monogamy (2016-04-28)

The majority of historically known human societies have allowed, and still allow, polygynous mating. Yet, in some of the most successful agricultural societies, polygyny was gradually replaced by socially imposed monogamy, beginning with the transition from hunter–gatherers to agriculturalists and the resulting larger residential groups.

Subsequently, a disproportionate amount of the world’s population growth in recent millennia has been driven by a relatively small number of societies, many of which practice socially imposed monogamy.

Some societies, such as the ancient Romans, imposed monogamous marriage but permitted extramarital sexual relations in brothels, so long as moderation was exercised. On the other hand, some partible paternal societies in South America encourage extramarital polyandrous mating. Hence, although marriage and mating are closely related, there is not always one-to-one correspondence.

These examples illustrate the enormous variability in sexual norms, which should be expected if norms are complex phenomena determined by multiple mechanisms. It is therefore improbable that any single hypothesis could explain the entire range of observed behaviours.

Socially imposed monogamy in humans is an evolutionary puzzle because it requires costly punishment by those who impose the norm.
Moreover, most societies were — and are — polygynous; yet many larger human societies transitioned from polygyny to socially imposed monogamy beginning with the advent of agriculture and larger residential groups.

Researchers used a simulation model to explore how interactions between group size, sexually transmitted infection (STI) dynamics and social norms can explain the timing and emergence of socially imposed monogamy.

Polygyny dominates when groups are too small to sustain STIs.
However, in larger groups, STIs become endemic (especially in concurrent polygynist networks) and have an impact on fertility, thereby mediating multilevel selection.

Punishment of polygynists improves monogamist fitness within groups by reducing their STI exposure, and between groups by enabling punishing monogamist groups to outcompete polygynists.

This suggests pathways for the emergence of socially imposed monogamy, and enriches our understanding of costly punishment evolution.

For more information
Nature Communications
Chris T. Bauch & Richard McElreath
Disease dynamics and costly punishment can foster socially imposed monogamy