Is human monogamy 鈥渘atural鈥�? No. To be sure, monogamy is possible and in many ways, desirable, but it isn鈥檛 the default biological mating system for Homo sapiens. Polygyny, in which a male mates with numerous females, is.
We know this for several reasons. They include traits linked to competition between males such as men on average being larger than women, a pattern that characterises nearly all polygynous species, along with males being more inclined to violence.
Then there is the fact that girls reach sexual and social maturity years earlier than boys 鈥� again a feature of polygynous species, with the entry of males into the arena of social competition being delayed until they are larger, older and perhaps wiser.
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We also know there is than maternally inherited mitochondrial DNA which means our ancestral fathers were less diverse than our ancestral mothers. And before the cultural homogenisation that came with Western colonialism, .
Model diseases
Knowing all this, a visiting Martian zoologist would surely conclude that humans are 鈥渘aturally鈥� polygynous. And this, in turn, leads to a mystery: why did monogamy end up being socially imposed?
The latest attempt to answer that question is published today in Nature Communications by Chris Bauch of the University of Waterloo in Canada and Richards McElreath of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. It relies on a fascinating mathematical model showing how monogamy could have created a selective advantage as bigger communities emerged that practised it. Two factors are key to this 鈥� the impact of sexually transmitted infections (STIs) and monogamists punishing polygyny.
The model assumes that, in larger communities, polygyny exposes men to greater risk of STIs, making such polygynous groups more vulnerable to serious downsides such as reduced fertility. So monogamy would benefit larger groups, allowing them to outcompete their polygynous rivals.
However, the analysis is far from the final word on this. For one, it relies on natural selection operating at the level of groups, a notion that is 鈥� to put it mildly 鈥� controversial among evolutionary theorists.
More plausible alternatives
Most important, although the authors find a positive correlation between group size and monogamy, which is consistent with their STI hypothesis, this correlation is also consistent with other, more plausible explanations.
For example, monogamy facilitates , because it increases a man鈥檚 confidence that he is the father of any children.
Such involvement is especially important for humans, because we are helpless at birth and require substantial parental investment. In addition, polygyny is typically disadvantageous to women, often leading to lower maternal reproductive success, largely because of conflict between co-wives.
The STI hypothesis assumes that women have no say when it comes to mating and marital arrangements; experience suggests otherwise. Most important, perhaps, there are additional possible explanations for the prevalence of monogamy, at least in modern Western societies.
Notable among them is the fact that in a species with an equal sex ratio, polygyny necessarily results in a large number of sexually and reproductively excluded males. The greater the degree of polygyny, the more sidelined bachelors there are.
These frustrated individuals are liable to be socially disruptive, leading to the possibility that monogamy developed as a trade-off whereby powerful potential harem-keepers agreed to forego some or all of their advantages in return for a degree of social peace, because monogamy is essentially a great democratising institution. Thus, by permitting 鈥� even mandating 鈥� monogamy, modern societies may be benefiting women, children and even most men.
Where, then, are we? It seems deceptively easy to explain human monogamy: there are lots of hypotheses, but as yet, no definite answers.
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