‍The vast majority of mammals are not monogamous. Primates are more likely to be at least socially monogamous, i.e. prone to living in couples, but it’s not ubiquitous in the way that it is for most humans. So what’s behind the anthropological tendency to pair up for life? 

In Ancient Egypt, kings routinely had multiple wives. Historians speculate that this was partly due to the need to forge diplomatic alliances, as there isn’t much evidence that commoners enjoyed similar privileges: whilst polygyny (the practice of having multiple female partners) was legal, it was expensive. It was also expected that men would have only one true ‘wife,’ and other female partners would have the status of servants. On top of that, both men and women could work, own land, and petition for divorce, meaning that a man would very likely have to secure his wife’s agreement before taking on other partners. 

We know that these norms governed almost all of the eras in ancient Egypt, meaning we’ve travelled about 5,000 years back in time. But what about earlier humans? 

Taking another step back to circa 10,000 BC, or the end of the Palaeolithic era, we catch the first human civilisations in ancient Mesopotamia – just before farming became widespread. A more strictly patriarchal society than ancient Egypt, marriage was nonetheless a covenant between one man and one woman. Conflicting reports about male homosexuality exist: in some contexts it was seen as a way to humiliate peers, and punishable by castration; in others, sexual relationships between men were considered unremarkable. As is often the case, no historical record of lesbianism exists. Adultery and remarriage after death are both mentioned in the oldest surviving law code that exists, the code of Ur-Nammu – but there is no discussion of polygamy, leaving the picture unclear.

In fact, most of what we know about ancient Mesopotamian civilisations comes from fragments of relatively late writing (from the third century BC onwards). To get a picture of social structure before then, we have to look elsewhere.

Non-industrial societies that rely on foraging, some of which still exist today, see low but reasonably consistent rates of polygyny. Interestingly, this is particularly true of societies in the tropics that face “high pathogen stress,” or a lot of dangerous environmental contaminants: it makes evolutionary sense for women to be more selective about their mates in these surroundings, because only those who can contribute good immune systems are likely to have healthy children. Healthy and pathogen-resistant men, therefore, may be able to attract multiple female partners.

The reverse is true in environments that are food-scarce. Women are far less able to provide for themselves when pregnant and nursing than men, and so finding a mate who’ll contribute resources is essential. In circumstances when men contribute relatively few resources and women are expected to provide for themselves, women are more likely to share their partners – because a male partner becomes a liability, not an asset, and having another woman who takes responsibility for feeding and sheltering him is actually a benefit. 

This makes sense of what we see in non-primate mammals, which as we know are almost universally polygynous: males typically contribute nothing but sperm to the development of offspring. In these cases, females are even more selective about their mates, and successful males may secure multiple short-term partners.

But both the effects described above are small. The biggest predictor of how prevalent polygyny is likely to be in any given society is how coercive that society is towards women. Arranged marriages, particularly between younger women (or children) and older men, are highly correlated with polygyny – as is the general level of violence within a society (although polygyny is actually a better predictor of violence than vice versa, suggesting that the causation isn’t one-way here). It’s likely that polygyny often results from male intimidation, particularly given the relationship between polygyny and male provision of resources: when men contribute less food, they rely on coercion to secure female companions instead. 

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Polyamory in women

So far, we’ve talked about men having multiple simultaneous partners – but not women. This is because cooperative polyandry, i.e. the same process in women, is both rarer and, we suspect, less interesting to the majority of (male) 20th-century anthropologists who contributed heavily to literature on human relationships. But it does exist, and the situations that give rise to it are fascinating.

In some societies, there is a cultural belief that a child can have multiple fathers. Two men can be socially recognised as the fathers of a child and the partners of its mother. This is common in parts of South America – much more common, in fact, than the biologically correct understanding – and children are often understood to have one female parent and several male parents. 

This can be a very convenient view to hold: in classical societies where land was both valuable and scarce, dividing it between one’s children (and so on between their children) resulted in many small, unproductive farms that failed to sustain families. In Europe, this problem was solved by allowing only first or last children (usually sons) to inherit; in Sri Lanka, India, Nepal, and other Eastern geographies, polyandry provided an answer instead. Rather than being fragmented, land held by men could be united, meaning that even if multiple heirs existed land would remain in larger tracts than it might otherwise. 

Often in polyandrous societies, a woman’s partners will both or all be brothers (just as a man’s partners will often be sisters or cousins in polygynous societies). Deuteronomy, the final book of the Torah and the fifth book of the Old Testament, dictates that men should marry their brothers’ widows and provide for them, rather than allowing the women to marry outside the family or social group. The same command is echoed in the New Testament, written more than half a millennium later. 

The desire to ensure that women are provided for in the absence or death of male partners is often a driver of polyandry. One example is found in some Inuit communities. The Netsilik people, one of the last northern indigenous groups to encounter Christian missionaries, historically practised female infanticide. As a result, wives were in short supply, and wife-stealing became a serious hazard. The choice for men in cases like this was between losing one’s wife to an enemy, or allowing one’s wife to be polyandrous with other related men, such as a husband’s younger brother (the most common choice). Choosing the latter option may have allowed men to save face, protect their wives (by incentivising other men to safeguard them too), and preserve at least part of their genetic heritage.

In 1950s America, similar motivations are believed to have given rise to swinging. Air Force pilots, not knowing whether they’d ever return to their wives, formed (very) close bonds with other pilots and their partners – under the provision that whichever pilot survived would care for the wives and children of the group.

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Infanticide, monogamy, and human evolution

We’ve learnt that polyamory of some description pervades human history, and is still found in a number of modern societies. Perhaps the question, then, is not “why polygamy?” but “why monogamy”? 

One hypothesis is to do with the evolution of sexually transmitted infections. With the arrival of agriculture, human societies were able to grow from a few dozen people to hundreds and thousands. This dramatically increased the rate at which venereal disease could spread and cause infertility, creating a powerful social incentive to enforce monogamy and punish those who violated the norm. Moreover, on this hypothesis monogamous societies would outlast polygamous societies, simply by virtue of being able to reproduce more successfully.

Unfortunately, this has so far proved impossible to test empirically. An alternative (though not contradictory) hypothesis was proposed in 2013 by a team of British and Australian academics: monogamy evolved because without it, males routinely slaughtered infants. The researchers assessed 230 primate species, and found that the single best predictor of monogamy was high levels of infanticide (right before monogamy evolved).

But why? To start with, we need to understand why males might kill infants to begin with.

A simple fact drives this behaviour: women cannot become pregnant whilst breastfeeding. This means that men who want to reproduce with breastfeeding women have two choices; patience or violence. Men chose violence, and infanticide became ubiquitous.

But murder in this context only makes sense if the offspring you dispose of are not your own. As many have discovered, killing one’s own children is a sure way to become a genetic dead-end. 

But what if the male isn’t sure whose offspring they are? A strange fact about humans, and indeed many other primates, is that female members of the species have concealed ovulation. It’s very hard for men to tell when women are fertile each month, and if lots of men are mating with one woman – as they would in polygynous societies – then paternity would almost always be unclear.

This is likely to be an evolutionary strategy itself: if men can’t tell which children are theirs, infanticide is a bad idea. It’s possible, then, that women (and many other female primates) evolved the ability to conceal ovulation as an anti-infanticide measure.

This in turn is a direct motivation for monogamy. If men didn’t know whether women were fertile or not – particularly if, say, women’s menstrual cycles tended to synchronise when they spent time around each other, as some researchers believe they do in humans – then a scattergun approach to mating would become unreliable. Men could expend great effort on courting (or subduing) multiple women only to find that no offspring would be produced.

A better strategy would be to select one woman and mate with her continuously, guarding her from other males. Not only would that guarantee children; it would also guarantee their paternity, and guarantee (or at least strongly enhance) their safety from rival males. If men can be sure that children belong to them – in fact, even if they can’t be sure but suspect the child probably belongs to them – their incentive to kill infants diminishes. And so monogamy began.

Women, and indeed the whole species, benefit from this enormously: they are able to secure additional resources from male partners, significantly lowering their own risk and their children’s risk of death, miscarriage, stillbirth, or infant mortality. 

In fact it’s possible that monogamy, and as a result paternal investment in childrearing, is what allowed humans to become such a successful species. Monogamy in humans evolved roughly 3.5 million years ago – just before human brain size began to increase significantly. Human pregnancy and childbirth are uniquely dangerous as a result of our large brains (because large brains require large heads); additionally, human infants are much weaker and less capable than those of other species. Both these factors mean that if they and their children are to survive, women need particularly high levels of support and resources during pregnancy and nursing. This support only became available after the development of monogamy.

There’s another reason monogamy has been good for humans: when men accept some responsibility for parenting, infants can be weaned more rapidly. The analysis of 230 primate species we mentioned above found that in species where some level of male parenting was normal, the proportion of females who were lactating at any given time was sharply reduced. When some resources were provided for them, females could devote more energy to nursing their children, and children were able to be weaned sooner. It’s no surprise, then, that in primates paternal care is associated with shorter inter-birth intervals: when men invest in their offspring, women are able to have more of them in a shorter timespan.

This is another factor caused by monogamy that lowers the risk of infanticide: males have no incentive to kill infants when the infants are not interfering with their reproduction. 

Monogamy, then, seems like an emergency solution for unstable societies. In the phylogenetic tree of primates, monogamy consistently and rapidly follows high levels of infanticide.

What’s interesting, however, is that as a rule those societies do not switch back once they’ve stabilised. In fact, what tends to happen is that male parental investment increases, infanticide rates stay low, and the society sees higher rates of successful reproduction.

This doesn’t mean that polygamy can no longer exist, but it does mean there might be social taboos about it – as we see in Western societies today. Even in ancient civilisations, polygamy was typically limited to the super-rich or powerful, and it was expected that at least some degree of consent would be sought from all parties involved. 

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Polyamory today

As might be expected given what we’ve learnt about its history, there is a sharp divergence between the number of men who are interested in having multiple girlfriends and the number of women interested in having multiple boyfriends. About 39% of British men are open to the idea of polygyny (compared to 5% of British women), but only about 10% of British women are interested in polyandry. 

This echoes what we see at Swan: a far greater proportion of men than women express interest in some form of open relationship. 

Of course, open or polyamorous relationships may be very different in theory to practice. The survey that provided the percentages above also found that virtually none of its 700+ participants had any real-life experience of polygamous relationships. There are lots of possible reasons for this: perhaps polyamorous people find it hard to meet each other (although specific dating apps exist for this purpose); perhaps social taboos get in the way, or perhaps – as discovered by the ancient Egyptians – managing multiple partners who expect to be cared for and treated respectfully just turns out to be an awful lot of work.

We don’t yet have a full picture of what predisposes some modern humans to be more interested in polyamory (also sometimes referred to as ethical non-monogamy, or consensual non-monogamy) than others. In cases that involve someone having two partners, there’s a trend for the primary partner to serve more ‘romantic’ functions and the secondary partner to serve more sexual functions. Being male is the best predictor of interest in non-monogamy, though sexual minorities, along with people who have less formal education, are more likely to have actually practised it. About a third of people who have practised non-monogamy would do so again – the same proportion as those who’ve tried it and would not do it again.

One study of 550 Italian adults who practised polyamory found that over 76% of the men who chose polyamory did so because of previous negative experiences of betrayal in their personal relationships; for women, 62% chose polyamory because of negative family experiences (such as seeing their parents divorced). When asked whether the choice to be polyamorous was their own or their partner’s, 52% of men stated that it was exclusively their own – compared to only 14% of women. 40% of women reported that their relationship was polyamorous at their partner’s request, compared to just under 19% of men.

Other studies, however, find that polyamorous relationships are used by women to explore their sexuality in ways that monogamous and heterosexual relationships may not allow. Researchers have also suggested that polyamory may help individuals develop a better understanding of their own identity, find new communities of people, and express their political values in ways that would be impossible within ‘traditional’ relationships.

Whatever the motivation, polyamory is inescapably part of human history, and – just like monogamy – it’s unlikely to disappear any time soon.

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