We tend to talk about monogamy as if it’s both sacred and obvious – the default way of being within our romantic relationships. But behind closed doors, monogamy can sometimes feel fragile, negotiated and even improbable.
If being sexually exclusive is meant to be natural for us humans, why does it seem to take so much effort and external reinforcement to maintain?
Scientists, it turns out, have been asking this question for decades, and their answers are far from simple.
Strategic powerplay?
There are plenty of examples of non-monogamy in human culture, from polyamory to infidelity to casual dating.
A study from the 1960s that is still widely cited today found that 87 per cent of pre-industrial human societies permitted some form of polygyny (where a man may have multiple wives concurrently), suggesting that the fixation on monogamous marriage across much of the world today may be a reflection of rapid and recent cultural change.
In reality, however, only a small fraction of the people living within those societies were actually partaking in non-monogamous partnerships. For most people, monogamy was still the norm.
Among mammals, though, monogamy is rare, with less than 10 per cent of species forming long-term pair bonds. Although the figure is higher among primates (about 25 per cent), our closest relatives – chimpanzees and bonobos – are outright promiscuous, with both males and females routinely mating with many members of their group.
Which begs the question: why do humans so often attempt monogamy when most other mammals don’t?
One prominent theory is that monogamy evolved as a defensive strategy against male infanticide. In a landmark paper published in 2013, scientists looked at 230 species of primates across various evolutionary lineages and found that male members of promiscuous species frequently killed other males’ offspring to force females back into the mating pool.
Over time, in groups where infanticide was the most pervasive, males began sticking around to help protect their young, prompting a societal shift towards monogamy.
Other primates appear to have taken a different tack. Female chimpanzees will often mate with every male in the group during their fertile window, confusing the paternity of their offspring and reducing the chances of infanticide.
Bonobos, on the other hand, are a notably more peaceful species that engages in frequent casual sex to cement alliances, diffuse social tension and even resolve conflict.
Alternative theories for why monogamy evolved propose that males who provide food and safety for females boost their odds of being selected as mates; or that in species where females disperse over a wide area, males are forced to choose between guarding their mate from rivals or risk roaming in search of additional partners.
Monogamous mating is also closely linked to the evolution of paternal care in mammals, which in turn raises the chances of offspring surviving.
Wired for wedlock?
Whatever winding route we took to get here, it’s clear that our evolutionary inheritance has left us with a biological drive to form closely bonded pairs.
It’s important to note, though, that being pair-bonded does not necessarily equal being monogamous in the way that we generally understand it culturally today (being emotionally and sexually exclusive with one individual for sustained periods of time). In fact, biologists define three distinct types of monogamy: social, sexual and reproductive.
A socially monogamous pair operates as a unit, living together and teaming up to mate, raise young and share resources. A sexually monogamous pair has sex exclusively with each other. And a reproductively monogamous pair breeds exclusively with each other.
Reproductive and sexual monogamy are essentially interchangeable in the animal kingdom, where contraception doesn’t exist.
Socially monogamous pairs may also be sexually (and reproductively) monogamous – but not necessarily so.
“For humans, our romantic pair bonds are really, really strong,” says Prof Agustín Fuentes, a bioanthropologist at Princeton University and author of the book Race, Monogamy, and Other Lies They Told You. “We tend to, on average, have one very tight sexual or social bond, sometimes both in the same connection – but sometimes in different connections at the same time.”
It’s these bonds, says Fuentes, that gave rise to our physiological and cognitive capacity for compassion, cooperation and extended networks of care, such as shared child-rearing.
Scientists know quite a bit about the potent neurochemistry that underpins these kinds of intense bonds.
Studies in prairie voles, which form committed monogamous relationships, reveal that a cocktail of feel-good brain chemicals, including the “love molecules” oxytocin and vasopressin, floods the female’s brain after she mates with a male for the first time, leading the two to form a tight pair bond.
When researchers used a drug to stimulate the nucleus accumbens (a brain region involved in reward and pleasure) in unpaired females, they found that it boosted the expression of genes that govern these potent neurotransmitters – prompting the females to pick a partner and begin huddling (the prairie vole version of cuddling) with him exclusively.
Monogamy by the numbers
“We’re very happy to characterise the mating system of, say, meerkats in one word; we say they’re a monogamous species,” says Dr Mark Dyble, an evolutionary anthropologist at the University of Cambridge. “Chimpanzees are promiscuous, gorillas are polygynous. But what if we were forced to summarise humans in the same way?”
The major challenge facing anthropologists is that getting good data on people’s sexual behaviour is notoriously fraught.
Even today, it’s difficult to get a clear read on how many partners people have had – men tend to exaggerate up, women tend to exaggerate down. Much of the ethnographic data we have was collected by white men striding into remote communities and asking male elders to be frank about their sex lives.
Dyble decided that the neatest way to quantify humans’ inclination towards monogamy was not to attempt to unpick our behaviour but to look instead at the outcome of that behaviour. “It was a way to cut through some of the complexity,” he explains.
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Using genetic data extracted from archaeological sites and ethnographic data from small-scale hunter-gatherer and agriculturalist societies, he analysed the proportion of full siblings (with the same mother and father) versus half-siblings (who have only one parent in common) across more than 100 human societies – and compared the results with equivalent data from 34 other species of mammal.
Overall, he found that 66 per cent of human siblings were full siblings – suggesting we’re a species with a strong tendency towards monogamy but with plenty of room for manoeuvre.
Today, nearly a quarter of UK marriages end in divorce within 10 years. Meanwhile, 20 per cent of American men and 13 per cent of American women admit to cheating on their spouse while married.
But the new findings put humans solidly within the set of socially monogamous mammals included in the study – ranking 7th, behind the Eurasian beaver (73 per cent) and ahead of meerkats (60 per cent).
The most committed creature of all was the California deermouse, with a full-sibling rate of 100 per cent.
Meanwhile, our close relatives, gorillas and chimps, scored just 6 and 4 per cent respectively, suggesting that human monogamy likely evolved from living in non-monogamous groups, a shift that Dyble calls “highly unusual among mammals”.
The study does have its limitations. Offspring data can only tell us about heterosexual couples.
And it can’t tell us whether half-siblings were conceived via occasional covert “extramarital” copulations; a sustained pattern of overt promiscuity within a polygamous structure where men (and sometimes women) have multiple spouses; or following the death of an exclusive partner.
Nevertheless, says Dyble, the result makes a simple but important point: “If we were any other species, we would readily characterise humans as monogamous.”
Cultural construct?
Of course, there is vast cross-cultural diversity in human sex and marriage practices. A lot of that arises out of differences in resource availability, inheritance practices, power dynamics, inequality and economics, says Fuentes, and is further scaffolded by Judeo-Christian religious norms and civil legal structures.
In the UK, the popular understanding of monogamy used to mean ‘one person for life’, but for many of us today it means ‘one person at a time’ – otherwise known as serial monogamy.
“Mostly, people don’t have lots of sex with lots of people at the same time – despite what they may claim,” says Fuentes.
But promiscuity is also something that shifts profoundly with age: “As people mature, they tend to end up being semi-monogamous, whether or not that’s required by society. But when they’re young, they’re all over the place.”
For some people, monogamy comes easily. For others, it’s a suffocating and outdated expectation. Floating somewhere in the middle are those for whom monogamy is a conscious but sometimes challenging choice.
Then again, not everything we choose to do is easy, but we choose these behaviours nevertheless. Monogamy may be similar – less a fixed human trait, and more a choice we return to again and again; an agreement whose terms are continually redrawn by the world around us.
So, how monogamous are humans really? The honest answer: it depends.
We’re a species capable of deep attachment, supported by ancient brain chemistry that makes pair-bonding both possible and powerful.
We’re also a species that has built elaborate cultural rules around those bonds, enforcing monogamy in some contexts and relaxing it in others. And we are – undeniably – a species that frequently bends or breaks those rules.
It seems that while humans may practice monogamy, we’re not especially good at it.
Put simply, says Fuentes: “We’re monogamish.”
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