How Monogamous Are Humans Vs. Other Mammals? Somewhere Between Beavers And Meerkats, Apparently

Humans are pretty monogamous, all things considered. That’s not a judgment call – it’s the conclusion of a new study from a researcher at the University of Cambridge’s Department of Archaeology, which places Homo sapiens a healthy seventh out of 35 species in terms of faithfulness to a single reproductive partner.

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We’re also, to put it bluntly, weird. “The vast majority of other mammals take a far more promiscuous approach to mating,” evolutionary anthropologist Mark Dyble, sole author of the new paper, pointed out in a statement this week. 

“Almost all other monogamous mammals either live in tight family units of just a breeding pair and their offspring, or in groups where only one female breeds,” he explained. “Whereas humans live in strong social groups in which multiple females have children.”

It’s a ranking that places us above, say, meerkats in monogamy (somehow not surprising) but below African wild dogs and Eurasian beavers. But what’s the logic that puts us in among these species? Well, it all comes down to your half-siblings.

Measuring monogamy

How do you figure out how monogamous somebody is? Well, one pretty direct way is to look at their offspring: “While previous work has relied on inferences from the fossil record, or cross-cultural comparisons of marriage norms, I […] focus on measuring the outcome of mating systems: the relative proportion of full and half-siblings born into a population,” writes Dyble – describing the metric as a “theoretically salient, but relatively overlooked approach.” 

Basically, the logic is this: if your parents are super-monogamous, then all your siblings should be full siblings, sharing around 50 percent of their DNA (though it should be noted that this figure is an average; you could share anything from 42 to 58 percent without it being super-weird). The less monogamous your parents are, the higher the proportion should be of siblings sharing half that connection with you, by virtue of having a different mum or dad.

It sounds simple, right? But it’s not quite: “Although it is clear that random mating will result in very few full siblings and that exclusively monogamous mating will result in only full siblings,” Dyble writes, “the nature of the relationship between the proportion of full siblings and the extent to which mating is monogamous is not obvious.” 

In other words, let’s say your parents play away from each other half the time – does that mean half your siblings will be half-siblings? More? Fewer? What are we dealing with here?

Well, why not try it and see? Not literally, of course – but through the miracle of computer models. “I varied rates of mating within a simulated population between exclusive monogamy (e = 0) and random mating (e = 1),” Dyble explains. “Varying rates of e in this scenario produces a nonlinear negative relationship between e (characterized as ‘deviation from monogamy’) and the proportion of full siblings, with relatively modest deviations from monogamy having a disproportionate effect on the production of half-siblings.” 

So, for example, this model found that, in a population for whom just one in four babies are born outside of a monogamous pairing, about 40 percent of sibling pairs should share both parents. Raise that to half of all children, like in our question from before, and it’s only 15 percent of pairs who share both mom and dad.

Then, “extrapolating back from the observed rates of full siblings, we can estimate e across the human sample,” Dyble explains. So, what did he find?

Unknotting the data

By now, you’ve probably spotted a problem with this question. 

Evidently, we live in a society that, with few exceptions, aggressively promotes monogamy as both normal and virtuous. If Dyble were to simply survey most families in, say, the US, he’d likely get a proportion of full-siblings fairly close to 100 percent – but is that because humans are naturally quite monogamous? Or is it because marrying multiple people simultaneously and raising children communally is illegal and taboo?

“[When] religions lose their force, serial monogamy, or polygamy by any other name, quickly emerges,” Robin Dunbar, a professor of evolutionary psychology at the University of Oxford who was not involved in the study, told The Guardian. Humans, he argued, live “right on the cusp between monogamous and polygamous species” – we “desire polygamy but are constrained into a grudging form of monogamy by social or religious threat.”

That’s not mere projection on the part of anthropologists. Previous research has shown that about 85 percent of pre-industrial societies – that’s roughly six out of every seven of them – allowed polygynous marriage, where one husband has multiple wives. Statistically speaking, therefore, we’re currently living in a bit of a weird time and place, monogamy-wise – but is that because we’re deviating from some natural rule, or reverting back to type?

Luckily, there’s something of a control here. As an archaeologist, Dyble drew his data on human monogamy not solely from, say, modern census data, but from DNA analyses and ethnographic data of ancient and pre-industrial societies around the world. The resulting dataset drew on evidence from Bronze Age Europe, Neolithic Anatolia, the Hazda people of Tanzania, the Toraja of Indonesia, and 99 other equally diverse populations.

Even without statistical analysis of the results, it’s clear that monogamy in humans is far from the default. Out of the 103 populations considered, only four showed 100 percent of siblings sharing both rather than one parent – and in one society, from Early Neolithic Britain, the proportion was only about one in four. Working back from these with Dyble’s monogamy-to-sibling-strength formula, that resulted in monogamy levels that varied from 65 to 100 percent, with an average of about 88 percent monogamous.

That sounds fairly high. But is it, really?

Humans: weird, but basically monogamous

Averaged over all the societies analyzed, human sibling pairs are about twice as likely to be full siblings as half siblings. And here’s the headline: considering our mammalian peers, that’s firmly “monogamous”, Dyble says.

It’s “obvious” from his datasets, he says, that “humans are not universally monogamous” – but “human rates of full siblings overlap with the range seen in socially monogamous mammal species.”

Moreover, they’re far higher than the rates you’d expect to see in a solidly non-monogamous species. The black rhino, for example – the non-monogamous species with the highest rate of full sibling pairs in the study – only comes in at 22.2 percent. That’s roughly one-third our own rate, and equivalent to about seven in every nine sibling pairs sharing only one parent.

Chimps, meanwhile – our closest cousins, evolutionarily speaking – have just 4.1 percent of sibling pairs sharing both parents, which explains their massive balls (not a joke. Real science.) Gorillas’ full-sibling rate is six percent; various macaques range from one in 100 to one in 40. And that, in turn, raises an interesting question: what makes us so different?

“Our closest living relatives, chimpanzees and bonobos have an entirely different mating system,” Kit Opie, an evolutionary anthropologist at the University of Bristol, told The Guardian – noting that in those species, females “try to confuse paternity, through promiscuity, so that all males in the group might be the father of the offspring.”

Humans likely began the same way, Dyble argues, evolving over time into our now essentially monogamous state. It’s a transition that’s highly unusual – but Opie says the end goal is pretty much unchanged.

“I would argue that both the promiscuity of chimpanzees and bonobos and monogamy in humans are counterstrategies to male infanticide, which is acute in large-brained primate species,” he said. In monogamous humans, “a single male is invested in the offspring and protects them.”

Still, there’s one other aspect to keep in mind here, and that’s that – well, we’re human, aren’t we? It’s perhaps not totally fair to measure us against other species in this way: “This study measures reproductive monogamy rather than sexual behavior,” Dyble admits, and “in humans, birth control methods and cultural practices break that link.”

Despite its limitations, though, Dyble argues that this rough method is still a pretty good way to gauge human monogamy patterns. And if we’re weird, with our big societies filled with stable, mixed-sex, multi-adult groups with many breeding pairs – well, the more the merrier.

“Monogamy is the dominant mating pattern for our species,” he said. But “humans have a range of partnerships that create conditions for a mix of full and half-siblings with strong parental investment, from serial monogamy to stable polygamy.”

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.