The mystery of monogamy: scientists claim to have the answer

Study shows how monogamy has risen and fallen for different species over 75m years. Photograph: Alamy

As an enduring mystery of the human condition, it has been praised and damned in equal measure. It is a bridge over the abyss of isolation but can be a bit like croquet: easy enough to grasp the rules but a hard game to enjoy.

Now scientists in Britain have taken the puzzle of monogamy and boiled it down to one big question: how did it come about in the first place? A new study claims finally to have an answer.

"Humans have ended up monogamous to some extent, it's the predominant way we live," said Christopher Opie, an anthropologist at University College London. "What we have now is an evolutionary pathway for the emergence of monogamy."

How males and females come together is as varied as anything in nature. Around 90% of birds live in pairs, but less than 3% of mammals do. Humans and other primates fall somewhere in the middle, with a quarter of species pairing up, according to Opie.

What leads some species to monogamy and others not has prompted scientists to come up with three possible explanations.

One is that when offspring are demanding, two parents might be better than one. The second, known as "mate guarding", proposes that males need to stay close to their mates to ward off rival males. The third is that males stick with females to defend their offspring against the violence of other males, who want to kill existing offspring so that females become fertile again and can be impregnated.

Opie's team drew up a plan to find out which hypothesis most likely led to monogamy. They took a family tree of 230 animals, including lemurs, bushbabies, monkeys, apes and modern humans, and collected information on their mating behaviour, rates of infanticide and paternal care.

Next, they used simulated evolution from 75m years ago to modern day. As the simulation ran, it showed how monogamy rose and fell for different species. Having run the program millions of times, they found that the evolution of monogamy in primates was preceded by one thing only: infanticide by males.

"You do not get monogamy unless you already have infanticide, and you do not get a switch to paternal care if you don't already have monogamy," said Opie, in research published in the journal Proceedings of the National Academy of Sciences.

Opie says the findings are linked to increases in brain size. In primates, brain size expanded as social groups grew larger, which meant that mothers were infertile for longer and more males were close by. For males, infanticide was a strategy to make females fertile again.

"Monogamy is only one strategy for dealing with infanticide. But it's not the only one," said Opie. "Chimps mate with all the males in their group to confuse paternity so males won't attack. But in others, humans included, males stick with females to protect them." Once a species becomes monogamous, paternal care and other behaviours evolve that help offspring to thrive, he said.

Robin Dunbar, a co-author on the paper and director of the social and evolutionary neuroscience at Oxford University, said : "It highlights the risk of infanticide and harassment for primates in general and it simply gets worse and worse as you get bigger brains, because the reproductive cycle gets longer." Dunbar believes monogamy evolved in our human ancestors only after modern forms emerged 200,000 years ago, and perhaps in the past 100,000 years.

For some scientists, however, the mystery of monogamy remains unsolved. Dr Maren Huck, who studies animal behaviour at the University of Derby, said the findings should be treated with "extreme caution". She said the authors had labelled some animals as monogamous that were not in the wild, and made false assumptions about the impact of infanticide. "Owl monkeys give birth once a year. So if the female loses her offspring, she will not resume oestrus again until the next breeding season the following year. Hence, killing the offspring would not hasten reproduction," she said.

She also queried the classification of monogamy, saying the numbers seemed very high. "Very few old world monkeys, for example, are monogamous."

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So monogamy works for some animals. Doesn't mean it's 'natural' for us | Meg Barker

'It seems that our focus on monogamy in animals reflects our current cultural preference for internal explanations of human behaviour.' Photograph: Carl De Souza/AFP/Getty Images

There has been a good deal of press coverage surrounding a research study that addressed the question of how human monogamy came about, from an evolutionary perspective. This suggested that males in monogamous mammal species remain with female partners to protect their families from other males, who would otherwise kill the young and mate with the females.

This is no doubt an interesting study, as is other recent evolutionary research highlighting how unusual monogamy is across animal species (reported in the book Sex at Dawn). However, more interesting to me is the focus of such research – and the journalists who report it. If we are genuinely interested in human monogamy, I wonder why our main focus is how it evolved in animals.

It seems that this reflects our current cultural preference for internal explanations of human behaviour. We seem drawn to neuroscience and evolutionary biology to address our questions, rather than to the social sciences or philosophy for example. Undoubtedly the ways in which humans evolved, and the ways in which our brains and bodies now function, are part of the picture of how we relate to one another. However, there are clearly many more contributing factors than that. We require a biopsychosocial approach, rather than one that engages with biology alone.

As a researcher I use a biopsychosocial approach to give a fuller picture of human relationships. As a therapist I also find such an approach useful in helping people to make sense of their relationships. People in the therapy room grapple with questions of how to maintain their freedom while in a relationship, how to form loving connections, and how to remain in relationships when they can be so painful. Here, it is helpful to have a sense of the diverse ways people can structure their relationships, and the common human dilemmas around freedom, belonging and craving the approval of others, which pervade such concerns.

In relation to social aspects, when we look back through time and across cultures, we see a diversity of relationships, structures and rules. More societies are polygamous than monogamous, and the forms that polygamy takes vary greatly. In many seemingly monogamous societies, secret non-monogamy and/or serial monogamy are actually the norm.

The basis of western partner relationships has shifted over the past century from pragmatic and economic concerns to an overwhelming emphasis on romantic love. There have been related shifts in living arrangements from extended to nuclear families, and with increasing numbers of people living alone and not having children. Of course the recent extension of marriage to same-sex couples reflects another change which research on male/female relationships alone will struggle to inform. Increasing gender equality is a further shift which has a huge impact on how we now conduct relationships. The internet too raises new questions around what counts as sexual or emotional fidelity. With new technology there has been a proliferation of forms of open non-monogamy: hook-up culture, friends with benefits, monogamish relationships, polyamory, and relationship anarchy, to name but a few.

Turning to psychological elements, we find that people experience their relationships in a diversity of ways, even within one culture or community. Ask people their reasons for getting married, or swinging, or having an affair, and you will get a wide range of responses. For example, some openly non-monogamous people emphasise their individual freedom or hedonistic pleasure; some have political reasons to do with the patriarchal and capitalist history of monogamous marriage; some feel it is an inherent part of their being, akin to a sexual identity; some wish to connect with multiple people, to find a sense of belonging, or to avoid the risks inherent in pressurising one relationship to meet all of their needs.

We need to get beyond our cultural obsession with what is "natural" when it comes to human relationships, and the common assumption that this equates to what is "normal" and also to what is "good". Instead we should turn our attention to the diversity of ways in which humans connect, and ask ethical questions about how we relate to each other in a world of ever-changing relationship rules.

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