‘‘Turkish princes have a dozen wives each” muses Jane Seymour in The Mirror and the Light, the final volume of Hilary Mantel’s trilogy on the life of Thomas Cromwell, chief minister to Henry VIII:

If the king had been of their sect, he could have been married to the late queen, God rest her, and Katherine, God rest her, and at the same time to me, if he liked. For that matter, he could have been married to Mary Boleyn, and to Mary Shelton, and to Fitzroy’s mother. And the Pope could not have troubled him about it.

There are many different ways of being polyamorous. Polygyny, historically the most common form, involves a single man partnered with multiple women, typically in a “hub and spokes” structure, in which the women are romantically linked to the man but not to each other. This might indeed have been a model that Henry VIII would have enjoyed, if it had been formally recognised in the England of the sixteenth century. High-status men, including Seymour’s “Turkish princes” within the Islamic tradition, have in some times and places been permitted to acquire many wives, occasionally forming harems consisting of thousands of women.

But in the modern West, polyamory looks very different. In a recent documentary presented by Louis Theroux, viewers were introduced to polyamorous people involved in a diverse range of innovative relationship models. One “throuple’”, Bob, Nick and Amanda, showed Louis their sleeping arrangement: a super-king bed, topped with three single duvets in blue, brown, and dinosaur print. With Amanda in her usual spot in the middle, the three of them chastely demonstrated the various cuddling options available (“If I were being the little spoon this time around and Nick wanted to join in . . . ”)

Polyamory is defined as taking part in multiple romantic relationships simultaneously, with the full knowledge and consent of all parties. Polyamorists may favour a hierarchical structure in which a primary partnership takes precedence over secondary relationships. Or, like Bob, Nick and Amanda, they might prefer a “vee” model in which one person — the “hinge” — is romantically involved with two people who are not involved with each other. A wide range of relationships fall under the polyamory umbrella, each described using terminology that may seem esoteric to outsiders.

Polyamorous relationships are not legally recognised within the UK, or indeed any other Western country. Polyamorists do not have access to any of the legal privileges that married people enjoy when it comes to matters like child custody and property inheritance. Nor are they protected from discrimination in the way that gay, lesbian, and bisexual people are. Not everyone in the polyamorous community is interested in acquiring marriage rights — thus turning polyamory to polygamy — but there are some who most certainly are.

“Welcome to the exciting world of the slippery slope,” wrote the American academic Fredrik deBoer in 2015, in response to Obergefell, the landmark civil rights case in which the US Supreme Court ruled that all 50 states were obliged to recognise same-sex marriages. Such a judgment was almost inconceivable a generation ago.

In 1989, at the height of the Aids crisis, Andrew Sullivan was the first person to make the argument for same-sex marriage in a major American publication. He provoked outrage, including from some other members of the gay community, who saw Sullivan’s proposal as anathema to their own ambition to radically overhaul relationship norms — an ambition shared by some polyamorists. Others, though, are eager for recognition within the institution of marriage, and suggest that their own community might now be at the stage that the LGB community was at in 1989. It’s possible that the legalisation of polygamy may indeed be the next slip down a thrillingly slippery slope.

A wide range of relationships fall under the polyamory banner, each described in esoteric terms

Marriage used to be defined as a conjugal union, “a holy mystery in which man and woman become one flesh” in the words of the marriage service of the Church of England. While of course there were cracks in this edifice — infertile people were permitted to marry, as were people beyond reproductive age — marriage was nevertheless understood to be based around, as Robert P. George, professor of jurisprudence at Princeton, puts it, a couple’s “sexual-reproductive complementarity . . . [which was] specially apt for, and would naturally be fulfilled by, their having and rearing children together.”

Our modern conception of marriage, at least among secular liberals, is closer to what philosopher John Corvino describes as “your relationship with your Number One person”. Rather than a bond based on economic or reproductive compatibility, the relationship with one’s spouse is instead intended as a source of sexual and emotional fulfilment. And now that we have let go of the idea that your Number One person ought to be a member of the opposite sex, it becomes difficult to argue against the idea of expanding the definition of marriage still further to include other consenting adults who would like to be legally joined in union with their Number One people.

Those liberals who insist that polyamorous relationships should be honoured and respected, but should not be granted recognition through marriage, have got some explaining to do. As Robert P. George writes, arguments against polygamy are starting to sound “more and more like mere rationalisations for stigmatising what many people (for now, at least) still find icky”.

Survey data suggests that polyamory is indeed viewed as “icky” by many people. A 2013 study found that polyamorous people were often viewed as immoral and untrustworthy by their fellow Americans, and were even more likely than Black Americans to report experiencing overt prejudice. It doesn’t help that polyamory is often associated with New Age, countercultural lifestyles, which are viewed with suspicion in much of mainstream society.

Polyamorists skew left — far left, in fact — and media portrayals often emphasise the non-conformism of the community. A typical article in Quartz quotes a non-binary demigirl called Indigo who is part of the polyamorous community of Brooklyn: “I think I’m changing the world . . .  I’m creating a long- and short-term community in which people can know their truest selves.” No wonder conservatives are wary.

And yet there are a lot of people, from across the political spectrum, who tell researchers that they are interested in pursuing polyamorous relationships, and the proportion of people who describe themselves as polyamorous is surprisingly high, particularly among Millennials and Gen Z. In fact, the number of Americans who identify as polyamorous (between 4 and 5 per cent) is larger than the number of Americans who identify as gay or lesbian (2 per cent).

Sceptics are wrong to suggest that polyamory is somehow not a “real” sexual orientation, and therefore not comparable to being lesbian, gay, or bisexual. Polyamory correlates with a stable, moderately heritable trait that psychologists call “sociosexuality”. People high in this trait tend to be more sexually adventurous, have more sexual partners over their lifetimes, and are more interested in casual sex. This is a fundamental component of one’s sexual identity, and many people high in sociosexuality report feeling unhappy and frustrated within monogamous relationships. As the novelist Anita Cassidy describes it in the Guardian (a newspaper which seems to publish an article on polyamory every other week):

It was the hardest thing I’d ever had to say to my husband, Marc. Three years ago, I sat down and told him: “The idea of having sex just with you for the next 40 years — I can’t do it any more.” But I had come to realise that my life was built around something I didn’t believe in: monogamy.

Cassidy and her (now ex) husband were fundamentally dissimilar in their sociosexuality — a conflict that proved impossible to resolve because some people do seem to be innately more inclined towards monogamy or polyamory.

And polyamorists are right to argue that institutionalised monogamy is neither natural nor inevitable. Only about 15 per cent of societies in the anthropological record have been monogamous. Monogamy has to be enforced through laws and religious customs, and even within societies in which it is deeply embedded, plenty of people defy convention by having affairs, buying sex, and getting divorced. To date, monogamy has been dominant in only two types of society: small-scale groups beset by serious environmental privation, and some of the most complex civilisations to have ever existed, including our own.

Non-monogamous societies have almost always been polygynous, in that men have been permitted to marry more than one woman. Polyandry — the marriage of one woman to multiple men — is astonishingly rare. When it does occur, it is typically an arrangement borne of economic necessity, rather than female desire. For instance, some societies permit two brothers to marry the same woman in order to simplify inheritance arrangements. In a few cultures, mating customs look very strange to us. The Na in China, for instance, are famous for having no institution of marriage and deliberately suppressing long-term pair bonding. A few Amazonian groups believe that a child can have two or more biological fathers. Such societies are notable precisely because they are so unusual.

So polyamory is not new, exactly. It’s just that it has historically appeared almost exclusively in one form. Stepping back and looking at the whole sweep of human history, it seems that our ancestors typically faced a choice between two different mating models — monogamy or polygyny — and, poised at a moment in which we are asked to choose whether or not to redefine our own marriage institution, it is worth pausing to reflect on why Western culture settled upon monogamy in the first place, how it became so dominant, and thus what might be the consequences if it were to be formally abandoned.

The proportion of people describing themselves as polyamorous is surprisingly high

Traditional polygyny suits the interests of one group of people in particular: high-status men. It is bad news for low-status men, who find it far more difficult to attract women when richer, more attractive, and more powerful men are permitted to continue accumulating wives. We know that unmarried men, feeling themselves to have no stake in the future, are twice as likely as married men to commit property and violent crime.

We know this not only because researchers have compared crime rates across contemporary African countries with different marriage systems, but also through ingenious studies that investigate the effects of an excess of unmarried men in contexts including the American Wild West, nineteenth-century Utah, and China in the aftermath of the one-child policy.

Another group that fares badly in a polygynous society is women. Polygyny results in all sorts of unwelcome demographic trends: higher birth rates, greater age differences between spouses, younger brides, and an increase in domestic violence, including between co-wives, who — far from engaging in sisterly cooperation — are in fact more likely to come into conflict with one another. On average, women in polygynous households have less autonomy and less influence over their husband’s decision-making. Young, unmarried women are also considered to be a more valuable “commodity” in polygynous societies, which encourages men to tightly control their female kin, disguising these restrictions as “protection”.

Polygyny may also have a negative effect on children. Living with unrelated adults has long been known to be a risk factor for child abuse, so much so that Steven Pinker has written that “step- parenthood is the strongest risk factor for child abuse ever identified”. One famous study found that step-parents (mostly fathers, but also mothers) were more than a hundred times more likely to murder children compared with biological parents, a phenomenon sometimes described as the “Cinderella effect”.

Of course, correlation is not the same as causation, and it is important to emphasise that there is no evidence to suggest that child abuse and domestic violence are any more prevalent in modern polyamorous households than monogamous ones. However, when we look to the past, there is good reason to suspect that, although monogamy may be tough for women and children in many traditional societies, polygyny is a whole lot worse.

When monogamy is imposed on a society, it tends to become richer. Birth rates and crime rates both fall, which encourages economic development. Wealthy men, denied the opportunity to devote their resources to acquiring more wives, instead invest elsewhere: in property, businesses, employees and other productive endeavours.

This is, it seems, the solution to what anthropologists have called “the puzzle of monogamous marriage”. How is it that a marriage system that does not suit the interests of the most powerful members of society — high-status men — has nevertheless come to be institutionalised across so much of the world? The answer is that, although monogamy is less satisfactory for these men, it is better for societies as a whole.

Our own monogamous practice can be traced back to Ancient Athens and Rome, down through the Christian tradition, to later be spread to other parts of the world through European colonialism. It is no coincidence, so the argument goes, that some of the wealthiest societies in the history of the world have been monogamous — monogamy comes first, and wealth follows after. And wealthy countries are able to spread their influence far afield, bringing marriage customs along with them.

Polyamorists are therefore quite right to argue that monogamy is a cultural relic. They are also right to associate monogamy with some of the most hated enemies of the liberal left: colonialism and Christian conservatism. Most of our ancestors did not live monogamously, including our very earliest ancestors. The old homophobic slogan, “God made Adam and Eve, not Adam and Steve” gets things wrong for more than one reason. The archetypical heterosexual relationship ought really to be Adam, Eve . . . and Eve’s co-wife, or even co-wives.

And perhaps, in the wake of the sexual revolution, monogamy is on the way out. New cultural norms that allow both men and women to pursue casual sexual relationships, combined with a wealth of data from dating websites and apps, allow us to analyse what we might unromantically refer to as the heterosexual “dating economy” when the restrictions imposed by Christianity and traditionalism are removed. These insights lend greater weight to the claim that, left to our own devices, human beings tend to settle at a point of polygyny, not monogamy.

As the data scientist Bradford Tuckfield points out, based on the way people behave on dating apps, “The great majority of women are only willing to communicate romantically with a small minority of men while most men are willing to communicate romantically with most women.” What we tend to see is that the most desirable men attract a lot of interest from women, which they are usually willing to reciprocate, even when women fall “below” them in the brutally Darwinian hierarchy of physical attractiveness.

Meanwhile, the least desirable men attract no interest whatsoever from women. When the sexiest, richest, funniest, smartest men are not expected to commit themselves to just one woman, they will tend to play the field. The result is a dating world that looks an awful lot like a polygynous society.

An important factor to consider here is the difference between men and women when it comes to sexual behaviour. Sociosexuality — the taste for sexual variety that forms part of a person’s sexual orientation — is not evenly distributed between the sexes. Men are, on average, higher in sociosexuality than women are, a finding that holds across a diverse range of cultures.

This means that, all else being equal, men are more likely to be interested in having more than one sexual partner, and surveys do indeed find that an interest in polyamory is disproportionately found among men. This may well be part of the explanation for why, in straight couples, men are more likely to cheat. It may also explain why gay men are more likely than lesbians to choose open relationships, and also accumulate a larger number of sexual partners across their lifetimes. There is no doubt that culture has a role to play here. Women are stigmatised for high socio-sexuality in a way that men are not, so it may be that women are more reluctant to risk being labelled “sluts”,  even in anonymous surveys. And although the male mean may be higher, there is substantial overlap between the two bell curves, with plenty of women falling towards the higher end of the spectrum of sociosexuality.

Nevertheless, the average differences between the sexes go some way towards explaining why the modern dating market has a tendency to drift towards polygyny. At the moment, and for most people, this state of affairs is time-limited. Eventually, in a culture that still prizes monogamy, high-status men will commit to a single partner and so remove themselves from the market. But what if they don’t? If polyamory were to become increasingly mainstream, as the evolutionary biologist Geoffrey Miller expects it to, might we end up sliding back towards the relationship model of our ancestors?

I put this question to Miller, who suggests that the modern world may well be different enough to permit entirely new forms of relationship structure to emerge:

I think human nature doesn’t change much, but I think that mating markets, reproductive costs and benefits, sexually- transmitted organism prevalence and risk, contraceptive technology, economies, social norms, and moral ideals can change dramatically enough that mating patterns can change quite substantially.

He sees the shift towards a greater degree of polyamory as both feasible and desirable, since it allows people higher in sociosexuality to seek out sexual variety while also allowing others to remain monogamous if they wish. He suspects that within 30 years we might well see as many as 25 per cent  of Westerners identifying as polyamorous, and he argues that such a shift needn’t entail any radical change in norms. In fact, it might well be less revolutionary than other changes we have already lived through, such as the rise of women working outside the home in the post-1960s era. And as for the skew in sociosexuality between the sexes, Miller observes that although a lot of men are eager to try polyamory, many go on to discover that it is not all that they had hoped for:

It’s hard to frame it as a new cause when polyamory is, historically, a step backwards

It’s true that when monogamous couples are first considering opening up their relationship, it’s often the male who initiates the discussion, imagining it will bring him a lot of sexual variety without much work. Many women agree reluctantly — but then, a common pattern is that once the women start using online dating apps, the women realize it’s much easier for them to attract outside men (“secondary partners”) than it is for the men to do that. The women revel in the courtship and attention that they’re getting, and the men who first asked for the open relationship realize that they now face a very tough learning curve in terms of managing their sexual jealousy. Some succeed in mastering it, but many don’t.

The key task facing those in the polyamorous community, as Miller sees it, is to disassociate themselves from the far left, countercultural reputation they have developed, and start appealing to centrists and conservatives. “Polyamory is going mainstream, like it or not,” he writes, and “it won’t be an existential threat to Western civilisation.”

Looking at the likes of Bob, Nick and Amanda, spooning under their dinosaur print duvet, it’s hard to believe that polyamory could ever be considered objectionable. Our world has changed so much that the lives of modern Westerners are almost unrecognisable when set against the lives of our ancestors. It’s perfectly possible that today’s polyamorists might succeed in forming entirely new relationship models, compatible with feminist and egalitarian principles. There are many who are determined to try.

But what makes it difficult to frame polyamory as a new civil rights cause, akin to the gay rights movement, is the fact that the history looks so remarkably different. Polyamory is, from an historical perspective, a step backwards, not forwards. It is tempting to take a
naive view and assume that, as the stigmatised sexual minority that they surely are, polyamorists ought to be treated as equivalent to LGB people, who have traditionally been punished and pushed underground within repressive societies.

But the polyamorists of the past were powerful men, not an oppressed minority, and the marriage customs that suited their interests did not suit the interests of women, children, and men of lower status. Is it possible that, in legalising polygamy, we might inadvertently stumble back towards that kind of society? We simply don’t know.

Fredrik deBoer, writing in 2015, gives an argument that will be familiar to liberal readers:

Progressives have always flattered themselves that time is on their side, that their preferences are in keeping with the arc of history . . .  given what you know about the advancement of human rights, are you sure your opposition to group marriage won’t sound as anachronistic as opposition to gay marriage sounds to you now?

No, I’m not sure. How could I be? But let me sound just one note of caution. “Progress” is not linear. Perhaps the legalisation of polygamy is the next step in a positive movement towards greater tolerance and respect for the sexual autonomy of consenting adults.

Or perhaps the 2,000 years of institutionalised monogamy that we have just come to the end of, in which societies have been able to produce unprecedented levels of societal affluence and gender equality, will prove to have been a blip, to be swallowed up within the 200,000 years of our species’ history. And perhaps we’ll miss it once it’s gone.