Recent momentous events in Ireland and the United States regarding the legal status of same-sex marriage have triggered a debate reverberating around the world – what’s next?

The Chief Justice of the United States Supreme Court, John Roberts, has set the hares running with his opinion that there is now no reason for governments to prohibit polygamy. ‘It is striking how much of the majority’s reasoning (in support of gay marriage) would apply with equal force to the claim of a fundamental right to plural marriage,’ Roberts stated in his dissenting report on the historic Supreme Court judgment legalising same-sex marriage.

The push for polygamy has been gathering pace in recent years, and from diverse and unexpected quarters. Some economists, for example, such as the highly influential Nobel-prize winning Professor Gary Becker (now deceased), argue that government caps on spouse consumption are inefficient and a free market in marriage would maximise societal well-being. As Patri Friedman, the libertarian grandson of another Nobel-prize winning economist, Milton Friedman, dismissively puts it, ‘monogamy equals marriage Marxism.’

The political pressure for legalising polygamy might appear to be small – an unlikely coalition of Muslims, Mormons, libertarians, and some economists – but ideas can be powerful, and if the idea of plural marriage finds no coherent resistance then the prospects of change in the medium term are real.

To date commentators across the board appear to be struggling with compelling reasons for opposition to polygamy; conservatives have discovered that invoking tradition is no longer sufficient to win the public debate, while those on the left are uncomfortable in the role of saying no to unorthodox practices. What has become clear is that many people are so steeped in the familiar arguments around sexual freedom that they fail to see that polygamy entails an entirely different set of issues at stake, and that these issues revolve around Darwinian competition. Let me explain these deeper reasons why allowing polygamy would be a very bad idea.

In every society where polygamy is allowed there arises a hoarding of wives by generally higher wealth males and a corresponding deficiency of wives among generally lower wealth males. It is a mathematical rule that as soon as soon as some people take more than one wife, there are going to be others that miss out. Spouse-acquisition, in the terminology of economics, is a giant zero-sum game.

When we look at polygamy-permitting Arab and African cultures, for example, the rates of polygamy are generally between around five and twenty-five per cent, meaning that there are enormous numbers of males in those countries who simply never find a partner and never have children. Obviously this is deeply frustrating to those individual males, but on a large scale it is highly destabilising for those countries to have millions of angry, sexually-deprived men unable to access and enjoy the psychological, social and economic benefits that marriage and family can bring.

Recent anthropological research published by the Royal Society has found crime rates to be significantly higher in polygamous cultures, with the authors drawing attention to the subsequent loss of economic productivity compared to monogamous cultures. It is clear that the culture of monogamy that emerged in Ancient Greece and Ancient Rome and consolidated through Christian Europe has helped to make those populations more competitive and therefore productive. Economists that support polygamy should be highlighting African and Arab societies as shining beacons of marital welfare-maximisation – the fact that they don’t points to the limits in applying laissez-faire principles without regard to healthy competition.

If a Western country were tomorrow to introduce polygamy we would undoubtedly see the same phenomenon here as occurs in other polygamous societies: higher wealth males, and the sons of higher wealth families, would immediately start attracting extra mates, and their gain would be at the expense of other males. At first, the numbers involved would be limited because the cultural stigma still exists, but over time as people became familiar with the practice there is no reason to think that we wouldn’t eventually end up with rates resembling countries where the practice is already legal.

These arguments around Darwinian competition may be unfamiliar to many, but when we note these competition aspects we can see why the old arguments around sexual freedom don’t apply. When polygamy advocates ask ‘Who would it hurt? What is the harm?’, it is supposed to be the argument clincher, the unarguable appeal to the philosopher John Stuart Mill’s golden rule of liberalism.

What these advocates fail to realise is that their decision to legalise polygamy would entail real harm to people: it would immediately condemn millions of people to lives without marital partners and therefore without children. Their decision to render this large group of people childless would effectively be, in blunt terms, a form of mass castration.

Polygamy advocates might respond that we also need to take into account other factors such as the current harm to some females who are denied the opportunity of being part of a harem, and the current harm to some males who might otherwise be the head of these harems. My view is that of course we need to take account of all factors, but that these arguments are clutching at trivial straws. Similarly, some advocates might argue that the number of female-headed harems will cancel out the male-headed ones, ignoring the reproductive asymmetries that make them perennially unpopular (female-headed harems leave male members frustrated with a lack of progeny, male-headed ones don’t).

Clearly, the inroads that the polygamy movement is making present an historical challenge to the West. A number of very clever intellectuals, from Nobel prize-winning economists to Supreme Court judges, believe that polygamy’s time has come. For the general public, however, it would be wise not to let these new intellectual enthusiasms interfere with old-fashioned common sense.