Working as a photographer for a charity a few years back, I was travelling through Malawi and stopped overnight in a mining town. It was a Wednesday, and I headed out to a bar. Other than a woman serving, everyone else there was male. Some were playing pool. Some were drinking, but most were doing neither. I asked the bargirl why there were no women in the place. With a look that suggested I was being dim, she explained: “The men get paid on Friday.”

On the surface, in a mining town, the gender pay gap is huge, with the vast majority of money officially going to men. And yet, by Saturday morning, much of the cash has been transferred to bar owners, prostitutes, girlfriends, and wives. A privileged observer might suggest that women in such a town ought to be liberated to earn their own money. But the point is that they already are. While most fair-minded people would no doubt agree that women should be free to take mining jobs if they choose, it’s unlikely that many women want such gruelling, dangerous, and unhealthy work when being a bar prostitute, a girlfriend, or a wife to a miner is available as an alternative.

The total value of the sex trade could be said to be the value of the net transfer of wealth from men to women. How can we begin to value an industry this big, ancient, and diverse, especially when much of it—probably most of it—is undocumented and untaxed?

During another African trip—this time to Bamako in Mali—I asked a young man whether he had a girlfriend. He explained “Non… pas moto, pas copine.” He had no moped, and so, no of course he didn’t have a girlfriend. He told me that the girls back home in his village were friendly and open, but the big-city Bamako girls had higher expectations. So, naturally, buying a moped became a priority for any aspiring young Malian. I had noticed that Bamako’s streets were filled with mopeds, mostly driven by men, but often with women riding pillion. For a man there, a moped means sex as well as transport. These anecdotes point towards the difficulty involved in calculating the extent of the sex trade. Some percentage of Bamako’s moped sales represent a hidden transfer of wealth from men to women: men buy mopeds, and women get free transport. What is this transfer of value worth?

Now consider how many similar transfers of wealth take place. The UK flower industry was worth almost £1bn last year. Of course, flowers are bought by many people for all sorts of reasons, but many are bought romantically by men for their female partners, or for courtship. What about restaurant meals, hotel rooms, concert tickets, diamonds, taxi fares, cocktails, vacations…? What proportion of money spent by men on these things is related to a promise, a hint, or a mere hope, of sex, whether fulfilled or unfulfilled? Historically, what proportion of the silk carried (invariably by men) along the Silk Road found itself worn by the wealthy wives and mistresses of Europe?

Males (in our species, and others) are, by definition, the low-value sex. The key difference between males and females in reproduction is that males are low investors and females are high investors. Female birds and reptiles lay big, nutritious eggs. Female mammals have to carry (and feed) infants for weeks or months of pregnancy, and then suckle them afterwards. Even in plants (at least those species that produce separate male and female flowers), the females are forced to invest more. It is no coincidence that marijuana farmers destroy male plants, and retain the females for their big, resin-heavy flowers. Females are more valuable, almost everywhere.

This truth about sex displays itself very differently in different species. In humans, it is expressed in a trade that is fundamental to us, and has shaped our recent evolution. In an essay entitled Why Do Men Hunt? (published in a collection entitled Why Is Sex Fun?), the science writer Jared Diamond hypothesises that hunting skills evolved in human males in order to acquire meat that could be traded for sex. Our recent evolution was heavily shaped by trade. Humans may not have the speed, strength, teeth, or claws of most predators, so our brains evolved instead. Our ancestors developed language, teamwork, advanced weapons, and the ability to strategise, because these abilities improved our chances of reproducing. A man who was a good hunter brought meat back to the clan, and a man with meat will mate more often and produce more children. The children in turn inherit the skills of their hunting fathers. The evolution of the modern human brain coincided with the extinction of the largest mammals (megafauna) on every continent, starting around 125,000 years ago. Until the rise of modern man, being big was a tried and tested survival mechanism. Humans changed that, and the largest mammals became an early casualty of the human sex trade.

If Diamond has answered the question Why Do Men Hunt?, the answer to the corollary question Why Don’t Women Hunt? ought to be obvious. Women didn’t hunt (in the traditional sense at least) because they didn’t have to. Hunting was dangerous and required a large investment of valuable calories. Why hunt when men will bring you meat? This does not mean, of course, that women were freed from intra-sexual competition. While men competed with each other in terms of hunting abilities such as strength, agility, and technical innovation, women competed to win the best meat (and sperm) from successful hunters. While female competition was less physical than the male variety, it was no less intense, and was focused on presenting attractiveness and youth (which are proxies for fertility and genetic health). Women therefore took a lot of interest in their own, and their rivals’ appearances, in order to copy techniques that other women employed to maximise their attractiveness, and to socially shun and stigmatise younger and better-looking rivals.

And so, a primitive economy emerged. The sex trade launched technological and economic growth, and the sex roles continued broadly as they had begun. Men relied on innovation, risk-taking, and social status to attract mates, and women became skilled in the arts of attracting (and preferably keeping) a mate. As the male-led industries evolved and diversified from their origins in hunting and fishing, thousands of new industries, roles and professions were spawned.

The original female industry—the sex trade—was undoubtedly far bigger than any of the other (male-created) industries, because its role was to collect a dividend on all male-led activity. The greater the innovation and diversification of male-run industries, the larger the sex trade became.

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As civilisation evolved, so did the sex trade. It began with “primitive prostitution”—straightforward trades of meat (and other rare gifts such as honey or decorative items) for sex, but with technological advances such as private property, money, and contracts (verbal or written), it became increasingly sophisticated. Private property allowed a man’s social status to be evaluated by his wealth (which, of course, he took care to display). Publicly acknowledged contracts allowed the development of marriage, in which women could offer exclusive access to their fertility in exchange for a male promise to provide for them (and their children). Sexual exclusivity was valuable to men, because it provided certainty over paternity. In exchange for this guarantee, men paid far more for sex with wives than they would for casual sex.

The price of (female) sex is driven by men’s ability to pay, and by availability. Unlike virtually any other commodity, the supply is fairly inelastic, since biology mandates (approximately) a 50-50 population balance between men and women. This means that, as the male-led industries have grown exponentially, the price of sex has kept track. While the average price of sex is very hard to estimate accurately, the price of prostitution is a good proxy and it is easy to measure. Cultural, economic, and demographic changes have all had the effect of increasing or reducing the price of sex. Wars and famines that reduce the male population more than the female population will naturally affect the ratio of supply to demand and lower the cost of sex. The sex-selective abortion of girls in countries like China and India, on the other hand, will increase it. Similarly, mass migration will tend to raise or lower the price, depending on the culture and gender balance of the migrants. When Polish people won free movement across the EU in 2004, I heard complaints from both low-skilled male friends in the building industry, and from sex workers that rates were being undercut by the new arrivals. The Economist suggested in 2014 that German sex workers had felt a similar effect. But (the same article reports), the price of sex has declined globally in recent decades, reflecting other trends, including online advertising for sex workers, hook-up culture driven by dating apps, and reduced social stigma for sex workers and women engaging in casual sex.

Unsurprisingly, sex workers are better aware than most of the value of sex, and less ashamed to discuss it without euphemism. I’ve seen social media posts from sex workers asking for free services, from photoshoots and car repairs, to video editing and rodent removal.

How does one value all this free stuff, given willingly by men in exchange for sex, or in the hope of sex, or merely to impress an attractive woman? This question is probably unanswerable to any degree of certainty. One thing is sure though: for men, sexual and romantic relationships can be expensive. In 2017, the Institute for Fiscal Studies found that men from poor backgrounds in their forties were twice as likely to be single as men from wealthy backgrounds. Another British study revealed that men spend about £1,300 a year more than women on dating. Dating is not just a way to discover a person’s personality, but a way to assess a man’s wealth and generosity. Women are advised by friends to “value themselves” and not sleep with men on a first date. The latest generation of dating apps produce data that reveals the extent of difference between male and female courtship behaviours. A study on Tinder, for example, found that men have to swipe right about 15 times more than women to get a similar level of response. These are not marginal differences, and they shine a light on an old reality: that female sex is vastly more valuable than male.

We may not be able to calculate the extent of the wealth transfer from men to women, but we can gauge the scale of the trade by examining male and female relative outcomes. The gender pay gap has become well known, and is widely (and falsely) presented as evidence of female disadvantage. The gap is typically calculated at between ten and 20 percent. Not only do women earn less than men on average, but they work fewer years of their lives than men. On this basis women must—surely—be poorer than men? If they are, this will be easy to demonstrate via various metrics. A naive researcher might expect the outcomes for gender to be similar to the racial disparities in the United States, where African Americans are paid less (on average) than Whites or Asians. Predictably, this racial pay gap is represented in other metrics: African Americans are more likely to be jailed, to be shot by police, and (most important) to die younger than other groups. Life expectancy is an excellent proxy measure of general wellbeing.

And yet, when the same measures are applied to gender, the outcomes are the reverse of what might be expected. American men are more than ten times more likely to be imprisoned than women and around 20 times more likely to be shot dead by police. Similarly, women outlive men significantly. How is it that women should be nominally poorer than men (based on pay differences, at least), yet by all metrics of wellbeing appear to be better off? This difference in lifespan tends to be blithely dismissed as “biology,” but this alone is no explanation. Yes, biology is the underlying reason men have worse outcomes than women. Not because men are inherently prone to die younger, but because the sex trade requires men to take the greatest risks and the toughest jobs.

The reality of these outcome gaps that black people represent a disproportionate proportion of America’s least successful 20 percent, and so do men. America’s prisons are full of poor people (disproportionately black and disproportionately male) who broke the law, in many cases, because they could find no other way to survive. Middle-class women with little understanding of how poor communities function, may instead be predisposed to find “toxic masculinity” a satisfactory explanations for male criminality. But working-class women tend to see things more clearly. When I interviewed Lady Andromeda, a black, south London sex worker, she explained simply: in poor communities, women can sell sex and do relatively well. In fact, working-class women in London who sell sex can easily earn more than most middle-class men. But what options do men have in the poorest communities? “They steal cars, or sell drugs,” she said. It is not, of course, that women cannot do these things. But they have a safer, better-paying, and (in London, at least) legal alternative. This is why poor men are far more likely to end up in prison, or murdered than either poor women or wealthier men.

So, women, thanks to the sex trade, have better outcomes than men. This still leaves the chicken-and-egg question: does the sex trade exist because women choose it, or because (as feminist theorists may claim) it is forced upon them by systemic misogyny and glass ceilings? Clearly, women do much better than men in poor communities and mining towns, but what about at the high end of society? We are often told that the gender disparity in corporate board positions is proof of a male-rigged system. Wouldn’t women become CEOs too, given the opportunity? It appears not. The book Superfreakonomics outlines a study of male and female MBA graduates. While women earned similarly to men early in their careers, the wage gap rapidly increased. It was found that women “…who leave the workforce are disproportionately those with very high-earning husbands.” It appeared that female MBA graduates often used their MBA to marry high-earning men rather than pursue long-term business careers. On paper, their earnings fell behind men, but in practice, their lifestyles were upheld by switching some of their corporate earnings for sex trade earnings. After all, being a senior manager of a large corporation is punishing, involving long hours, endless travel, and missing out on social and family time. Is it better to be a CEO or a CEO’s wife? Each occupation shares the same wealth, home, vacations, but the CEO’s wife arguably has a better lifestyle than her husband.

From Jared Diamond’s question Why Do Men Hunt? to the modern versions: Why Do Men Mine?, Why Do Men Sell Drugs? or Why Do Men CEO? the answers are similar. But the direction of travel looks positive for equality. The social trends of recent decades—for women to join the once-male economy, for increased sexual freedom, and for the price of sex to fall—point towards a narrowing of the gap in outcomes between men and women. Economic innovations such as Universal Basic Income may help narrow the gap further. Conversely, the current trends towards conservatism and nationalism may halt and reverse the liberal revolutions of the twentieth century, with potentially unhappy consequences for men and women.