The poet Philip Larkin famously proclaimed that sex began in 1963. He was inaccurate by 800 million years. Moreover, what began in the 1960s was instead a campaign to oust sex—in particular, sex differences—in favor of gender.

Why? Because biological differences were thought to spell genetic determinism, immutability, anti-feminism and, most egregiously, women's oppression. Gender, however, was the realm of societal forces; "male" and "female" were social constructs, the stuff of political struggle; so gender was safe sex.

The campaign triumphed. Sex now struggles to be heard over a clamor of misconceptions, fabrications and denunciations. And gender is ubiquitous, dominating thinking far beyond popular culture and spreading even to science—such that a respected neuroscience journal recently felt the need to devote an entire issue to urging that sex should be treated as a biological variable.

And, most profoundly, gender has distorted social policy. This is because the campaign has undergone baleful mission-creep. Its aim has morphed from ending discrimination against women into a deeply misguided quest for sameness of outcome for males and females in all fields—above all, 50:50 across the entire workplace. This stems from a fundamental error: the conflation of equality and sameness. And it's an error all too easily made if your starting point is that the sexes are "really" the same and that apparent differences are mere artifacts of sexist socialization.

Consider that 50:50 gender-equal workplace. A stirring call. But what will it look like? (These figures are UK; but ratios are almost identical in all advanced economies.) Nursing, for example, is currently 90% female. So 256,000 female nurses will have to move elsewhere. Fortunately, thanks to a concomitant male exodus, 570,000 more women will be needed in the construction and building trades. Fifteen thousand women window-cleaners. One hundred twenty-seven thousand women electricians. One hundred forty-three thousand women vehicle-mechanics. One hundred thirty-one thousand women metal-machinists. And 32,000 women telecom-engineers.

What's more, the most dangerous and dirty occupations are currently almost entirely 100% male—at least half a million jobs. So that will require a mass exodus of a quarter of a million women from further "unbalanced" occupations. Perhaps women teachers could become tomorrow's gender-equal refuse-collectors, quarry workers, roofers, water-and-sewage plant operators, scaffolders, stagers and riggers?

And perhaps gender-balanced pigs could fly? At this point, the question becomes: If that's the solution, what on earth was the problem? Gender proponents seem to be blithely unaware that, thanks to their conflation of equality and sameness, they are now answering an entirely different set of concerns—such as "diversity," "under-representation," "imbalance"—without asking what on earth they have to do with the original problem: discrimination.

And the confusions ramify. Bear in mind that equality is not sameness. Equality is about fair treatment, not about people or outcomes being identical; so fairness does not and should not require sameness. However, when sameness gets confused with equality—and equality is of course to do with fairness—then sameness ends up undeservedly sharing their moral high ground. And male/female discrepancies become a moral crusade. Why so few women CEOs or engineers? It becomes socially suspect to explain this as the result not of discrimination but of differential choice.

Well, it shouldn’t be suspect. Because the sexes do differ—and in ways that, on average, make a notable difference to their distribution in today's workplace.

So we need to talk about sex.

Here's why the sexes differ. A sexual organism must divide its total reproductive investment into two—competing for mates and caring for offspring. Almost from the dawn of sexual reproduction, one sex specialized slightly more in competing for mates and the other slightly more in caring for offspring. This was because only one sex was able to inherit the mitochondria (the powerhouse of cells); so that sex started out with sex cells larger and more resource-rich than the other sex. And thus began the great divide into fat, resource-laden eggs, already investing in "caring"—providing for offspring—and slim, streamlined sperm, already competing for that vital investment. Over evolutionary time, this divergence widened, proliferating and amplifying, in every sexually reproducing species that has ever existed. So the differences go far beyond reproductive plumbing. They are distinctive adaptations for the different life-strategies of competers and carers. Wherever ancestral males and females faced different adaptive problems, we should expect sex differences—encompassing bodies, brains and behaviour. And we should expect that, reflecting those differences, competers and carers will have correspondingly different life-priorities. And that's why, from that initial asymmetry, the same characteristic differences between males and females have evolved across all sexually-reproducing animals, differences that pervade what constitutes being male or female.

As for different outcomes in the workplace, the causes are above all different interests and temperaments (and not women being "less clever" than men). Women on average have a strong preference for working with people—hence the nurses and teachers; and, compared to men, they care more about family and relationships and have broader interests and priorities—hence little appeal in becoming CEOs. Men have far more interest in "things"—hence the engineers; and they are vastly more competitive: more risk-taking, ambitious, status-seeking, single-minded, opportunistic—hence the CEOs. So men and women have, on average, different conceptions of what constitutes success (despite the gender quest to impose the same—male—conception on all).

And here's some intriguing evidence. "Gender" predicts that, as discrimination diminishes, males and females will increasingly converge. But a study of 55 nations found that it was in the most liberal, democratic, equality-driven countries that divergence was greatest. The less the sexism, the greater the sex differences. Difference, this suggests, is evidence not of oppression but of choice; not socialization, not patriarchy, not false consciousness, not even pink t-shirts or personal pronouns … but female choice.

An evolutionary understanding shows that you can't have sex without sex differences. It is only within that powerful scientific framework—in which ideological questions become empirical answers—that gender can be properly understood. And, as the fluidity of "sexualities" enters public awareness, sex is again crucial for informed, enlightened discussion.

So for the sake of science, society and sense, bring back sex.