A BEAUTIFUL man with high cheekbones, fluttering eyelashes and a galaxy of silver glitter in his hair strides into the room. He is wearing a wedding dress and dirty trainers. The gender-bending at this club night in east London is not new: Shakespeare’s comedies are filled with cross-dressers; Gladys Bentley stomped the boards of 1920s Harlem in a tuxedo; Ziggy Stardust, David Bowie’s ambiguous interstellar alias, landed in the 1970s. What is new, though, is that convention-defying statements of gender identity are moving from stage and dance floor to everyday life.

The word “gender” is used by prudes to avoid saying “sex”, and restricted by purists (and, until recently, The Economist’s style guide) to speaking about grammar. In the 1970s feminists described the restricted behaviour regarded as proper to men and women as “gender roles”. But in recent years “gender identity” has come to mean how people feel or present themselves, as distinct from biological sex or sexual orientation. Growing numbers of young people describe themselves as “non-binary”. Others say gender is a spectrum, or that they have no gender at all. Facebook offers users a list of over 70 gender identities, from “agender” to “two-spirit”, as well as the option to write in their own.

New and old notions of gender identity collide most starkly in transgender people: those who do not identify with the sex on their birth certificates. They may transition from a male identity to a female one, or vice versa, perhaps taking sex hormones and having surgery to make their bodies match how they feel and want to be seen. Some have become celebrities. Laverne Cox, the transgender star of “Orange is the New Black”, appeared on the cover of Time in 2014. Vanity Fair profiled Caitlyn Jenner, formerly Bruce, an Olympic gold-medallist, the following year. Last December National Geographic put transgender children on its cover.

This growing prominence is in some ways surprising. Though clinics that treat gender dysphoria—distress caused by a mismatch between felt and perceived gender identity—report a soaring caseload, transgender people are still rare. The Williams Institute, a think-tank in Los Angeles, recently came up with an estimate of 1.4m Americans—0.6% of those aged 16-65. Moreover, young people say that gender matters less than it used to, which sits oddly with the spreading belief that gender dysphoria can be severe enough to justify the upheaval and risks of transitioning.

But transgender identities raise more general questions, and not only for those cultural conservatives who regard them as transgressing the natural, perhaps God-given, order. There is a tension between believing that it is possible to feel, act or look so much “like a woman” that you should be acknowledged as one, and believing, as feminists do, that a woman can act in any way she wishes without casting doubt on her womanhood. A war of words has broken out between some transgender activists and women they call TERFs (trans-exclusionary radical feminists) about who should be let into women-only spaces, from domestic-violence refuges to women’s literary and sports competitions.

Such questions are most urgent for people who question their gender identities. But they also illuminate the extent to which gender identity is a meaningful human characteristic. And they have made transgender rights an issue in America’s culture wars, most recently in battles over who gets to use which public toilets.

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Some parents face a more visceral question: what to do with children who say they have been classified as the wrong sex? Should parents resist, telling them that whatever they think they could do if they switched sex, be it dress differently, play different games or hang around with different friends, they can also do without switching? Or should they support their children to transition? How to predict which children will later decide they are in the right body after all?

The answers to such questions depend on what it means to be male or female. The starting point is genetic. As well as 22 pairs of matched chromosomes, female humans have two X chromosomes. Males have one X and a smaller Y. From this follow hormonal differences that shape female and male bodies, with most of the work done in the womb and during puberty. By every physical criterion—chromosomes, genitals, blood hormones, appearance—most people can easily be classified as one or the other.

Human females and males differ little in regard to most abilities and behaviour. The most marked difference, says Melissa Hines, a professor of psychology at Cambridge University, is in fact gender identity, though few notice, since deviations from the norm are so rare. Next most marked is sexual orientation, with all but a few percent of people mostly or exclusively attracted to the other sex. Differences less clearly linked with reproduction are even blurrier. The best-supported is that, allowed to choose between wheeled toys and dolls, boy toddlers choose the wheels slightly more often, and girls the dolls. (Since monkeys show similar preferences, this could be part of evolutionary history.)

But as many as 1% of people have a “disorder of sex development”. Most suffer only a minor genital anomaly, but doctors will struggle to classify a few as male or female. The genitalia of some such “intersex” people are a combination of male and female. Some XX people produce unusually high levels of androgens (male hormones) in the womb, and some XY ones do not respond to androgens in the usual way. They may be born with bodies that are more typical of XY or XX people, respectively. Their birth registrations may clash with their genes.

Lost in classification

Until recently intersex children usually received the surgery doctors thought most likely to produce a body typical for one sex or the other. Now many think doctors should wait until children can decide what to do themselves. In 2013 the UN special rapporteur on torture condemned gender-normalisation surgery for children. Eric Vilain, a medical geneticist in Washington, DC, is leading a longitudinal study on the treatment of intersex children. “Right now, we’re exploring a lot of diagnoses, without the appropriate research,” he says.

Intersex people are unusually likely to switch gender identity at some point, perhaps because those identities are less stable or they were misclassified in childhood. Their existence, and their varying gender expressions, show that biological sex is neither cleanly binary nor inseparable from gender identity. But most gender-dysphoric people have no known anomaly of hormones, physique or brain structure. Some neuroscientists think they have found atypicalities in such people’s brains; others are unconvinced.

Lacking an observable cause, trans people can find it hard to convince others of their felt identities. Something so inward is hard to demonstrate. It is also hard to explain. Danielle Castro, who works at the Centre of Excellence for Transgender Health in San Francisco, is a trans woman. Asked why she transitioned from the male identity on her birth certificate, she searches for words: “my own innate sense of self…I feel more comfortable; this is who I am.” It is harder to explain transgender identities to “cisgender” people (“cis” is the Latin prefix for “on this side of”) than to convince heterosexual people of the reality of homosexual desire, she says. “It’s easier to accept that ‘love is love’.”

Gender-dysphoric adults may be offered gender reassignment. The established protocol is to take cross-sex hormones and live in the target identity for a year or two. If psychiatrists agree, they may then be offered the delicate surgery whereby genitals are reshaped. Many trans people do only “top” surgery—breast reduction or enlargement. “Some of the most helpful surgeries are chest and facial because that is what people see,” says Colt Keo-Meier, a psychologist (and trans man).

Doctors naturally worry that a patient may regret such life-altering treatment. Not all the changes wrought by cross-sex hormones are reversible, and genital surgery may cause sterility. Conversely, some trans people find it frustrating to have to convince doctors to permit them to transition. This may feel like pandering to stereotypes. Sam Blanckensee, a 23-year-old Irish trans man, says he resented having to act hyper-masculine to get surgery. After having top surgery and no longer needing to convince anyone, he feels closer to non-binary. “In the eyes of my doctors I would have been seen as binary. I stuck with that idea because it was easier to get the right medicines and procedures.”

And yet trans people themselves can also fall back on gender stereotypes—provoking furious rows with feminists. In March Jenni Murray, the host of BBC Radio 4’s “Woman’s Hour”, wrote of interviewing two trans women: India Willoughby, a television presenter, and Carol Stone, an Anglican vicar. Ms Willoughby endorsed workplaces requiring women to wear makeup, and said unshaven legs on women were “dirty”. The Rev Stone said her main concern after transitioning was what to wear to meet parishioners.

“‘Feeling like’ a woman seems to imply feeling like wearing mascara, heels, hair extensions and stockings,” wrote Lionel Shriver, a novelist (who has written for this newspaper), last year in an essay titled “Gender—Good for Nothing” in Prospect, a British magazine. “The version of femininity offered up by Caitlyn Jenner is foreign to me—exaggeratedly coiffed, buffed and corseted.” That “version of femininity” riles many feminists. Simone de Beauvoir’s famous remark that “a woman is not born, but made” was intended as a criticism of the arduous feminine ideal that deformed women’s lives, not as a promise that attaining that ideal conferred womanhood.

But a woman who takes such a line now risks being called a TERF, as Ms Murray and Ms Shriver have been. Indeed, any exploration of transgender identities can be risky (as trans people know better than anyone). Rebecca Tuvel, a philosopher at Rhodes College in Memphis, was pilloried for her article, “In Defence of Transracialism”, published in March. It argued from a viewpoint sympathetic to transgender identities that Rachel Dolezal, a white woman who described herself as black, should be accepted in her chosen racial identity. More than a hundred academics called for its retraction, saying that it caused “harm” to trans people, for example by “dead-naming” a trans woman, that is, referring to her by her former male name. They omitted that the trans woman in question was Ms Jenner, who often talks about life as Bruce.

Attempts to make language more inclusive of trans people mean that in some quarters the very words “man” and “woman” are falling out of use. Some sexual-health clinics now talk about “people with prostates”, “people with vaginas” and so on. An article in the Tab, a student magazine, about stress and the menstrual cycle avoided the words “female” and “women”, noting that over a third of “students with uteruses” at Cambridge had missed periods. Such redefinitions can be merely a way of signalling political virtue. And they cause more trouble for women than for men, since it is women who more often need to organise and speak collectively, for example about maternal and contraceptive services, discrimination and harassment, and sexual violence.

Rows in America over which lavatories trans people should use, and whether trans women should be allowed into women-only events, have aligned some feminists with the conservatives they normally oppose in the culture wars. Though the issue may seem trivial, and the vitriol disproportionate, feminists value spaces where women are safe and not crowded out or interrupted, or forced to make nice or conform. But for trans women exclusion from the group qualified to enter such spaces strikes at their self-conception.

A crucial concept for those who work with trans people, says Ms Castro, is “gender-identity threat”—an attack on a trans person’s identity. As an example, she describes projects she works on to reduce the number of trans women who are HIV-positive. They may engage in risky sex to shore up their sense of femaleness, she says, in response to remarks or situations that threaten that sense, for example being treated in anti-HIV programmes designed for gay men—or excluded from women-only spaces. Cutting HIV transmission requires “gender affirmation”—reinforcing their identities in constructive ways, for example with support groups or counselling.

In the early days gender-reassignment clinics saw more males wishing to change sex than females. Many people thought this reflected a psychological difference between the sexes. The conventional wisdom now is that the reason was social. Parents were more bothered by “sissy” sons than “tomboy” daughters. And men who dressed and acted like women faced mockery, more so than the other way round.

In recent years the balance has shifted hugely. The British gender-dysphoria service now sees four times as many girls who are suffering as boys. This may be because constraints on male behaviour have loosened. It may also be because having a female body has become more onerous for children. Some girls seem unable to find a place for themselves in a sea of sparkly pink princess dresses, and then, after puberty, in a hypersexualised pop culture.

“If the mind cannot be changed to fit the body, then perhaps we should consider changing the body to fit the mind,” ran the press notice when America’s first gender-reassignment clinic, at Johns Hopkins Hospital in Baltimore, opened in 1965. That wording, which seemed so progressive at the time, would now be regarded by some activists as starting from a false premise. The “gender-affirmative” approach takes lasting gender dysphoria to signal a cross-gender identity that needs no explanation. The Johns Hopkins clinic’s implication—that an attempt to change the mind should precede one to change the body—is seen as akin to discredited “conversion therapies” to turn gay people straight.

In 2015 Kenneth Zucker, a Canadian paediatrician specialising in gender dysphoria, was sacked and his clinic shut after a campaign by activists. His starting point had been to try to help gender-dysphoric children become more comfortable with their biological sex, and to wait and see if they changed their minds. In a BBC documentary last year, he drew an analogy: “A four-year-old might say that he’s a dog—do you go out and buy dog food?” The comparison caused outrage. But Dr Zucker was making an important point: gender-dysphoric children are not all set for transgender adulthood. In the late 1960s and early 1970s Richard Green, a psychiatrist and sexologist then at UCLA, studied boys with markedly feminine identities. Some four-fifths of those followed to adulthood matured into gay or bisexual men. Only one was transgender. Studies in Canada and the Netherlands have since found rates of 12-39% for persistence of transgender identity into adulthood. This is puzzling: gender identity is distinct from sexual orientation. Most gay people never doubt their gender identities. Plenty of trans people are homosexual; Professor Green estimates that a third of the post-surgery trans women he saw between 1995 and 2007 while working at Charing Cross Hospital in London, which had the world’s largest transgender treatment programme at the time, were attracted to women. Bruce Jenner was heterosexual; in Vanity Fair Caitlyn said she didn’t yet know where her sexual interests lay and that “if you have a list of ten reasons to transition, sex would be number ten.” And yet much evidence suggests that gender dysphoria depends partly on a society’s attitudes, not only towards gender nonconformity but towards homosexuality. The penalty for male homosexuality in Iran is death, for example, but the ayatollahs believe that a person can be “trapped” in the wrong body. Gay Iranian men are pressed to accept cross-sex hormones and gender-reassignment surgery. Though some flee the country to avoid changing sex, others find doing so allows them to live more comfortably.

The majority view among those who see them in clinical settings is that children with settled gender dysphoria should be given drugs that delay puberty, so they have more time to decide what to do before their bodies grow into what may be the wrong adult form. “We can’t identify with certainty which cases will persist,” says Polly Carmichael, the director of Britain’s national gender-identity service. “So we have to pace treatment carefully.” But little is known yet about the life courses of children who start such “puberty blockers”. Delaying puberty may be harmful, if many children who say they “feel like” the opposite sex are expressing what will, post-puberty, settle into an unconventional gender identity or same-sex attraction. It may short-circuit the process whereby some gender-identity issues would naturally have been resolved.

All this puts doctors in a quandary. Transitioning earlier means better physical results. Waiting means needless distress for children who will not change their minds. But what about the others? Will some of those whose identity switches have been reported in television shows and magazines end up feeling regret? Will they be able to find a way back? And some of those who do not regret transitioning might also have been content had they not done so, in which case they will, on balance, have been harmed. Professor Green cites “the medical consequences, the hormones, the risk of imperfect surgery and perhaps unwanted infertility.” As for the probability that some would otherwise have grown up cisgender and gay, he says: “I’ve been seeing transsexuals for 50 years. I can tell you that being a gay man or lesbian woman is one hell of a lot easier.”

Gender-mending

Most people are comfortable with their gender identities, perhaps without having any strong sense of being male or female. Ms Shriver writes: “I have no idea what it ‘feels like’ to be a woman—and I am one.” As traditional and legal constraints on men’s and women’s behaviour loosen, that group may grow and, with luck, the number of children who feel stifled by their gender roles will fall. But there will probably always be a few people whose felt identities are at odds with what the world sees, and who will need to do something about it if they are to be at ease.