The question “What is a woman?” has haunted politics on both sides of the Atlantic Ocean for a decade. In Britain, where I live, Prime Minister Keir Starmer has given a range of tortured answers, before eventually settling on “an adult female.” This week, the country’s highest court has endorsed that view.

Interpreting Britain’s flagship civil-rights law, the Equality Act of 2010, the Supreme Court ruled Wednesday that the “concept of sex is binary, a person is either a woman or a man.” By unanimous decision, the five justices hearing the case decreed that the terms man, woman, and sex used there “refer to biological sex.” They also stressed that their ruling applied only to interpreting anti-discrimination law, and was not a wider judgment on the validity of transgender identities.

In everyday life, many Americans and Britons, including me, are happy to use people’s chosen names and pronouns, and take their asserted identities at face value. But in some situations covered by the Equality Act, individuals’ biological sex—a matter of verifiable fact—is more relevant than the gender with which they identify.

This week’s ruling most immediately affects women-only services, such as domestic-violence shelters, single-sex hospital wards, and prisons—places where women’s dignity, privacy, and physical safety are paramount. Men could already be legally excluded from such services. The implications of the ruling also extend into other emotionally fraught situations: Is a rape victim entitled to request a forensic examination by someone of the same biological sex? Are lesbian dating meetups allowed to exclude trans women without risking a legal challenge? The court now says yes.

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At the same time, the court was keen to stress that transgender people have their own legal protections. If a trans woman is discriminated against by someone who reads her as a woman, that can be sex discrimination. And if she is discriminated against simply for being trans, then that falls under another protected characteristic in the Equality Act, “gender reassignment.” Critics of the British ruling have focused on the fact that trans women will be denied access to certain women’s facilities. However, the ruling enhances the rights of trans men, who have reassurance that they can sue for pregnancy and maternity discrimination based on their biology.

The U.K. judgment shows that society can separate out the distinct forms of discrimination faced by biological females and trans women, without denying that either group has a valid claim to protection under the law, or that trans women may deeply feel themselves to be women. “It is not the role of the court to adjudicate on the arguments in the public domain on the meaning of gender or sex,” the ruling states, “nor is it to define the meaning of the word ‘woman’ other than when it is used in the provisions of the EA 2010.”

For more than a decade, the LGBTQ movement has argued that “trans women are women” and that no attempt should ever be made to treat the two groups differently. But are women discriminated against because of their biology, or because of what the trans theorist Julia Serano has termed femmephobia? One definition would apply to trans women, and the other would not. The British gender-critical feminist movement—which maintains that sex should take precedence over gender in law—has argued that biology, rather than identity, is what stops employers from hiring women in their 30s, because bosses are worried about having to pay for maternity leave. Biology is why no woman can run a 100-meter race as fast as Usain Bolt. Biological males commit the overwhelming majority of sexual and violent offenses. The research on gender identity and criminality is limited, but the most relevant study found that trans women followed a pattern more typical of males than of females.

In Britain, unlike in America, the debate about whether and when gender matters more than sex has not been strictly a matter of right versus left. In the United States, the debate is divided in a more classically partisan way: the best-known critics of trans inclusion include the swimmer Riley Gaines, who attended the signing of one of President Donald Trump’s executive orders on gender; Representative Nancy Mace, who demanded that her trans colleague, Sarah McBride, not use women’s bathrooms at the Capitol; and the conservative influencer Matt Walsh, who made a 2022 documentary called What Is a Woman? As a result, liberals and leftists approach the issue with extreme trepidation. Many try to dodge it altogether, as Kamala Harris did in the 2024 election, without acknowledging that there are legitimate grounds for disagreements. The U.S. debate has been shaped around the analogy between gay rights and trans rights for the purpose of civil-rights legislation. Any mention of bathrooms is also more fraught, because it evokes a time when Black citizens were denied the right to use whites-only facilities.

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Here in the U.K., however, the earliest high-profile proponent of the gender-critical movement was Julie Bindel, a radical feminist lesbian whose background is in campaigning for women in prison, and whose partner is a human-rights lawyer. Many of the women who have won employment-tribunal cases over harassment and dismissal for their gender-critical views were employed at NGOs, at universities, or in social work—all left-leaning sectors. Every major party of the left has a gender-critical caucus, such as the Labour Women’s Declaration and Liberal Voice for Women. True, the question “What is a woman?” has become a reliable gotcha for right-wing newspapers and TV channels in the U.K. But the gender-critical feminists who celebrated outside the court this week were mostly lifelong left-wingers.

The question of who counts as a woman reached Britain’s highest court thanks to a relatively minor issue. In recent years, the left-wing, pro-independence Scottish National Party has made transgender inclusion one of its flagship issues—evidence that it is more liberal and enlightened than its counterparts in England. In 2018, Scotland passed a law to encourage 50–50 gender representation for nonexecutive positions on public boards. The SNP defined women to include anyone “living as a woman.” Wait a minute, said the grassroots feminist group For Women Scotland. Did that mean a company could have a “gender-balanced” board that was composed of 100 percent biological males? Yes, said the Scottish government.

For Women Scotland then launched a crowdfunded legal challenge, to which the author J. K. Rowling contributed £70,000. The group’s appeal reached the Supreme Court in October—and ended in victory this week.

From a British perspective, one thing that has been immediately striking is how broadly this week’s judgment has been accepted across the political spectrum. This is a dramatic reversal. To obtain a gender-recognition certificate in Britain, an applicant needs a medical diagnosis of dysphoria and assessment by an independent panel. Ten years ago, almost all the main British parties—Labour, the Conservatives, the SNP, the Liberal Democrats, the Greens—supported changing this to a policy of self-identification of gender, which would have eliminated the need for outside review. Today, only the Liberal Democrats and the Greens support self-ID.

The Conservative leader Kemi Badenoch, who took strongly gender-critical positions when she was in government, described the ruling as a “victory for all of the women who faced personal abuse or lost their jobs for stating the obvious.” Labour’s Keir Starmer did not interrupt his Easter vacation to make a statement—in keeping with his longstanding aversion to getting involved with this issue—but his minister for women and equalities, Bridget Phillipson, said the ruling brought “clarity and confidence.” So great is Labour’s reversal on this issue that the leader of Scottish Labour, Anas Sarwar, even incorrectly claimed he had “always” been in favor of single-sex spaces. (In 2022, he whipped his party to vote for the Scottish bill that would have made gender a matter of pure self-identification. The law passed, but was blocked by the British government.)

Most notable of all, the left-wing SNP, whose legislation led to the judgment, also accepted the ruling. Under its former leader Nicola Sturgeon, the party disdained gender-critical feminists, to the point that some left for a rival pro-independence party. However, the SNP’s current leader, John Swinney, has taken a more conciliatory approach and said the new ruling will now “underpin” his government’s approach to gender.

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LGBTQ organizations have responded more strongly, with Stonewall expressing “deep concern” about the ruling’s implications. What activists really fear is that the judgment will license MAGA-style attacks on the trans community—or that it is the first step toward banning legal gender changes altogether. (Under the Trump-friendly autocrat Viktor Orbán, Hungary has done this, along with passing a suite of other anti-LGBTQ measures.) “I fear this will be the beginning of a maximalist agenda to further undermine and often insult trans people, and their status in society,” the podcaster Lewis Goodall wrote on Substack. “With Trump’s victory, the ideological winds are finely attuned to this purpose.”

But the court’s careful judgment in London is a world away from recent executive orders from Washington. Liberal Americans who are tempted to depict the British court ruling as another outrage from Rowling’s coven of witches on “TERF island” should note this difference. The “adoption of a gender identity inconsistent with an individual’s sex conflicts with a soldier’s commitment to an honorable, truthful, and disciplined lifestyle, even in one’s personal life,” declared Trump’s executive order on the military—a statement that amounts to calling all trans people deliberate liars. By contrast, the first paragraph of the British judgment states that “the trans community is both historically and currently a vulnerable community.”

That seems right to me. It is entirely possible to condemn the language and overreach of Trump’s executive orders on gender while also supporting the judgment in London. Women face discrimination, and so do transgender people of both sexes, and the law can recognize that. At times, their rights must be balanced. This careful ruling has done exactly that.