In a widely anticipated defeat for transgender rights, the Supreme Court upheld state laws in Idaho and West Virginia that ban transgender girls from playing on girls’ school sports team. The decision, issued Tuesday, does not impose a nationwide ban on trans athletes. But it does preserve laws passed in 27 states by GOP politicians and anti-trans activists who argued that transgender women threaten safety and fairness in women’s athletics.

All nine Supreme Court justices agreed that Title IX, the federal law forbidding sex discrimination in schools, allows states to ban trans girls from girls’ sports. They also ruled 6–3, along ideological lines, that such bans do not violate the equal protection clause of the Constitution.

“He’s not only creating a bad precedent for trans people, he’s significantly lowering the protection all women get under equal protection.”

The science is far from settled about whether trans girls who have received gender-affirming treatment actually have a competitive advantage or pose a greater risk of injuring other players. But the majority opinion, by Justice Brett Kavanaugh, glosses over those unknowns—reasoning that “biological sex” is a good enough proxy for athletic ability for states to categorically ban trans girls from girls’ sports.

“Separate sports teams for biological males and biological females are reasonable,” Kavanaugh writes. “Given the inherent physical differences between the sexes, allowing only biological females to play on women’s and girls’ teams can reduce the risk of physical injury and ensure fair competition.”

Yet the ruling has much broader implications. In her dissent, Justice Sonia Sotomayor points out that the majority opinion is allowing states to make laws based on broad differences between boys and girls, without looking closer at the subcategories of people who may not fit into those generalizations. “In so concluding,” she writes, “the Court…lowers the State’s burden for justifying the use of sex classifications in potentially all cases.”

In other words, the decision makes it easier for states to justify treating men and women differently. In the past, Sotomayor argues, the court has overturned laws that used “overbroad generalizations” that suited most men and most women but failed to make exceptions for a minority who did not conform to sex stereotypes. But this case breaks that long-standing pattern: The court on Tuesday failed to account for the minority of students who have received gender-affirming treatment and thus may not conform to sex stereotypes about their athletic performance.

As a result, the ruling could threaten decades of progress on gender equality, Sotomayor warns. “The majority applies its diminished view of equal protection to the sports context today,” she writes. “One can only hope that the same misguided approach does not and will not extend to other contexts tomorrow.”

The legal cases, known as Little v. Hecox and West Virginia v. B.P.J., began in 2020 and 2021, when trans students’ participation in sports had not yet become a culture-war flashpoint or presidential campaign–defining issue. Back then, conservative political strategists had just........

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