The Court of Arbitration for Sport (CAS) this week upheld the International Association of Athletics Federations (IAAF) regulations governing eligibility for the women’s category in international elite athletics competition. In effect, CAS decided the question “who is a woman” for purposes of elite sport. And it ratified the IAAF’s preferred answer: A woman in sport is anyone whose legal identity is female—whether they personally identify as such or not—and who has testosterone (T) levels in the female range. That may seem like a mere technical ruling. But as I’ll explain in this article, the ramifications are profound for female athletics everywhere—a cause that has been central to my life and to the lives of millions of girls and women worldwide. The female range for testosterone is categorically different from the male range. In general, males have 10 to 30 times more T than females. Most females, including most elite female athletes, have T levels in the range of 0.5 to 1.5 nanomoles per liter (nmol/L). For men, typical values are 10 to 35 nmol/L. The …