Each year, the world’s best time in the women’s marathon is surpassed by hundreds of men. The women’s world records in all of the races on the track from 100 meters to 10,000 meters are also surpassed by many men each year, including by many high school boys. For example, in 2017, 36 boys ran faster than Florence Griffith Joyner’s seemingly unassailable 100-meter record of 10.49.

There is no characteristic that matters more than testes and testosterone. Pick your body part, your geography, and your socioeconomic status and do your comparative homework. Starting in puberty there will always be boys who can beat the best girls and men who can beat the best women.

Because of this, without a women’s category based on sex, or at least these sex-linked traits, girls and women would not have the chance they have now to develop their athletic talents and reap the many benefits of participating and winning in sports and competition. Eric Vilain, a geneticist who specializes in differences of sex development, has been blunt about it: removing sex from the eligibility rules would “be a disaster for women’s sport … a sad end to what feminists have wanted for so long.”

This may sound like hyperbole but it isn’t. In competitive sport, winning and room at the top are what ultimately matter, so relative numbers are irrelevant. It doesn’t matter that there are 100 females and three males in a girls’ race if the three males win spots in the final or on the podium because they are males. The unusually high incidence of intersex athletes in the women’s middle distances and their reported 100 percent win share in the women’s 800 meters at the Olympic Games in Rio show their disproportionate power. Indeed, it is because they clustered in the middle distances that these events are the initial focus of the rules. Their supremacy was proof of principle. Testosterone readings outside of the female range were also found in the throws, but these were attributed to doping, not intersex conditions.

The I.A.A.F. is requiring that affected athletes lower their testosterone levels to within the female range if they want to continue competing in the middle distances in the women’s category. By definition, the required hormone therapy causes medically unnecessary physiological change, and no one should be forced to take drugs they don’t want or need.

But the I.A.A.F.’s requirement isn’t rogue or reckless. It simply adopts the standard of care in transition medicine, which is very much embraced by those who seek to match their bodies to their gender identity. Intersex athletes who have gone through puberty may not want to drop their testosterone levels to female levels if they don’t identify as female or don’t see the need to feminize their bodies for reasons other than sport. This is especially true for athletes who are in the game to win rather than to express themselves through participation as women. This is properly their choice and it’s a real one.

When we’re not focused on intersex athletes we tend to understand the fact of sex differences and their relevance. In the United States those differences are the basis for Title IX and its hugely popular mandate that separate and equal funding be set aside for girls’ and women’s sport. Under Title IX, sex remains the principal and generally uncontroversial basis for classification and legal protections.

Given this, the compromise reflected in the I.A.A.F.’s rules is significant. Using testosterone as a proxy for sex may seem like subterfuge, but it’s not. If it were really sex that mattered and not testosterone, then athletes who had gone through puberty as biological males would be categorically banned from the women’s category as they used to be. Instead, today, if their legal documents identify them as women they can compete as women in the events they prefer if they transition the single sex trait that determines capacity for the win. They can keep their bodies otherwise intact. The process offers fairness both to the affected athletes and to the field.