Today, the United States Supreme Court will hear oral arguments in two cases that go to the heart of fairness in athletics. The question before the Court is whether states may enact laws preventing biological males from competing in girls’ and women’s sports. These laws, passed in Idaho, West Virginia, and more than two dozen other states, reflect common sense and biological reality. They are also supported by a strong majority of Americans across the political spectrum.

At stake is a simple question: Do sex-based differences matter in sport? The answer, grounded in eons of human experience and decades of biological and sports science research, is an unequivocal yes.

It is not a novel or controversial scientific claim to say that males — boys and men — and females — girls and women — are biologically different. These differences begin at conception, are evident at birth, persist throughout childhood, and become substantially magnified at puberty. Importantly for the cases before the Court, sex-based differences relevant to athletic performance exist even before puberty. On average, boys run and swim faster, jump higher and farther, and throw with greater speed and distance than comparably aged, trained, and talented girls.

These performance gaps are the predictable consequence of biological differences in body composition, skeletal structure, muscle mass, and cardiopulmonary capacity. That is precisely why competitive sports have long been separated by sex, to ensure fair competition and meaningful opportunities for girls and women.

Yet in the cases before the Supreme Court, it will be argued that because some males identify as girls or women, they should be permitted to compete in female sports categories. This argument asks the Court to disregard established science in favor of subjective identity.

Biology, however, does not yield to feelings or identity. Being male confers advantages that are directly relevant to athletic performance. These include greater muscle mass, lower body fat percentage, larger and stronger bones, greater height and limb length, and larger hearts and lungs. These traits translate into superior strength, speed, and endurance in virtually every sport dependent on strength, speed, or endurance. Across running, swimming, cycling, rowing, throwing, and jumping events, male performance exceeds that of females by margins that girls and women cannot overcome through training, nutrition, and desire.

Crucially, these advantages are not erased by self-identification. Nor are they eliminated by medical interventions such as puberty blockers, testosterone suppression, or estrogen administration. Suppressing testosterone in males may reduce performance relative to their own prior baseline, but it does not reverse the structural and developmental advantages accrued through male anatomy and physiology. Many of these advantages are established before puberty and further solidified during it. Lowering testosterone does not undo greater height, longer limbs, larger bones, greater muscle mass, or the cardiopulmonary benefits that drive athletic capacity.

Sport, by its nature, relies on categories to promote fairness and safety. Weight classes exist in wrestling and weightlifting because body mass confers a clear, measurable advantage. Age categories exist because growth and maturation influence both performance and injury risk. These classifications are not viewed as discriminatory: They are widely accepted as necessary for meaningful competition.

Notably, the performance differences between an eighth-grade student and a high school freshman, differences that justify strict age-based divisions, are far smaller than the average differences between males and females. Yet no one seriously argues that older athletes should be allowed to compete against younger ones simply because they identify as being younger. Such a proposal would rightly be dismissed as incompatible with safety, fairness, and common sense.

And yet, that is precisely the logic now being advanced with respect to sex-based sport categories.

Protecting girls’ and women’s sports is not about exclusion or animus. It is about recognizing biological reality and preserving the very purpose of female athletic categories. These categories exist to ensure that girls and women can compete on a fair playing field, earn championships, break records, and access scholarships and opportunities that would otherwise be claimed by males.

The Supreme Court is not being asked to decide a matter of personal identity or social courtesy. It is being asked to determine whether states may rely on objective, well-established biological facts when crafting policies that govern competitive sport. The science is clear, the rationale is sound, and the stakes for fairness, safety, and opportunity in women’s athletics could not be higher.