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Faith Kipyegon is the greatest female middle-distance runner in history. The Kenyan star is a three-time Olympic gold medalist and the current world record holder in both the women’s mile and 1500 meters. In a high-profile event last month, she gave us something rare: a transparent and widely-broadcast reminder of the physiological differences that separate male and female athletic performance—differences that are too often downplayed or denied in today’s debate.

It was a Nike-orchestrated, globally livestreamed spectacle—complete with supershoes, aerodynamic pacers, custom speedsuits, and a team of ten male runners tasked with pacing her to history. The goal was for Kipyegon to become the first woman ever to run a sub-4 minute mile. Nike set her up with the very best conditions that any athlete could ever expect.

Kipyegon ran a mile in 4:06—a remarkable performance by any measure, and a personal best, but well short of the sub-4 minute goal. While Kipyegon wasn’t directly racing her pacers, they were there to pull her to a time that hundreds of male athletes have already achieved. Rather than charging down the final straightaway alone, leaving the best women in her wake, as she so often does, we saw Kipyegon straining to hang on behind a group of male runners who weren’t even near their limit, as they turned around to cheer her on.

This race matters because it offered something exceedingly rare: an honest, direct comparison of male and female performance at the highest level. It was a window into the physiological differences that shape athletic performance, raising critical questions about how to ensure fairness in women’s sports while respecting all athletes’ identities.

What the Race Revealed

The most talented female runner of our time still wasn’t close to the standard that elite high school boys can now achieve. The 25th fastest U.S. high school boy this year has run 4:04—faster than any woman in history—and seven of them have run it in under 4 minutes. Sam Ruthe, a 15-year-old boy in New Zealand, joined the sub-4 club this year. Even with Nike’s full arsenal of technological support behind her, Kipyegon still couldn’t match the time he achieved. That’s not a knock on her—she’s amazing. It’s a reality check, rooted in biology.

We’re living in a moment when the boundaries of sex-based categories in sport are increasingly contested, with sports allowing the inclusion of transgender athletes in women’s competitions. These include cases from swimming, track and field, weightlifting, cycling, and fencing, and other sports where biological males are competing with biological females—often following testosterone suppression, but sometimes not.

Why does this matter?

Sex-based sport categories exist to contain predictable physiological systems—not to eliminate all individual-level advantage, but to manage how advantage is distributed across shared developmental contexts. The female category is not meant to ensure equal outcomes. It’s meant to exclude one massive system-level advantage: male-pattern development shaped by testosterone exposure through puberty.

The advantage males carry, especially after puberty, is not confined to just one or two variables—it is the result of an entire developmental system affecting multiple traits simultaneously: limb proportions, bone density, cardiovascular capacity, neuromuscular function, and more. These are not isolated attributes that can be easily matched or offset, even if testosterone is suppressed.

And while the most significant performance gaps between males and females emerge after puberty due to testosterone-driven physiological changes, research shows that athletic differences begin even earlier. Studies have found that biological males often demonstrate 1-5% advantages in speed, strength, and power beginning in childhood. Even pre-pubertal testosterone suppression may not eliminate all male-performance advantages—though current evidence is limited and not sufficient to draw definitive conclusions.

Some critics of biological sex-based categories argue that all elite athletes have biological advantages which makes them champions—Michael Phelps’ long arms and large lung capacity are often cited as examples of such an advantage. But those are individual anomalies within a sex category. They are not the result of massive biological system-level advantages—ones which cannot be overcome by even the best training, coaching, or other resources.

Others argue that defending biological sex-based categories risks reinforcing sexist stereotypes, as if acknowledging male athletic advantage implies all biological women are somehow socially inferior. Yes, sexism remains a serious and persistent problem in the world of sports, from unequal pay to lack of media coverage and support. But recognizing physiological differences between males and females isn’t sexist; it’s a necessary step toward fairness. The biological differences between males and females reflect average, well-documented physiological traits that significantly influence performance outcomes. It’s precisely because sex-based categories exist that women—of all shapes, sizes, and sexual orientations—can compete on a level playing field, without being overwhelmed by the systematic performance advantages typically conferred by male development.

Some point to examples of cisgender women who have defeated transgender athletes—such as Lia Thomas falling short of NCAA records or Laurel Hubbard failing to medal at the Olympics. But aside from the fact that Hubbard was much older than her competitors, these examples miss a crucial distinction. A truly elite, world-class female athlete can often outperform well-trained, non-elite biological males (such as Thomas or Hubbard), but not national-level or world-class biological males. In Kipyegon’s case, she can comfortably outrun more than 99.999% of men, including many highly-trained male runners. But against the very best men (and boys), the gap is simply unbridgeable. Telling women they should simply be “training harder” is naïve and condescending to those already giving their very best.

Advocates of identity-based participation often highlight the many sociocultural benefits of sport—building confidence, fostering belonging, and promoting health and well-being. The idea that sport is about more than just podium finishes and medals is a valid and valuable perspective. Indeed, all athletes should be able to compete in the sports that they love, regardless of their gender identity.

But when it comes to competitive sport, performance does matter. Victories lead to scholarships, prize money, records, and endorsements. In these arenas, fairness isn’t just symbolic—it’s material. And that’s precisely why sex-based categories were created in the first place: to ensure that female athletes have a level playing field in the competitions where winning carries real-world consequences.

The Hard Question We Need To Ask

That doesn’t preclude transgender inclusion in sport, but it does raise a pressing question: At what level of competition does fairness need to take precedence over inclusion? Is it only at the Olympic level, or does it also matter in college and high school championships, where scholarships and future opportunities are on the line?

These are difficult questions, but if we want to maintain competitive integrity in women’s sports, we need to be willing to ask them—and answer them honestly.

This isn’t about demonizing transgender or intersex individuals or denying the value of women’s athletic achievements. It’s about acknowledging that, at every competitive level, biological males are more likely to reach the upper limits of athletic performance—and that preserving fairness in women’s sport depends on recognizing that reality.

Nike may have missed the mark with its marketing campaign, but the event inadvertently proved something far more important than any world record—that even the most gifted female athletes cannot overcome the male physiological advantage. Any policy that ignores that will inevitably render the playing field uneven, particularly at the expense of women who’ve trained their entire lives, not to simply participate in sports, but to win at the highest level. If fairness is truly the basis for women having their own sports leagues and competitions, organizations must anchor those competitions in biological reality.

James Smoliga is a professor in the Department of Rehabilitation Sciences at Tufts University School of Medicine and writes the Substack Beyond the Abstract.

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