Is it fair that some female athletes are born with male traits? No. But it wouldn’t be fair to disqualify them either

Open this photo in gallery:

Eero Mäntyranta was a seven-time Finnish Olympic medalist with a genetic advantage. That wasn’t known in the 1960s, when he took home three Olympic golds for cross-country skiing, but his body was producing extraordinarily high levels of hemoglobin, which gave him enhanced oxygen-carrying capacity: a clear advantage in endurance sports. It wasn’t until 97 members of his family were assessed in the 1990s that scientists discovered a familial mutation in the EPOR gene, which resulted in an abnormally high production of red blood cells. The condition was so mild in some family members that they didn’t know they were affected, according to the research paper, but others …well, one in particular, went on to be one of the best cross-country skiers in the world.

Olympic athletes are physically exceptional, which happens when you devote your life to gruelling training regimes. But many are also innately physically suited to their sport – some, unusually so. American swimmer Michael Phelps has an absurdly long “wingspan,” and it has been reported that his body produces far less lactic acid than others, staving off fatigue for longer. American gymnast Simone Biles has the perfect petite body to reign in gymnastics. British cyclist Chris Froome can absorb oxygen at a volume believed to be at the top of what is possible for humans. These genetic advantages are all allowed, and even marvelled at, in professional sports. So why is it that genetic differences in sex – which have ignited controversy in these Olympic Games around Algerian boxer Imane Khelif and Taiwanese boxer Lin Yu-ting – are perceived as cheating?

The answer, really, is obvious. To make professional sports as fair as possible, we segregate athletes into competition with their anatomical peers, which most of the time means segregating by sex. It should be uncontroversial to acknowledge that male bodies are generally faster and more powerful than female bodies, so we divide sports along the line of biological sex and sometimes into sub-categories such as weight class in boxing. Perhaps if athletes were made to compete against other athletes with similar red blood cell production counts, for example, then Mr. Mäntyranta’s genetic abnormality would have been disqualifying. But we instead use the much broader category of sex, which means variations therein are bound to create controversy.

To clear up misinformation that has been swirling around these two boxers these past weeks, Ms. Khelif and Ms. Lin are not transgender; they were born female, raised female, and have never lived as men. Ms. Khelif was, however, disqualified by the (notoriously corrupt, Russian-run) International Boxing Association after she beat previously undefeated Russian boxer Azalia Amineva in 2023. IBA president Umar Kremlev claimed that DNA test results showed that she (and Ms. Lin) have XY chromosomes. (International Olympic Committee spokesman Mark Adams called the tests by the IBA “so flawed that it’s impossible to engage with it.”) The IOC, for its part, released a new framework on gender inclusion in 2021, noting that individual sport governing bodies should determine who is eligible to compete, and dropping a requirement that female athletes’ testosterone levels be below a certain threshold.

It is easy to suggest that any woman who has XY chromosomes, or high testosterone levels, should be disqualified from competing against other women. But the reality is not so straightforward. Many girls with Swyer syndrome, for example, don’t know that they have XY chromosomes until the age of puberty. Women with androgen insensitivity syndrome (AIS) might be genetically male, but their bodies cannot process testosterone, meaning they get no athletic advantage from testosterone circulating in their bodies. And some women, such as those with congenital adrenal hyperplasia, may have increased testosterone levels but XX chromosomes. So where do we draw the line? At chromosomes? Testosterone? Other physiological differences that give men athletic advantages over women, such as muscle mass, lung capacity or heart rate?

There is a case to be made that trans female athletes – that is, athletes who were born male and transitioned to female – should not be eligible to compete against cis female athletes, particularly if they went through male puberty, because they will still possess these myriad physiological advantages (the transition process does lower testosterone levels in trans women, but not necessarily to the level of cis females, and it doesn’t reverse some of the physical changes that confer athletic advantage). But it is harder to make that case for women who have one or two advantages through an accident of birth, and not through an active process of transitioning or through scientific intervention.

Is it entirely fair? Not really. But it’s also not fair that Mr. Phelps has a wingspan like a wandering albatross or that Mr. Mäntyranta could reoxygenate better than any of his competitors. Exceptional physical traits, combined with great skill, technique (check out Ms. Khelif’s footwork) and training, make for extraordinary athletes. Ms. Khelif and Ms. Lin might have been born with some sort of advantage, but they had to do the rest of the work.