In the last few years, women’s sports have been a highly visible arena for the broader cultural and political debate over the rights and inclusion of transgender people. As transgender athletes come out, pursue gender-affirming medical care like hormone therapies, and try to compete in the division that aligns with their gender identity, many sports’ governing bodies have developed policies restricting participation on the basis of sex assigned at birth or other biological markers. Sometimes, these policies amount to outright bans on transgender women competing in the women’s category.

　　Even though proponents of bans often argue using the language of biology—men have more testosterone, women are less muscular—there’s actually very little scientific research into how hormone therapies commonly used in transition-related care affect athletic performance. Most of these bans are based on the assumption that the physical traits resulting from boys’ testosterone-saturated puberty, like increased muscle mass, strength, and height, are retained by someone assigned male at birth when they choose to transition.

　　But a new study published in the British Journal of Sports Medicine, which compared athletic performance between cis- and transgender men and women, suggests it’s a lot more complicated than that. “Trans women are not biological men,” Yannis Pitsiladis, one of the study’s authors, told the New York Times. 

　　The only measurement in this study in which transgender women categorically outperformed cisgender women was grip strength, which can be an indicator of overall strength. But the trans women who were surveyed had lower lung capacity, VO2 max (a measure of how efficiently oxygen is transported throughout the body, a marker of endurance capacity), and jump height.

　　This suggests that a transgender woman competing at, say, volleyball or long-distance running, could actually be at a disadvantage compared to her cisgender counterparts. This is likely because once someone assigned male at birth starts hormone replacement therapy, their strength and muscle mass relative to their frame declines, leaving them to “carry this big skeleton with a smaller engine,” as Pitsiladis puts it.

　　These findings are in line with real-world performances. The few trans athletes out there have been competitive without being head-and-shoulders superior to their cisgender peers. Transgender gravel cyclist Austin Killips won a couple of races in 2023—and finished somewhere in the top ten in a few others—which led the Union Cycliste Internationale, the sport’s governing body, to institute a sweeping ban of transgender women racing in the women’s category. Laurel Hubbard, a transgender weight lifter from New Zealand, qualified for the Tokyo Olympics but couldn’t execute a lift in her event.

　　More than anything, this study highlights just how little we actually know about how transition-related medical care affects an athlete’s body and performance. It’s not a definitive piece of research by itself—the sample size is very small, with only 23 trans women, 21 cisgender women, 19 cisgender men, and 12 trans men. All of the transgender athletes included were at least one year into hormone replacement therapy, but the researchers suggest collecting long-term data, following athletes over the course of their transition, to confirm that the measured differences are caused by gender-affirming care.

　　“This fear that trans women aren’t really women, that they’re men who are invading women’s sports, and that trans women will carry all of their male athleticism, their athletic capabilities, into women’s sports—neither of those things are true,” Joanna Harper, who researches transgender athletes at Oregon Health and Science University and was not involved with the study, told the New York Times. 

　　The International Olympic Committee, who funded the study, has gone through several different iterations of their gender inclusion policy. They presently defer to the rules of the international governing bodies of respective sports, after replacing fairly strict guidelines that required medical examinations and a cap on women’s testosterone levels in 2021.