President Donald Trump has brought the effort to ban trans women and girls from participating in athletics to the White House. 

What are the stakes when a fraction of a second, the length of an extended arm or the weight of a body in motion can mean the difference between victory and defeat?

The question – at the center of disagreements over transgender athletes’ participation in sports – has echoed from high school running tracks to Olympic arenas as lawmakers and sports bodies face intense pressure to weigh in on a debate over what fair play looks like.

Although few trans athletes have reached elite levels of sports competition and even fewer have taken home top prizes, the success of a small group of trans women – particularly NCAA swimmer Lia Thomas – has fueled an increasingly vocal movement to ban trans women and girls from participating on teams consistent with their gender identity. Since 2020, more than half of US states have implemented bans on trans athlete participation.

President Donald Trump has brought the effort to the White House, issuing an executive order aimed at eradicating trans women and girls from participation in women’s sports and punishing institutions that are inclusive of trans athletes. Earlier this week, Trump threatened to withhold federal funding from California over the participation of a high school track and field athlete.

Opponents argue that adult transgender women – even those who have undergone treatment to lower their testosterone levels – retain unfair physical advantages after puberty that would deprive cisgender women of opportunities to succeed.

Trans athletes and advocates, in turn, point to a lack of consistent, conclusive research to support this claim and the wide-reaching bans it’s led to. They say trans people deserve the right to compete alongside their peers and reap the proven social, physical and mental benefits of sports.

Research on trans people’s athletic performance is scarce, and there have been no large-scale scientific studies on the topic or on how hormone therapies may affect their performance in specific sport categories, such as running or wrestling.

And although existing research hints at how puberty and hormone therapy may affect a person’s physical abilities, some experts say that far more data is needed to make confident conclusions about whether trans people in general hold advantages in their respective sports. Even among cisgender athletes, bodies and physical abilities vary widely, and traits that may be an advantage in one sport – such as grip strength or bone density – may not be an advantage in others.

Even so, the fraught environment has driven sports bodies such as the NCAA to reverse its previous trans-inclusive policies and effectively ban trans women from women’s sports, while still allowing trans men to play on men’s teams.

Here is what the research does – and doesn’t – tell us about trans athletes.

How many transgender athletes are competing in sports?

Complicating this debate is a lack of reliable data on how prevalent trans athletes are, whether in recreational youth sports or in cutthroat international competition. This has led advocacy groups on both sides to make wide-ranging and often conflicting estimates.

In the most competitive arenas, however, figures indicate that transgender people make up a sliver of participating athletes and rarely take top prizes.

Since the International Olympic Committee began permitting trans and nonbinary athletes to participate openly in 2003, fewer than a dozen have qualified. Most have competed on the team consistent with their gender assigned at birth, choosing to forgo hormone replacement therapy in order to qualify for competition, including nonbinary Team USA track and field athletes Nikki Hiltz and Raven Saunders.

Only one out trans woman, New Zealand weightlifter Laurel Hubbard, has qualified for the Olympics, and she failed to complete a single lift.

In US college and university athletics, top leagues have struggled to quantify trans athletes even as they craft policies banning most trans women from competing in women’s categories.

NCAA President Charlie Baker testified before the US Senate last year that he was aware of “less than 10” transgender athletes competing in the league – a number amounting to less than .002% of its athletes. And when the NAIA, a smaller college association, effectively banned trans women from participating in most of its categories last year, a spokesperson told the Wash

More than half of US states also have laws banning school-age trans students from competing on sports teams that align with their gender identity.

The Williams Institute, a think tank at UCLA Law, estimates that as many as 122,000 trans youth age 13 to 17 could be participating in high school sports. It is unclear, however, how many play on the team that aligns with their gender identity, as many of them presumably live in states with bans in place.

How does puberty confer athletic advantages?

Puberty that is masculinizing can give an athlete substantial physical advantages, experts say. The difference-maker is testosterone, a sex hormone essential to male development.

Washington Post that it does not track whether any trans athletes participate on its teams.

Every body produces testosterone, regardless of sex – even in the womb. For cisgender women, testosterone generally affects bone and muscle health, mood and energy, the menstrual cycle and fertility as well as libido. For cisgender men, it does many of the same things but also helps develop secondary sex characteristics.

Some small studies have shown that when boys are exposed to higher levels of the hormone in the womb, it may confer a slight athletic advantage later on, but more research is needed.

The real advantages come with puberty, said Dr. Bradley Anawalt, an endocrinologist with the University of Washington who has advised the NCAA.

“For practical purposes, prepubescent boys and girls, before age 10 or 11, they can pretty much compete in the same sports, and there’s not big differences in size, power, speed,” Anawalt said. “But at the time of puberty, when boys have their testosterone concentrations skyrocket in the blood, then you start to get significant differences between boys, girls, men, young women.”

In childhood, boys’ and girls’ bodies and physical abilities are quite similar, experts say. When puberty begins, about age 11, a boy’s testes will produce 30 times more testosterone than they did previously. Levels of circulating testosterone will then exceed that of females of any age by 15-fold,

Exposure to high levels of testosterone spurs growth that may create an athletic advantage if a person does not undergo hormone therapies to limit its effects. A masculinizing puberty leads to thicker bones, more height, higher muscle mass, greater muscle strength, larger aerobic capacity and an increase in hemoglobin, a protein in red blood cells that carries oxygen to working muscles, which is important for endurance.

“All of these characteristics lead to very, very substantial advantages in sports, and that is regardless of your gender identity or how you were identified at birth,” said Dr. Joanna Harper, a postdoctoral scientist researching transgender athletes in Oregon who has advised several sports’ governing bodies, including the IOC. “This applies to both transgender and intersex women who may have undergone this testosterone-filled masculinizing puberty.”

The advantage that a masculinizing puberty may have on athletic performance depends on the sport, said Dr. Ada Cheung, an endocrinologist who has written extensively about trans athletes and has advised several sports organizations.

“Using upper-body strength as an example, there tends to be a rate of difference. So things like baseball pitching, the difference in speed, there may be a 40% to 50% difference. But if you are looking at something like swimming or rowing, it might be like a 10% difference,” said Cheung, a professional fellow in endocrinology at Melbourne Medical School. “Understanding physical difference requires some nuance, though.”

Cheung said sex differences also cannot account for all the varying abilities in sport. An individual’s sex won’t determine their success or failure in athletic competition, and trans athletes’ abilities vary just like those of cisgender athletes.

“There are many, many differences in humans, regardless of gender, that give people athletic advantages,” she said, such as taller height, a greater wingspan, a higher proportion of fast-twitch muscle fibers or more flexibility. An athlete will also have an advantage with more access to training or better nutrition.

“It’s hard to put the diversity of human experience into just two boxes,” Cheung said.

What difference does gender-affirming hormone therapy make for trans athletes?

Gender-affirming hormone therapy is a medical treatment that uses hormones to help people align their physical characteristics with their gender identity. For trans women, it includes estrogen plus a testosterone blocker.

Although a masculinizing puberty may confer advantages, an October 2023 review of trans women who seek out such therapy said that many of the advantages are “reduced, if not erased, over time” if they undergo gender-affirming hormone therapy.