For nearly a century, certain elite athletes have been subject to anatomical, chromosomal or hormonal testing to compete in women’s events.
At the Ivy League women’s swimming championships this week, many eyes in the crowd will be fixed on Lia Thomas, a star of the University of Pennsylvania team. In recent months, Thomas has made headlines not only for her speed — handily winning one Ohio race that went viral — but also for her gender identity.
In 2019, while competing on the men’s team, Thomas began to medically transition, taking testosterone blockers and estrogen. Although her swim times slowed considerably, she’s still a top competitor in several women’s events, raising questions about the role of testosterone in athletic performance.
Some have called for her to be barred or separated from regular competition, arguing that her body underwent changes during puberty that gave her a lasting, unfair advantage. But others have contended that there’s no justification for excluding transgender athletes like her.
The organizations that oversee college swimming have put out conflicting rules on transgender athletes in recent weeks. U.S.A. Swimming earlier this month announced a new requirement that transgender women must suppress their testosterone levels for three years before competing, a rule under which Thomas would have been excluded. But last week, the N.C.A.A., the national body overseeing college sports, said that instituting a new policy in the middle of the season would be unfair — allowing Thomas to compete at the N.C.A.A. championships in March.
These thorny questions over the nature of athleticism are not new in women’s sports. They have come up many times over the past century, typically when an athlete deemed too masculine started to win. Sports authorities have leaned on medical tests — whether anatomical, chromosomal or hormonal — to determine eligibility in women’s categories, while requiring no analogous tests for men.
But in the realm of elite physical performance, where extraordinary biology is the rule, science has never provided neat answers.
“In the end, it’s about how we think about who is a woman, right?” said Katrina Karkazis, an anthropologist at Amherst College and co-author of “Testosterone: An Unauthorized Biography.”
“And, of course, sport has for a very long period not been at the forefront of gender equality, inclusion,” she said. “So it’s no wonder that we’re having this kind of debate.”
In 1928, the Olympic Games introduced its first women’s category for track and field events. In the early days, there was public speculation that some of the athletes were too masculine, and even that men were posing as women in order to win.
“Women’s sports was controversial and involved women encroaching on a male domain,” Susan Cahn, a historian at the University at Buffalo, said. “So the womanhood of women athletes was always questioned when they were really good.”
By the 1960s, driven by Cold War suspicions over foul play, the International Olympic Committee established a new medical commission to oversee testing for doping, as well as “nude parades,” in which naked female athletes would be inspected by a panel of physicians, Jaime Schultz, a sports historian at Pennsylvania State University, said.
The I.O.C. replaced the physical exams with a genetic test for sex chromosomes in the late 1960s, claiming that the cheek swab would provide a “simpler, objective, and more dignified” way to verify gender.
In 1967, a Polish track and field gold medalist named Ewa Klobukowska became the first athlete to be disqualified from women’s races because of the chromosome test, after it reportedly found that she had a type of “genetic mosaicism” in which some cells carry a Y chromosome.
Genetic testing faced a major legal challenge in 1977, after Dr. Renée Richards, an ophthalmologist and transgender woman, sued the U.S. Tennis Association for requiring the chromosome test to compete in the women’s category of the U.S. Open. Richards, who had transitioned after playing tennis in the men’s division for years, won the suit.
“It seems clear that defendants knowingly instituted this test for the sole purpose of preventing plaintiff from participating in the tournament,” a judge on the New York Supreme Court wrote. Richards played in the U.S. Open that year, at age 43. She was quickly eliminated in the singles tournament but made it to the doubles finals.
At the 1996 Olympics in Atlanta, eight women failed the chromosome test because they carried Y chromosomes. All were allowed to compete after it was shown they had hormonal conditions that had prevented them from going through a typically male development. By the 2000 Olympics, international sports bodies had abandoned the chromosome test and reverted to ad hoc medical examinations.
Sex tests based on testosterone began about a decade ago, after the South African track star Caster Semenya won the 800-meter race at the 2009 world track and field championships by more than two seconds. Scrutinized for her strong physique, Semenya was forced to undergo examinations by an endocrinologist and a gynecologist, as well as mental health assessments by a psychologist.
The results of those tests were never publicly released, but Semenya later disclosed that she was forced to take medication to reduce her testosterone in order to compete. In 2011, World Athletics, the body overseeing track and field events introduced its first policy for women with high testosterone, limiting concentrations of the hormone to 10 nanomoles per liter. (The average testosterone level in women is below 3 nanomoles per liter, whereas in men it typically ranges from 10 to 35 nanomoles per liter.)
Many experts speculate that the rules were changed because of Semenya. “It was really a reaction to one phenomenal athlete,” said Schultz, the historian at Pennsylvania State.
Four years later, the I.O.C. loosened its policy on transgender athletes, which had previously required that they undergo genital surgeries and legally change their gender, to focus solely on testosterone. Transgender women would need to demonstrate reduced blood concentrations of the hormone for at least a year.
In 2020, after several challenges in international courts, the World Athletics testosterone rule was upheld. Semenya, who declined to take testosterone-suppressing drugs, was unable to compete in the 2020 Olympics.
“As a woman, I should be in control of my own body,” Semenya wrote last year. “Why should I have to take hormone-altering substances just so I can compete in my chosen profession?”
The I.O.C. adjusted its stance once again in November of 2021, abandoning its sex testing rules. But the organization also allowed individual athletic federations, like World Athletics, to set their own policies.
Testosterone’s role in physical strength and stamina is robust. Boys and girls are roughly equal in terms of athletic ability until they reach puberty, at which point testosterone surges in boys and their athletic performance typically catapults above girls’.
And when athletes take anabolic steroids — synthetic testosterone — they can recover more quickly after strenuous exercise and greatly increase their muscle mass.
But among elite athletes, men and women sometimes carry similar levels of the hormone.
In a 2014 study, funded by the I.O.C. and the World Anti-Doping Agency, researchers analyzed testosterone levels in nearly 700 elite athletes who played 15 different sports. The researchers found that 16.5 percent of men had low testosterone levels and 13.7 percent of women had high testosterone levels, with considerable overlap between the two groups.
“The I.O.C. definition of a woman as one who has a ‘normal’ testosterone level is untenable,” the study’s authors wrote.
Given the complex interplay of testosterone, sex and athleticism, some experts oppose all testosterone testing in women’s sports.
“T’s effect on athleticism isn’t straightforward,” said Karkazis of Amherst, who consulted for Semenya’s legal defense, using the shorthand for testosterone. “No study has ever concluded that you can predict the outcome of speed or strength events by knowing competitors’ T levels.”
And scant data exists on the performance of transgender women who went through male puberty but later suppressed their testosterone.
Joanna Harper, a competitive long distance runner in Britain, noticed that her pace began to slow just a few months after taking testosterone-suppressing drugs during her transition nearly 20 years ago.
Toward the end of her first year on hormone therapy, she recalled, she saw a marked decrease in her muscle mass, even though she was still running 100 miles per week. And when she ran in women’s races, she was about 12 percent slower than when she had been competing against men.
“There were plenty of women over the years who said that it was unfair for me to compete,” said Harper, a medical physicist who studies transgender athletic performance at Loughborough University in Britain.
In a review of the scientific literature on athletic ability and transgender women, Harper found some studies suggesting that after four months of testosterone suppression, transgender women have comparable levels of hemoglobin, a blood molecule that carries oxygen through the body, to cisgender women. And after one year on hormone therapy, transgender women see significant decreases in muscle mass.
Even after three years, though, transgender women still tended to have more strength than cisgender women. It’s not clear, however, that these results would hold for elite athletes.
Transgender women may have a disadvantage in some sports, given their heavier musculature, said Dr. James Barrett, the director of the Adult Gender Identity Clinic in London, who is helping lead a study for the I.O.C. that looks at how much athletic ability decreases in transgender women after they start hormone therapy.
“Trans women by and large aren’t winning across the board,” he said. “It’s not obvious that there’s necessarily an advantage at all.”
Still, because of development during puberty, transgender athletes may have some lasting physical advantages in a sport like swimming, such as a taller height and larger hands and feet. Coming up with a policy for sex-segregated sports therefore requires making a choice: Either exclude these athletes, or allow them to compete with potential advantages, said Jakob Vingren, an exercise physiologist at the University of North Texas.
“There’s no good answer,” Vingren said. “Someone is disadvantaged one way or the other.”
Others argue that these physical changes mean transgender women like Lia Thomas shouldn’t be allowed to compete in women’s divisions.
“If a cis woman gets caught taking testosterone twice, she’s banned for life, whereas Lia has had 10 years of testosterone,” said Nancy Hogshead-Makar, a three-time Olympic gold medalist in swimming and the president of the advocacy group Champion Women.
The creation and funding of separate competitive arenas for men and women has given women a fair shot, allowing them to be seen as powerful athletes, Hogshead-Makar argued. She suggested that Thomas could be allowed to race in a ninth swimming lane but not have her scores recorded in official counts.
“It’s about the principle of having sport continue to be sex-segregated: having a space where women are really honored and where they can triumph,” she said.
Harper works with Hogshead-Makar as a member of Champion Women but disagrees with the group’s stance on barring Thomas from competition.
Her own transition experience convinced her that testosterone requirements are a necessary, if imperfect, way to preserve fair competition in elite women’s sports. But she said that U.S.A. Swimming’s rule requiring three years of testosterone suppression was excessive and seemed to have targeted Thomas because of her success.
“If we allow trans women to compete, we have to allow them to win, too,” Harper said.