What is gender? It might sound like the kind of question that college students debate in a liberal arts class. But for the International Olympic Committee, it’s a practical question that demands a hard and fast answer. As at previous Olympic Games, athletes competing in Rio de Janeiro will be segregated into women’s events and men’s events, and that means the IOC needs a way to sort women from men.

New IOC guidelines issued in November allow athletes who have transitioned to another gender to compete without sex reassignment surgery. The rules allow athletes who’d previously identified as female to compete in the male category without restriction, because they would not gain an advantage from their previous gender. Those who transition from male to female, on the other hand, must meet several requirements. The athlete must declare a female identity, and this identity cannot change for at least four years. The athlete must also document that her total serum testosterone levels have remained below a certain limit for a minimum of 12 months before competing, and these levels must remain under the threshold as long as she’s competing.

The Olympic committee’s decision is a “huge step forward for everybody in the [transgender] community,” Caitlyn Jenner told me last week. “You can still have your old parts, which I think is very forward thinking.” Jenner is a trans woman who won the gold medal in the decathlon at the 1976 Olympics when she was Bruce Jenner, and she’s keeping the anatomical details of her own transition private. The public “is obsessed with — do you have it, or don’t have it?” she said, but “a trans person’s body parts is nobody’s business.”

It wasn’t long ago that sporting officials considered every female athlete’s parts to be their business. Up until the late 1960s, sporting officials used a sex verification process that required female competitors to parade, in the nude, in front of a panel of female judges. As horrible as these “nude parades” were, they were done in pursuit of a noble purpose — fair play.

People who go through male puberty are taller, have bigger bones and develop greater muscle mass than those who go through female puberty, said William Briner, a sports medicine physician at the Hospital for Special Surgery in Uniondale, New York, during a session on transgender athletes at the American College of Sports Medicine meeting in early June. Men also have more red blood cells than women, and their hearts and lungs are bigger too. Some of these advantages are irreversible, Briner said, even if testosterone levels are cut. “Insofar as there is an ‘athleticism gene,’” it’s the gene on the Y chromosome that signals the development of testes and the production of testosterone, writes David Epstein in his book, “The Sports Gene.”

These differences give men a distinct advantage over women, and they’re the reason that sport is segregated by gender. Which is all fine and great until it comes time to sort women from men.

The nude parades that women were once subjected to weren’t just humiliating. They were also problematic, because about 1 in 1,500 to 1 in 2,000 people are born with atypical genitalia. At the 1968 Olympics, anatomical checks were replaced with chromosome tests, and only athletes with XX sex chromosomes among their 23 chromosome pairs (so-called 46,XX females) could compete in female competitions. It may have seemed like a good idea at the time, but once again, the method ran into trouble, because some people are born with anatomical features that don’t match their chromosomes.

For example, María José Martínez-Patiño, a Spanish national champion hurdler, learned via a chromosome test that despite her life-long identification as a female, she actually had a 46,XY karyotype. Genetically she was male, but she was also born with an insensitivity to testosterone, which led her to develop as a female. The testosterone in her body didn’t give her an edge, because her body was incapable of responding to it. When her story leaked to the press, “I was expelled from our athletes’ residence, my sports scholarship was revoked, and my running times were erased from my country’s athletics records,” she wrote in the Lancet. “I felt ashamed and embarrassed. I lost friends, my fiancé, hope, and energy.” With almost nothing left to lose, she fought her disqualification. “I knew that I was a woman, and that my genetic difference gave me no unfair physical advantage,” she wrote. “I could hardly pretend to be a man; I have breasts and a vagina. I never cheated.”

The chromosome test ended Martínez-Patiño’s career. But her case drew attention to the issue, and the chromosome tests were discontinued for the 2000 Olympic Games in Sydney, although women deemed suspicious could still be subjected to a battery of testing. That’s what happened to Caster Semenya, a South African 800-meter runner who endured a highly publicized and humiliating ordeal in which her gender was questioned, and she was reportedly advised to undergo therapy to cut her serum testosterone levels.

In 2011, after the Semenya controversy, the International Association of Athletics Federations instituted new definitions for determining eligibility for women’s sport. Female athletes who had functional testosterone (in other words, not just high testosterone levels, but also functioning receptors that allowed their bodies to respond to the hormone) above a threshold number were not eligible to compete unless they did something to reduce their testosterone below the threshold. In 2012, the IOC adopted similar rules. The testosterone threshold was set at 10 nmol/L, a number arrived at by taking the average numbers from a sampling of female athletes with polycystic ovarian syndrome (a condition associated with elevated testosterone levels) and adding five standard deviations to it, in hopes of ensuring that it would only flag dopers and women with hyperandrogenism.

Not everyone agreed with the new rules. “The transgender community is split over the question of whether to use T or to use gender identity for eligibility for women’s sport,” said Joanna Harper in a Q&A with sport scientist Ross Tucker. Harper is an athlete, scientist and transgender woman who took part in the 2015 IOC consensus meeting that led to the group’s new guidelines on transgender and hyperandrogenism. “Many intersex activists thought it was wrong to force women to alter their bodies to compete in sports,” she told Tucker.

Harper supports a testosterone limit (she wrote that her running times fell markedly after she started hormone therapy), but the case for disqualifying athletes based on their hormone profiles isn’t straightforward. During his talk at the ACSM session on transgender athletes, Jonathan Reeser of Marshfield Clinic showed a Far Side cartoon depicting a shifty-eyed school kid opening up his coat to reveal a hidden brain. The caption: “Midway through the exam, Allen pulls out a bigger brain.” The implication is that Allen is pulling out this extra brain to get an unfair advantage. “But what if he wasn’t trying to cheat?” Reeser asked. “What if Allen just happened to have an ancillary brain? Would that be fair or not?”

A recent decision by the Court of Arbitration of Sport (CAS) implies that it is fair for athletes to use their natural-born advantages, at least if these advantages fall within a certain range. The ruling granted Dutee Chand, a sprinter from India, the go-ahead to compete, for the time being, anyway, without altering her hormones.

After some of her teammates questioned her gender, Chand was tested, found to have naturally elevated levels of testosterone and ruled ineligible to compete. She fought the ruling, and the Court of Arbitration of Sport issued a decision in her favor, saying that the IAAF had not shown that the degree of Chand’s advantage was more significant than the advantages athletes might derive from other natural physical characteristics.

The CAS decision essentially gives the IAAF, track and field’s governing body, two years to come back with science that supports the 10 nmol/L limit. If they can’t provide evidence that women with natural testosterone levels exceeding 10 nmol/L have an unfair edge, then the limit will be scrapped.

Although such a move wouldn’t immediately change anything for trans women, since they are subject to separate rules, it could potentially pave the way for them to challenge the hormone limits they face as well.

I asked Jenner whether hormones were a reasonable standard and if it was fair for her to compete as a female. “I have seen no indication to this point that trans people, male or female, have any advantage whatsoever at that level,” she said. “There’s no trans person out there, male to female, that’s out there dominating. It just doesn’t happen.” The idea that men would transition to women just to win competitions is just “a big N-O, no,” she said.

Despite Jenner’s certainty, these issues remain unsettled and are almost sure to make news in Rio, where Semenya is expected to vie for the gold medal. If Semenya wins the 800m in Rio, “I think we’re going to be seeing another public reaction to her accomplishments,” predicted Myron Genel, a Yale pediatric endocrinologist, during his talk at ACSM.

The testosterone limits that CAS suspended were already much higher than the average woman’s, and if the IAAF comes back and successfully argues that elevated testosterone levels confer an unfair advantage, there’s still the question of where to put the cutoff. Wherever that threshold might land, there will also remain the question of whether the same standards should be set for male-to-female trans athletes whose levels are set with hormone therapy in the first place.

At stake is a delicate balance between protecting the integrity of women’s sport while also honoring the rights of intersex and transgender athletes, who already face discrimination and marginalization.

This was an edition of Strength in Numbers, my column exploring the science of sports and athleticism. Got feedback, suggestions or a news tip? Email me, leave suggestions in the comments section or tweet to me @CragCrest.

CORRECTION (July 4, 12:30 p.m.): An earlier version of this article misidentified the organization that had found Dutee Chand ineligible to compete. It was the International Association of Athletics Federations, not the International Olympics Committee.