Editor’s note: An updated version of this article is available here

FEW ATHLETES have been as blessed and cursed as Caster Semenya. All that the 28-year-old South African runner has ever done is run as fast as her legs could carry her—fast enough to win back-to-back Olympic gold medals over 800 metres, and to triumph in each of the last forty 800-metre races that she has entered. But her extraordinary body has also been the subject of ridicule, speculation and censure. In 2009, when she breezed to a World Championship title as an 18-year-old, the International Association of Athletics Federations (IAAF), the sport’s governing organisation, announced that it was investigating whether she might be intersex—an umbrella (and misleading) term for people with a wide range of developmental conditions affecting the genitalia and gonads. To protect her privacy, the IAAF never published its findings. But it has spent the ensuing decade fighting a regulatory battle against Ms Semenya, about whether she must meet certain hormonal criteria to compete as a woman. On May 1st the Court of Arbitration for Sport (CAS), the highest legal body in sport, ruled against her in a decision that will have wide-ranging implications.

The court allowed the IAAF to impose a limit of 5 nanomoles of testosterone per litre of blood (nmol/L) on runners with certain intersex conditions that increase naturally occurring testosterone. The ruling covers women’s races between 400 metres and a mile. That testosterone threshold is far below the normal male range of 8-30 nmol/L, but considerably above the normal female range of 0.1-1.8 nmol/L. To continue racing over 800 metres, Ms Semenya would have to undergo hormone therapy, which can increase the risk of cardiovascular disease, along with other nasty side effects.

The direct impact of the court’s ruling might actually be quite small. Ms Semenya has undergone hormone therapy before, when the IAAF introduced a testosterone limit of 10 nmol/L for women across all track-and-field events in 2011. The court suspended that rule in 2015, when Dutee Chand, an Indian sprinter with abnormally high testosterone levels, disputed that there was proof that women like her received an unfair advantage in all athletic events. The IAAF has spent the ensuing years gathering data on the question.

Its researchers presented to the court an analysis of results at past athletics championships, which showed that women with high levels of testosterone did disproportionately well in middle-distance events. They did not find such evidence for most other competitions. The court therefore allowed the IAAF to impose a tougher 5 nmol/L limit, but only for those particular races. Ms Semenya is one of a handful of runners affected. Rather than undergoing another bout of hormone therapy, which added about 4% to her 800-metre time, she could simply switch to the 5,000-metre race. The event is not covered by the new rules, and she won it at the South African Athletics Championships on April 26th.

Only a few runners will have to make immediate career choices after the court’s decision. But the precedent set by the IAAF’s ruling could affect female athletes in every sport. It is by far the most prominent and detailed ruling that the court has delivered regarding biological sex, and it is a potentially far-reaching one. From now on, the CAS will almost certainly use testosterone levels to determine who should be allowed to compete in women’s events. These tests will apply not only to intersex athletes, but also to trans women, who were born male but identify as women. The International Olympic Committee (IOC) had already introduced a testosterone limit of 10 nmol/L for trans women in all sports in 2016, replacing its previous requirement for athletes to have undergone genital-reconstruction surgery. It is now considering reducing the limit to 5 nmol/L. This rule change has not been tested at the court, but after Wednesday’s precedent it looks likely to stand.

A binary code

How to distinguish between men and women? If sports administrators want to maintain separate competitions for different sexes, any attempt to draw a firm biological distinction between them will miscategorise some people. That leaves them with two options. The first is to pick a binary physical characteristic, such as having—or not having—testes or a Y chromosome, a typical (but not universal) biological marker for males. The second is to pick a physical characteristic that exists on a spectrum, such as endogenous testosterone levels, and to set a threshold.

The first approach might seem more robust. Officials at the Olympics used chromosome testing to verify athletes’ sex between 1968 and 1996. But it turns out that some women who possess both Y chromosomes and testes receive no performance-enhancing effects from them. Maria José Martínez-Patiño, a Spanish hurdler, was kicked off the national athletics team (and shunned by her colleagues and boyfriend) after failing a verification test in 1985. Three years passed before geneticists could prove that her body was insensitive to testosterone, and that her intersex condition thus conferred no athletic advantage. The IAAF subsequently stopped chromosome testing, but not soon enough for Ms Patiño to save her career.

Her case was extremely rare. Various estimates suggest that the share of intersex people in the general population ranges from 0.05% to 1.7%, depending on how broadly the concept is defined. But those figures include conditions that neither raise questions about a person’s biological sex nor confer any conceivable sporting advantage (as in Ms Patiño’s case). Over time, officials have decided that they are unwilling to exclude people who fall into this category. Their approach now is to include people in women’s sports unless they demonstrably have an unfair competitive advantage.

This has led governing bodies to the second option: picking a physical characteristic that exists on a spectrum. Scientists generally agree that testosterone is the best candidate. The hormone drives the development of male characteristics from puberty, such as bigger muscles, sturdier bones and less fat. Ross Tucker, a sports scientist who advised Ms Semenya on the case, has shown that the gap between boys’ and girls’ running times grows wider once they become teenagers, as males experience this physiological boost. In adulthood, the fastest men run about 10% faster than women. The discrepancy is even bigger for some other field events, such as javelin and pole vault. It is no coincidence that anti-doping bodies ban competitors from juicing up on testosterone artificially.

One advantage of setting a testosterone threshold is that athletes such as Ms Patiño, who produce lots of it without any benefit, can lower their levels without affecting their performance. But this approach also poses lots of problems. Working out where the threshold should be is tricky. Defining femaleness on a sliding scale means that trans women, as well as intersex athletes, can qualify after hormone therapy. That is unlikely to undo all the sporting advantages of a testosterone-charged puberty.

A lack of reliable data complicates things further. Whereas the hormone-induced gap between male and female performance is obvious, the correlation between testosterone levels and athletic prowess within the tiny group of elite female competitors is weak. This is a select group of highly unusual people who have a similar biological and sporting profile. (Some trans women have erroneously used the weakness of this correlation within a small sample to claim that testosterone does not matter at all across the two sexes, which Mr Tucker strongly disputes.)

So measuring the advantages of abnormally high testosterone levels among women is tricky, which could explain why the IAAF struggled to find any sign of such effects in most of the events that it studied. The organisation’s analysis might also have had methodological flaws. It compared women whose testosterone levels are in the top third of the range against those in the bottom third, when it might be more enlightening to look at the farther extremes of the range instead. And Mr Tucker was one of a trio of independent researchers who found that the IAAF’s figures were riddled with errors, such as athletes who had been double-counted.

It takes an extraordinary person, such as Ms Semenya, to demonstrate just how potent testosterone can be in women’s sport. A by-product of her first experience with hormone therapy is that we know roughly how much it hindered her performance: by 4%. But whether that is an appropriate adjustment is purely subjective. Ms Semenya thinks it is unfair: “I just want to run naturally, the way I was born,” she says. Sports officials have ultimately come to an arbitrary decision. The IAAF’s 10 and 5 nmol/L thresholds were largely based on estimating the maximum level that a non-intersex woman could naturally reach.

The law of comparative advantage

The guesswork becomes even sketchier for combat and contact sports, because measuring the performance of a wrestler or rugby player is much harder than timing a runner. Bone structure is also more important in these games, which gives anyone who develops a stronger skeleton at puberty a permanent advantage, regardless of how much hormone therapy they later go through.

The most controversial cases in these sports have involved trans women, rather than intersex ones. Such “transitioning” is a broad process that may include declaring oneself to be a woman, or undergoing hormone therapy or genital reconstruction surgery (transition goes the other way, too).

Fallon Fox, an American mixed-martial arts fighter, was pilloried by commentators and other combatants when she revealed in 2013 that she had undergone gender-reassignment surgery. One male fighter called her a “lying, sick, sociopathic, disgusting freak”. She last appeared in the ring in 2014. Hannah Mouncey, a trans woman who had represented the Australian men’s handball team before changing gender and switching to the women’s team, was barred from the women’s Australian Football League in 2017.

Reliable data about the change in performance for trans women are just as scarce as for intersex ones. Joanna Harper, a scientist who is also a trans runner, and who appeared as a witness for the IAAF in Ms Semenya’s case, has conducted one of the few studies to date. Her research covered eight non-elite endurance runners who competed first as men, then as women. She found that their slower times after hormone therapy put them in roughly the same position within women’s races as they had achieved within men’s.

But the data are far too scant to say that this pattern holds for all athletes in every sport. If a handful of trans women fare better in elite women’s events than they did in men’s—suggesting that they have maintained an unusually large athletic advantage, even after hormone therapy—then sports administrators could find that the testosterone threshold creates a bigger quandary than chromosome tests did. Rather than cruelly ending the career of a few athletes such as Ms Patiño, they could reduce the chances of victory for a large number of women who would struggle to beat disproportionately successful trans competitors.

Ms Harper points out that there has been no deluge of trans women gaming the system. The trans athlete who has come closest to dominating an individual sport is Laurel Hubbard, a New Zealander who held junior national weightlifting records as a man and then became an international contender as a woman. In 2017 she finished second in the World Championships, and was leading at the Commonwealth Games in 2018 before an elbow injury. It was only in 2016 that trans women could compete in women’s events without having undergone genital-reconstruction surgery—which only a small proportion of trans people do. Not a single trans athlete has yet competed in the Olympics.

Nonetheless, the possibility remains that the testosterone threshold could allow several pretty good male competitors to reign supreme after transitioning to become women. Many women worry that such champions are imminent. Some trans women accuse them of scaremongering. Only the passage of time and collection of more data will prove who is right.

Regardless of what happens, the court’s ruling on Ms Semenya’s case confirms that the definition of what it means to be female in sport will from now on be set on a sliding scale. If anybody thought that the decade-long dispute would clarify the matter, they were wrong.

Correction (May 9th 2019): An earlier version of this article said that the IAAF’s 5 nmol/L limit referred to nanolitres of testosterone per litre of blood. In fact, the unit of measurement is nanomoles per litre of blood.