If you take testosterone to supplement your body’s natural supply, it’s considered a performance enhancer. If your body makes an unusually high volume of the hormone on its own, that’s just a good old-fashioned competitive advantage—if you’re a man. But if you’re a woman, at least according to some of the biggest sports associations in the world, having more testosterone is just plain unfair.

In the last few years, a debate has flared up about whether it’s fair for people with naturally high levels of testosterone to compete in women’s sports. The International Association of Athletics Federations has previously forced athletes to undergo testing, and implemented a policy barring anyone with testosterone levels above 10 nanomoles per liter from competing in certain women’s track and field events (even if they are cisgender women). Dutee Chand, an Indian sprinter, challenged that ruling in court and won, but the IAAF came back a few years later with an even more stringent rule: A limit of just 5 nanomoles per liter, which trans athletes are required to demonstrate continuously for at least 12 months before they’re eligible to compete. South African sprinter Caster Semenya challenged that ruling and has had some small victories, but is still fighting to compete without having to take hormone suppressant drugs. It all essentially boils down to what’s fair, which is, of course, a massively complex issue.

But the debate hasn’t come to much of a conclusion about testosterone’s effect on athletic performance (even the IAAF report on the matter had flaws in its data). And that’s for a good reason: there isn’t much evidence one way or the other.

Researchers generally aren’t inclined to pump a healthy body full of hormones it doesn’t need just for the sake of studying athletic performance, which limits our ability to collect data on how much effect they actually have. But this week, researchers released a first-of-its-kind study in which healthy, physically active young women spent 10 weeks applying a testosterone-increasing topical cream daily to see how their athletic performance might change.

The main conclusion: Raising testosterone levels made the women better runners. But like many sports studies, the results weren’t actually that simple. Supplemental testosterone increased the amount of time the test subjects could run before hitting exhaustion by 21.1 seconds, or 8.5 percent, but that’s not the only metric the researchers looked at. The hormone didn’t have any apparent effect on VO2 max, a central measure of aerobic capacity. Increased testosterone also failed to significantly impact how high a test subject could jump, or how much power output they had on a bicycling test.

As one of the few double-blind controlled studies on the matter, the paper holds some weight—it certainly shows an effect on running performance in cis-women, even with a fairly small bump in testosterone. That lines up with what we know are demonstrable effects of testosterone on the body. It generally helps you increase lean muscle mass, which can help you become stronger and faster.

But the real question isn’t whether increasing your testosterone increases how long you can run before you’re exhausted—it’s whether it improves your athletic performance across the board. Unfortunately, we’re lacking on solid evidence to know for sure one way or the other.

dutee chand celebrating a bronze medal in the 4x100 meter relay
Dutee Chand, an Indian sprinter who was briefly barred from competition for having hyperandrogenism, a condition in which a female body produces high levels of testosterone. Wikimedia Commons

One study of professional male triathletes found no relationship between testosterone levels and performance (though cortisol levels did correlate). Another, looking at professional cyclists, found the same lack of correlation. Yet another, comparing cyclists, weightlifters, and controls to each other on a cycling test, actually found a negative correlation between testosterone levels and performance.

But other studies have found mixed or opposing results. One found that sprinters seem to get an advantage from testosterone, while runners specializing in longer events didn’t. Another came to the conclusion that higher testosterone levels helped female track athletes, but not male ones.

Some reviews of the existing data have come out suggesting that there’s not enough evidence to enforce any upper testosterone limit in women, while others conclude that we know enough to apply very specific hormonal guidelines to competitors.

Complicating all of this is the fact that testosterone levels vary quite a lot among professional athletes. One analysis found that [25 percent of elite male athletes have testosterone levels lower than what the International Association of Athletics Federations consider the limit for men. And those athletes weren’t just in less strength- or speed-oriented sports. Some of the events with the most men below the limit were powerlifting, rowing, track and field, ice hockey, and rowing. Basketball players and alpine skiers had some of the highest testosterone levels. At least to some researchers, that seems to imply that high testosterone isn’t a universal performance booster.

eero mantyranta at the 1964 olympics
Eero Mäntyranta at the 1964 Olympics in Innsbruck Wikimedia Commons

It may be that, in sports like sprinting where muscular strength and speed play such a pivotal role, extra testosterone really does make a meaningful difference. One analysis puts the competitive advantage around two to five percent for women with naturally high testosterone levels even within the standard limit, which is small but critical if you’re an elite athlete. Of course, speed and strength aren’t necessarily the most important aspects of every sport, and training and mental conditioning have a huge effect even for sprinters and powerlifters.

But none of this really addresses the issue up for debate: What’s fair?

Michael Phelps’ muscles produce half the lactic acid of a normal person, enabling him to push himself for much longer without muscle pain and fatigue. Finnish cross-country skier Eero Mäntyranta has an inherited mutation that increases his red blood cells’ oxygen-carrying capacity by 25 to 50 percent, which is the genetic equivalent of doping to increase stamina. Plenty of basketball players are, statistically speaking, very abnormal when it comes to their height. Some sprinters benefit from having more fast-twitch muscle fibers than average due to genetic variance. If having naturally high testosterone is cheating, then one could argue the same of any of the aforementioned adaptations.

As two researchers who study this issue pointed out in a New York Times op-ed earlier this year, the problem really only arises because we think of testosterone as being inherently male. That implies a woman with high levels of the hormone is somehow slipping into another sexual category, and that therefore it’s only fair to make her either compete with the men or deform her body to compete with other women.

But men and women both produce testosterone, just like they both produce forms of estrogen. Women can even develop health problems from having too little of it. So even if a particular hormone gives athletes an advantage, it makes no sense for one sex to claim testosterone’s benefits as fair and natural while the other has to try to keep it under control.

This is all also assuming that there is a clear and firm biological division between male and female bodies, which we are increasingly learning just isn’t true. Most folks are born either XY or XX, but some are XXY or just X (to name just a couple of the many possible variations), and plenty of others have one biological sex but identify as another gender. Even among people who are chromosomally male or female and identify accordingly, many of the internal differences once thought to distinguish one sex from another (including testosterone levels) follow much less of a binary than previously assumed. Trans women athletes have struggled with this in particular, since hormone therapy doesn’t always suppress testosterone to a point that doctors consider a “normal” level for women. It remains entirely unclear how excess testosterone from hormone therapy, even in the presence of added estrogen, might impact athletic performance, so matters are no less complicated for athletes transitioning to male.

There’s certainly a lot left for scientists to learn about sex, gender, and just how helpful testosterone might be to an athlete’s performance. But as those results trickle in, it’s important to recognize that this debate rests on subjective, complex issues that have nothing to do with a single hormone.