COMPARED WITH banner events like the 100-metre sprint or the marathon, Olympic weightlifting does not usually draw big crowds. This year, it is in the news even before the Tokyo games begin. On June 21st New Zealand announced its weightlifting team. One member is Laurel Hubbard, a 43-year-old who won gold at the 2019 Pacific games. She was born male, so her inclusion in the Olympics has made her a lightning rod for the debate over whether trans women athletes should be allowed to compete in women’s sports. But Ms Hubbard will not be the only trans woman at this year’s games. She will be joined by Stephanie Barrett, a Canadian archer. Chelsea Wolfe, an American cyclist, and Tifanny Abreu, a Brazilian volleyball player, are likely to be there too. And trans women have already seen success away from the Olympics in sports from cycling to running.

Critics decry all this as unfair. Sex is one of the most powerful and consistent predictors of performance. Males have significant advantages over females in nearly every sport (see chart). In running, they are consistently around 10% faster than females, whether in sprints or the marathon. In Ms Hubbard’s sport of Olympic weightlifting, the gap is 25% or more, even after correcting for an athlete’s size. The Olympic record for females in the 69kg bodyweight category, for instance, is a total of 286kg. For males of the same weight, it is 357kg. Some teenage males outperform women’s world-record holders. Without sex categories, females would be absent from most elite sport.

The male sporting advantage is rooted in biology. Males have bigger muscles than females. With no need to accommodate childbirth, their skeletons are better adapted for running. They are taller, have denser bones, bigger hearts and higher levels of haemoglobin, the molecule which shuttles oxygen around the body. Those advantages stem from male puberty, during which levels of testosterone—the main male sex hormone and the original anabolic steroid—rise sharply. With the importance of testosterone in mind, the International Olympic Committee ruled in 2015 that trans women would be allowed to compete in women’s events provided that they took drugs to suppress their testosterone levels for at least a year before the competition. The hope was that would remove the male advantage, allowing trans women to compete in their preferred category while keeping things fair for the biological females against whom they would be competing.

But two recent scientific reviews—aggregating data from other studies—suggest the IOC’s compromise does not work, because many of the advantages conferred by male puberty are permanent. One of the papers found that testosterone suppression does indeed reduce haemoglobin levels to those often found in females. But both found that the effect on strength and muscle size was far smaller—perhaps around 5%, even after more than a year of suppression. That is far less than the advantage granted by being male in the first place. And no amount of hormone therapy can reduce the size of an athlete’s body. Last year, such evidence persuaded World Rugby that it was both unsafe and unfair to let trans women play in the international women’s game. The IOC, for its part, says it is reviewing the evidence, but that amending the rules so soon before the Tokyo games would be unfair. It has promised new guidelines once the Olympics are over.