Each sport should decide for itself how to fairly regulate transgender women competing in women’s sport, rather than follow the blanket bans proposed in dozens of U.S. states, sports scientists said on Wednesday. The International Federation of Sports Medicine (IFSM), which represents 125,000 physicians in 117 countries, said data is scant on the advantages or otherwise of trans athletes, but that each sport needed rules to meet its own physical demands. The world of competitive sport is fiercely split over whether trans women hold an unfair advantage despite taking cross-sex hormones that lower their testosterone. Trans men have sparked less controversy, as the extra strength that comes from testosterone taken for transitioning is widely seen as no barrier to safe and fair competition. The global debate has united social conservatives and some top sportswomen against trans activists and supportive athletes. Opponents say trans women have advantages gained in male puberty that are not sufficiently reduced by hormone treatment. “All politics aside, all biases aside, science has to direct it,” said lead author Blair Hamilton, who researches trans athletes at Britain’s Brighton University. Hamilton, who is trans, rejected suggestions that sportswomen like her will inevitably come to dominate female sport, adding that she plays in the seventh tier of British women’s football. “I’ve played women’s sport now for four years,” said Hamilton, whose study backed testosterone limits for trans women in sport. “I’m not dominating the sport.” Trans athletes are at the heart of a U.S. culture war, with bills to ban trans women and girls from competing in college and school sport under debate or passed in 29 states, according to Freedom For All Americans, an LGBT+ advocacy group. Mississippi’s governor signed a ban into law this month, while Idaho passed a similar law last year that was eventually blocked by a federal court. U.S. President Joe Biden has pushed for greater LGBT+ inclusion, signing an executive order in January that banned discrimination based on gender identity in bathrooms, changing rooms and school sports. However, concerns about physical safety led World Rugby to ban trans women from international women’s games last year. Male puberty provides a 10-50% physical advantage, dependent on the sport, with the gap widest in activities that use “muscle mass and explosive strength”, according to a 2020 review of existing studies by the University of Manchester and Sweden’s Karolinska Institute. The muscular advantage enjoyed by trans women falls by about 5% after a year of testosterone-suppressing treatment, according to their research.