Why is it so controversial for trans women to compete in sports?Weightlifter Laurel Hubbard is set to become the first trans athlete to compete in the Olympics. Her selection has fuelled global debate about trans women's inclusion in women's sports.
New Zealand weightlifter Laurel Hubbard will become the first openly transgender athlete to compete at the Olympics after being selected for the women's super-heavyweight 87+kg category at the Tokyo Games next month.
Hubbard's selection is fuelling a fierce global debate about whether trans women should be allowed to compete in women's sporting events, and - if they are - what the rules should be.
Trans rights campaigners say excluding trans athletes is discriminatory and will stoke bias against trans people in general, but critics say trans women athletes have an unfair physical advantage in women's competitions.
The issue has been at the heart of a culture war in the United States between conservatives and supporters of US President Joe Biden's push for greater LGBT+ inclusion.
Why has controversy grown?
As more people come out as trans, the participation of trans women in women's sport has increasingly been called into question, including by well-known sports stars.
Tennis champion Martina Navratilova said last year the physical advantages for trans women competitors who had gone through male puberty were "pretty obvious".
Hubbard's gold medal wins at the 2019 Pacific Games in Samoa, where she topped the podium ahead of Samoa's Commonwealth Games champion Feagaiga Stowers, caused outrage in the host nation.
Hubbard, who will be the oldest lifter at the Games at 43, competed in men's weightlifting competitions before transitioning in 2013.
Two years later, she became eligible to compete at the Olympics in women's events in line with guidelines issued by the International Olympic Committee (IOC).
The IOC advises sporting bodies to let trans women athletes compete in women's events if their testosterone levels remain below a certain threshold for at least a year. Trans men face no restrictions.
What do other sports authorities say?
Around the world, sporting authorities are grappling with how to formulate their rules.
World Rugby ruled last year that trans women could not compete in elite and international women's rugby, citing safety and fairness concerns.
In the United States, school policies for trans athletes vary, but are usually set by bodies that govern athletics, rather than state laws. Several states have no policies at all.
The National Collegiate Athletic Association (NCAA), which runs inter-college sport in the country, says trans women must suppress their testosterone for at least a year before competing in women's contests
Trans men cannot compete with women if they start taking testosterone, NCAA rules state.
In March 2020, Idaho became the first US state to ban trans women and girls from women's sports leagues in schools and colleges, setting off a trend that swept more than 30 other state legislatures.
Idaho's landmark law was suspended after a court ruled that it was discriminatory.
This year, Alabama, Arkansas, Mississippi, Montana, Tennessee, West Virginia and Florida passed similar legislation and South Dakota's governor has signed an executive order supporting a sports ban. All have Republican governors.
In April, nearly 100 companies, including Facebook, Pfizer and Dell, said they were "deeply concerned" about the slew of trans-related legislative proposals presented in conservative states, calling the bills "discriminatory".
What do scientists say?
Trans women's muscular advantage only falls by about 5% after a year of testosterone-suppressing treatment, according to a review of existing research by the University of Manchester and Sweden's Karolinska Institute.
Britain's Loughborough University found that hormone therapy reduced trans women's haemoglobin levels, which affects endurance, to equal that of non-trans women within four months.
But strength, lean body mass and muscle area remained higher after three years of medication to block testosterone, it said.
Tommy Lundberg, who co-authored the first study, said male athletes gain their 30% muscular advantages during puberty, but there are no studies of trans adolescents who could take puberty blockers or cross-sex hormones before puberty finishes.
What do trans activists say?
LGBT+ activists say the US sports bills and World Rugby ban are discriminatory and dispute claims of a physical advantage.
Gillian Branstetter, a trans advocate and spokeswoman for the National Women's Law Center, said trans athletes have not consistently outperformed other female athletes in the 16 U.S. states that have trans-inclusive high school policies.
"There's not been the abolition of women's sport. The nightmarish rhetoric (of) the people proposing these bills simply hasn't come to fruition," she said.
Chris Mosier, a triathlete and the first trans man to represent the United States internationally, said the current debate was damaging for all trans people.
"These are very dangerous bills that are attempting to serve as an entry point to larger scale discrimination," he said.