The presence at the Paris Olympics of two boxers who were disqualified from last year's world championships has revived the discussion over whether athletes with Differences of Sexual Development (DSD) should compete in women's competition.
What is a DSD?
Differences of Sexual Development are a group of rare conditions involving genes, hormones and reproductive organs. Some people with DSDs are raised as female but have XY sex chromosomes, blood testosterone levels in the male range and the ability to use testosterone circulating within their bodies.
Why is that an issue in sport?
Women's sports categories exist in most sports in recognition of the clear advantage that going through male puberty gives an athlete.
That advantage is not just through higher testosterone levels but also in muscle mass, skeletal advantage and faster twitch muscle.
In combat sports such as boxing, this can be a serious safety issue.
What is the issue at Paris Olympics?
Boxers Imane Khelif and Lin Yu-ting at last year's world championships in New Delhi fell foul of International Boxing Association (IBA) eligibility rules that prevent athletes with XY chromosomes from competing in women's events.
Algeria's Khelif was disqualified hours before her gold-medal bout in New Delhi, while Taiwan's double world champion Lin lost her bronze medal after she too failed to meet the criteria.
Why are they allowed to compete at the Olympics?
The IBA was stripped of its status as the global governing body for boxing by the International Olympic Committee (IOC) in June last year because of its failure to complete reforms on governance, finance and ethical issues.
The IOC is therefore running the boxing competition at the Paris Games, as it did at the Tokyo Olympics, and its rules on the inclusion of athletes with DSDs and gender diversity in the women's competition apply.
The latest IOC guidelines issued in 2021 state that inclusion should be the default in such cases and that athletes should only be excluded from women's competition if there are clear fairness or safety issues.
"Federations need to make the rules to make sure that there is fairness but at the same time with the ability for everyone to take part who wants to. That's a difficult balance," IOC spokesman Mark Adams said on Tuesday.
"In the end it's up to the experts for each discipline. They know very well where there is an advantage, and if that is a big advantage then that is clearly not acceptable. But that decision needs to be made at that level."
Are there any similar cases at the Paris Olympics?
Zambia women's soccer captain Barbra Banda was ruled out of the 2022 Africa Cup of Nations after failing the Confederation of African Football (CAF) gender eligibility tests.
The 24-year-old did captain her country at last year's Women's World Cup and on Sunday scored a hat-trick in the first half of their 6-5 loss to Australia at the Paris Olympics.
FIFA, which organises the Olympic football competition, still uses rules first published in 2011. They state that only men are eligible to play in men's competitions, and the same for women. FIFA is reviewing its policy but there is no timeline for completion.
What happens in other sports?
South Africa's double Olympic 800-metres track champion Caster Semenya has unwittingly been at the centre of this issue for more than a decade.
Over the past couple of years, the governing bodies of athletics, cycling, swimming and rugby union have been tightening their regulations to exclude those with the advantage of going through male puberty from some or all women's competition.
Semenya has not been allowed to compete in any distance between 400m and the mile since 2018 unless she maintains lower testosterone levels, which requires medication.
She appeared at the European Court of Human Rights (ECHR) in May to continue her challenge against World Athletics. The case is still being considered.
What does the IOC say about boxers?
The IOC has defended its decision to allow the two boxers to compete.
"I would just say that everyone competing in the women's category is complying with the competition eligibility rules," IOC spokesman Adams said on Tuesday. "They are women in their passports and it is stated that is the case."
What do other boxers say?
"If you're talking about fighting an actual man, then no, I'm not cool with fighting a man. But in their case, I can't say specifically on them because I don't know their scenario or situation," Australian boxer Tiana Echegaray said on Wednesday.
"Biologically, genetically, they're going to have more advantages, and in combat sports, it can be dangerous," said Australian boxer Caitlin Parker.
"I think she is a very strong female boxer. She has a good chance of winning. I have confidence in her. She is very confident," said Lin's Taiwan teammate Chia Wei Kan.