Which sports will allow trans athletes at the Paris Olympics?

LONDON - A transgender non-binary runner is heading to the Paris Olympics this month after qualifying for the U.S. team by winning the women's 1,500-metre race in trials.

Nikki Hiltz, who was assigned female at birth but uses they/them pronouns, set a U.S. trials record during the meet in Eugene, Oregon on June 30.

They will now be one of the few trans athletes competing in the 2024 Games as they are not undergoing hormone therapy and are not subject to World Athletics' restrictions on trans athletes.

"This is bigger than just me. It's the last day of Pride Month (...) I wanted to run this one for my community," said Hiltz during a post-race interview with NBC Sports.

Over the past three years, multiple sporting bodies have moved to restrict the inclusion of trans athletes - particularly trans women - in international sport amid growing debate over which categories they should compete in.

The International Olympic Committee (IOC) changed its rules in November 2021 to allow individual sports to determine whether trans athletes can compete.

Here's what you need to know.

Why is there debate over trans inclusion in elite sport?

The participation of trans athletes in women's sport has become an increasingly contested issue.

Those against their inclusion in female categories typically argue that trans women who have gone through male puberty have an unfair physical advantage over non-trans women, due to the impact of testosterone on their bodies even after transition.

Men's higher testosterone means they generally grow taller and stronger than women. Critics say this means trans women could retain an advantage even after medical transition. Trans groups say taking female hormones evens the playing field.

A survey of elite British sportswomen, published earlier this year, found more than 70% were uncomfortable with trans athletes competing in female categories.

Those who oppose bans on trans athletes say they do not have a sporting advantage, and that bans are discriminatory and will likely lead to less participation among the community at all levels.

Trans women are already under-represented in elite-level sport, a report commissioned by the Canadian Centre for Ethics in Sport found in 2021.

What does the science say?

Research into how transitioning affects athletic ability remains limited.

In April, a study funded by the IOC stated that trans women were at a physical disadvantage compared to women born female across several areas, including lung function.

Bone-density was also found to be the same for athletes born female and those who transitioned.

"While longitudinal transitioning studies of transgender athletes are urgently needed, these results should caution against precautionary bans and sport eligibility exclusions that are not based on sport-specific (or sport-relevant) research," the researchers wrote in their conclusion.

In 2021, a study by Britain's Loughborough University found that hormone therapy decreased strength, lean body mass and muscle area in trans women after 12 months, but overall levels still remained higher than non-trans women after three years.

Which Olympic sports have banned trans women?

Since the IOC allowed sporting bodies to set their own policies on trans participation, multiple official bodies have updated their policies, largely in relation to trans women.

At least 10 Olympic sports have restricted the participation of trans athletes in female categories.

World Rowing has issued a near-blanket ban on the participation of trans women in female categories, while the World Boxing Council does not allow any trans athletes to participate.

The world bodies for athletics, cycling, swimming, rugby and cricket have banned trans women from competing in female categories if they underwent puberty before starting their transition.

Because Hiltz was assigned female at birth and is not undergoing any hormone therapy, they are still allowed to compete.

The governing bodies for the triathlon, tennis and archery require testosterone levels to be suppressed within a specified limit.

Other sporting bodies, such as the Badminton World Federation, have said they will consider trans athletes on a case-by-case basis, while FIFA, global soccer's governing body, announced in 2022 that it would review its policy.

Have trans Olympians competed before?

Weightlifter Laurel Hubbard from New Zealand became the world's first openly trans Olympian in 2021, when she took part in the Tokyo Games.

Ahead of the competition, she faced intense scrutiny about her possible advantages, but ultimately placed last in her group.

Canadian footballer Quinn, who does not identify as male or female, became the first non-binary person to win a medal at the same Games after the women's team took home the gold. Non-binary skateboarder Alana Smith also competed for the United States in the women's street skateboarding event.

American swimmer Lia Thomas, who became the first trans athlete to win a National Collegiate Athletics Association (NCAA) title in 2022, lost a legal battle against World Aquatics in June, after its updated policy excluded her from competing in the Olympics.

The result meant her ban from taking part in the U.S. swimming Olympic trials was not lifted.

French sprinter Halba Diouf, who had been training for the Olympics, told Reuters she felt "marginalised" after World Athletics banned trans women from competing in 2023.

(Reporting by Lucy Middleton; editing by Clar Ni Chonghaile.)

This explainer was updated on Thursday July 4 at 11:45 GMT to include Hiltz's qualification for the Olympics.