Women's boxing at the Paris Olympics underscores the complex challenges of crafting and enforcing sex eligibility rules in women's sports, revealing how athletes like Imane Khelif of Algeria and Lin Yu-ting of Taiwan become vulnerable in the aftermath.

Scrutiny of eligibility for women’s events often presents a legal quagmire for sports organizations, potentially subjecting athletes to embarrassment and mistreatment.

In the 1960s, the Olympics relied on demeaning visual tests to confirm athletes' sex, highlighting the invasive and often humiliating nature of these procedures.

The modern era of eligibility rules is widely considered to have begun in 2009, following South African 800-meter runner Caster Semenya's rise to stardom as an 18-year-old gold medalist at the world championships.

Semenya, the Olympic champion in the 800 meters in 2012 and 2016, is not competing in Paris due to a ban unless she medically reduces her testosterone levels. She remains engaged in a legal challenge to track’s rules, now in its seventh year.

Here’s a look at sex tests in sports and the complexity they create amid changing attitudes toward gender identity:

Testosterone levels – rather than XY chromosomes, which are typically seen in men – are the key criterion for eligibility in Olympic events where the sport’s governing body has established and approved rules.

Some women, assigned female at birth and identifying as women, have conditions called differences of sex development (DSD) that involve an XY chromosome pattern or naturally higher testosterone levels. Some sports officials argue that this gives them an unfair advantage over other women, but the science remains inconclusive.

Semenya, whose medical data proved impossible to keep private during her legal battles, has a DSD condition. She was legally identified as female at birth and has identified as female her entire life.

Testosterone is a natural hormone that increases bone and muscle mass after puberty. The normal adult male range is significantly higher than for females – up to about 30 nanomoles per liter of blood compared with less than 2 nmol/L for women.

In 2019, at a Court of Arbitration for Sport hearing, the track’s governing body argued that athletes with DSD conditions were "biologically male." Semenya said this was "deeply hurtful."

Semenya’s case was highly publicized before 2021 when gender identity became a major topic at the Tokyo Olympics and in society at large. She took oral contraceptives from 2010 to 2015 to lower her testosterone levels and reported a range of side effects: weight gain, fevers, nausea, and abdominal pain, all of which she experienced while competing at the 2011 World Championships and the 2012 Olympics.

Female athletes of color have historically faced disproportionate scrutiny and discrimination regarding sex testing and false accusations of being male or transgender.

Each Olympic sport’s governing body is responsible for drafting its own rules, from the field of play to eligibility.

Women’s boxing at the Paris Games adheres to the same eligibility criterion as the 2016 Rio de Janeiro Olympics – an athlete must be female according to her passport – following the International Boxing Association's permanent ban from the Games due to decades of governance issues and a lack of transparency. Much has evolved in science and debate over the past eight years.

Since the Tokyo Games in 2021, track’s World Athletics has tightened eligibility rules for female athletes with DSD conditions. Starting in March 2023, athletes must suppress their testosterone levels below 2.5 nmol/L for six months, commonly through hormone-suppressing treatments, to be eligible to compete.

This is half the 5 nmol/L threshold proposed in 2015 for athletes competing in distances from 400 meters to 1 mile.

World Athletics followed the lead of World Aquatics in prohibiting transgender women from competing in women’s races if they had undergone male puberty. The International Cycling Union also adopted this policy last year.

World Aquatics requires transgender women who did not experience male puberty to maintain testosterone levels below 2.5 nmol/L.

World Aquatics is not actively testing junior athletes. The first step for athletes is for national swim federations to "certify their chromosomal sex."

Similarly, FIFA defers to national member federations to verify and register the sex of players.

"No mandatory or routine gender testing verification examinations will take place at FIFA competitions," FIFA said in a 2011 advisory that is still in force and under review.

Many sports bodies strive to balance inclusion and fairness on the field of play. They also argue that in contact and combat sports, such as boxing, physical safety is a critical consideration.

In Semenya’s case, judges at the Court of Arbitration for Sport acknowledged in a 2-1 ruling against her that discrimination against some women was "a necessary, reasonable and proportionate means" to preserve fairness.

Male athletes are not required to regulate their natural testosterone levels, and female athletes without DSD conditions can also benefit.

"There are many women with higher levels of testosterone than men," International Olympic Committee (IOC) spokesman Mark Adams said in Paris amid the women’s boxing debate. "The idea that a testosterone test is some kind of magic bullet is actually not true."

The IOC wields considerable power but is also limited. The Switzerland-based organization manages the Olympic Charter, owns the Olympic brand, selects host cities and helps fund the Games through billions of dollars earned from broadcasting and sponsorship rights.

However, individual governing bodies, like FIFA and World Athletics, manage Olympic sports events. They establish and enforce their own athlete eligibility and field-of-play rules as well as disciplinary codes.

When Olympic sports reviewed and updated their handling of sex eligibility issues, including with transgender athletes, the IOC published advisory guidelines in 2021, rather than binding rules.

The IOC’s framework on gender and sex inclusion emphasizes creating a "safe, harassment-free environment" that respects athletes’ identities while ensuring fair competition.

Boxing’s situation has been different, with significant consequences in Paris.

The IOC has been embroiled in a yearslong and increasingly bitter feud with the International Boxing Association, now led by Russia, culminating in a permanent ban from the Olympics last year.

For the second consecutive Summer Games, Olympic boxing tournaments are being run by an IOC-appointed administrative committee, not a functioning governing body.

In this dysfunction, boxing eligibility rules have lagged behind other sports, and the issues were not addressed before the Paris Games.

At the 2023 world championships, Khelif and Lin were disqualified and denied medals by the IBA, which cited eligibility test failures for the women’s competition but provided little information. The governing body has repeatedly contradicted itself on whether the tests measured testosterone.

In a chaotic press conference Monday in Paris, IBA officials said they tested only four of the hundreds of fighters at the 2022 world championships and that Khelif and Lin were tested in response to complaints from other teams, acknowledging an uneven standard of profiling widely deemed unacceptable in sports.

Before Semenya, Indian sprinter Dutee Chand challenged track and field’s initial testosterone rules, passed in 2011 in response to Semenya.

A 2015 CAS ruling temporarily froze the rules and led to an update in 2018, which Semenya then challenged. Her career in the 800 meters stalled because she refused to take medication to artificially suppress her testosterone levels and was barred from elite events.

Semenya lost at CAS in 2019 but took her case to Switzerland’s supreme court and then the European Court of Human Rights, where she scored a landmark but partial victory last year.

In May, another ECHR hearing in Semenya’s case took place, with a ruling expected next year.

The case could be sent back to Switzerland or possibly to CAS in Lausanne. Other sports are watching and waiting.