The International Olympic Committee announced a new policy on March 26, 2026, for women’s competitions: Every athlete must be tested for a gene called SRY, usually found on the Y chromosome. Males typically have a Y chromosome and females typically don’t, so the IOC says this requirement will exclude “biological males.” This announcement comes as planning for the 2028 Summer Olympics, hosted in Los Angeles, is underway.

But the IOC statement hides the complexity of biological sex and continues the organization’s century of what the record shows is inconsistent and biologically unsound sports policies.

I’m a biology professor and author of an upcoming book, “The Binary Delusion: How Biology Defies the Myth of Two Sexes.” While the impetus for the new policy seems to be exclusion of transgender women from women’s athletics, it will more likely exclude and draw unwelcome publicity to many more women who are not transgender.

Few elite athletes are transgender

Transgender people have faced mounting legal and political attacks in recent years.

President Donald Trump issued an executive order on Jan. 20, 2025, asserting that biological sex is simple and binary – that everyone is unambiguously female or male – and another executive order precluding “males” from women’s competitions.

At least 29 U.S. states have excluded transgender girls and women from girl’s and women’s athletic competitions. These laws are built on the idea that men on average are superior to women in many sports, so women need to be protected from unfair competition.

But elite transgender athletes are rare. In a 2024 hearing before the U.S. Senate, the president of the NCAA testified that of the 510,000 athletes in U.S. colleges at that time, he was aware of fewer than 10 transgender athletes – less than 0.002%.

Only one known transgender woman has ever participated in an Olympic women’s competition since the committee allowed women to compete in the Games beginning in 1900: Laurel Hubbard, a weightlifter who competed for New Zealand in 2021 but did not medal.

The rarity of transgender athletes in elite competition suggests their exclusion is a solution in search of a problem.

Biological sex is complicated

The genetic test the IOC is requiring is more likely to identify intersex women.

Intersex people have a combination of typically female and typically male biological sex traits. These include sex chromosomes, internal and external reproductive anatomy, sex hormones and hormone receptors.

There are many variations of intersex traits, but three may be the most relevant for women’s athletic competitions: androgen insensitivity, 5-alpha-reductase deficiency and genetic mosaicism.

People with androgen insensitivity don’t respond as much to androgens like testosterone. Some believe that having high levels of testosterone can give athletes a competitive advantage. Athletes with this condition gain little or no advantage like muscle growth from androgens. This also means their visible sex characteristics, including their genitals, appear mostly or entirely female. The new IOC policy has an exception for “complete” androgen insensitivity but doesn’t say how athletes would demonstrate this.

The policy also doesn’t mention partial androgen insensitivity, where androgen receptors respond to testosterone but probably not enough to gain a significant advantage in performance. Nevertheless, athletes with partial androgen insensitivity will presumably fail the test and be excluded from participating under the new policy.

People with 5-alpha-reductase deficiency make and respond to testosterone but make little or none of a more potent androgen called dihydrotestosterone, or DHT. If they have no DHT, their genitals appear more female and they gain less athletic advantage from androgens. People with this condition who have a Y chromosome will fail the new sex test and be excluded.

People with mosaicism are born with some cells that have a Y chromosome and some that do not. Women can develop mosaicism during pregnancy, when fetal cells with Y chromosomes cross the placenta into her body. The woman will have some cells with a Y chromosome, perhaps for the rest of her life. Such cells could cause a previously pregnant athlete to fail the new test.

History of sex testing in women’s athletics

The IOC and associated sports federations have a long history of sex testing, especially for track events. Sex testing has switched from genitals to genes to testosterone levels, and now back to genes. While the stated goal for these policies was to uncover males pretending to be female, they have never found any. Instead, they identified and excluded intersex women.

For much of the 20th century, sports administrators examined genitals if a competitor was suspected of being male. In the mid-1960s, they began examining the genitals of all women participating in International Amateur Athletics Federation competitions in what were called “nude parades.”

The nude parades embarrassed athletes and sports federations and were replaced by newly available chromosome or gene tests in the late 1960s. These tests were often done without informed consent – athletes were instead told they were being tested for performance-enhancing drugs. The test results were then often revealed publicly without the athlete’s consent.

For example, in 1967, the Polish sprinter Ewa Klobukowska, who had won three gold medals and set three world records, was designated “male” by a chromosome test, though she had typically female genitals. She was excluded from competition and forced to return her medals. But the following year, she gave birth – she apparently had genetic mosaicism.

In 1985, a Spanish hurdler, Maria José Martínez-Patiño, found out through a public announcement that she was designated “male” by a genetic test and excluded from competition. “I felt ashamed and embarrassed,” she has said in a personal account. “I lost friends, my fiancé, hope and energy. But I knew that I was a woman and that my genetic difference gave me no unfair physical advantage. I could hardly pretend to be a man; I have breasts and a vagina.” She has complete androgen insensitivity.

Genetic testing largely gave way to testing testosterone levels in recent decades, which also excluded many intersex athletes. The 2026 IOC announcement states that there is no overlap in the testosterone levels of female and male elite athletes, but published research examining hundreds of elite athletes contradicts this statement.

In 2021, the IOC announced a new policy stating that “Every person has the right to practise sport without discrimination and in a way that respects their health, safety and dignity.” But the committee left it to each sport’s federation to regulate their own competitions, leading to a confusing mix of criteria that may have paved the way for the organization’s simplified 2026 policy.

SRY gene test is misleading

The IOC’s 2026 policy hints at the complexity of biological sex, stating that sex includes “sex chromosomes, gonads and hormones.” But it’s odd that genitals didn’t make their list, considering that genitals – external sex organs like the vagina and penis – are how most ordinary people define female and male, how physicians assign sex at birth, and how the IOC itself defined sex for decades.

Sex testing through genital inspection, though embarrassing and traumatizing for many athletes, may have been a better indicator of athletic advantage from androgens like testosterone than the SRY gene. During prenatal development, androgens cause initially undefined body structures to become a penis and scrotum; in their absence, or with androgen insensitivity, these structures become a clitoris and labia instead. Thus, typically female genitals indicate low androgen levels or low sensitivity to androgens, which suggests an athlete’s physical performance was not enhanced by these hormones.

Unless the IOC takes scrupulous care to screen for these exceptions, its new genetic test will likely exclude athletes who have not gained an advantage from androgens. Their new policy, however, states that “the need for consistency and fairness across sports” will not allow for “case-by-case consideration.”

As a result, it’s likely that another generation of intersex women will be excluded from the Olympics.

Ari Berkowitz receives funding from the National Science Foundation.