How Sex Testing in Sports Works—and Why It's So Hard

A Test With No Easy Answer

For nearly a century, sports governing bodies have tried to draw a bright line between male and female athletes. The methods have ranged from degrading physical inspections to chromosome analysis to, most recently, screening for a single gene. Each approach has promised clarity—and each has collided with the same stubborn reality: biological sex is not a binary switch.

From Physical Exams to Chromosomes

Formal sex verification in international sport dates to the 1940s, when female competitors were required to submit medical certificates confirming their sex. By the 1960s, the process had become far more invasive. At some events, women were asked to parade nude before a panel of doctors, according to research published in the Canadian Journal of History of Sport.

The 1968 Mexico City Olympics introduced the first laboratory-based approach: the Barr body test, which looked for a condensed X chromosome in cheek-swab cells. In 1992, the International Olympic Committee switched to a PCR-based test targeting genetic material on the Y chromosome. Both methods flagged women with rare but naturally occurring genetic variations—while never once catching the "male imposters" the tests were designed to find.

Polish sprinter Ewa Kłobukowska, a 1964 Olympic gold medallist, became the first athlete to fail a gender test in 1967. She was found to have XX/XXY chromosomal mosaicism—a condition that gave her no competitive advantage but ended her career. Cases like hers led the IOC to abandon universal sex testing in 1999.

The SRY Gene: Master Switch With Limits

Modern sex-testing debates centre on a tiny stretch of DNA called the SRY gene (sex-determining region Y). Discovered in 1990 by geneticist Andrew Sinclair and colleagues, SRY acts as a transcription factor that triggers the development of testes in a fetus. Without a functioning SRY, a fetus typically develops ovaries, a uterus, and fallopian tubes—regardless of which chromosomes it carries.

SRY works by activating another gene, SOX9, which drives the formation of Sertoli cells and, ultimately, the testes. The testes then produce testosterone, which steers the rest of male sexual development. It is, in the words of developmental biologists, the "master switch" of mammalian sex determination.

But a switch can malfunction in many ways. Some people carry an SRY gene that does not function, meaning they have XY chromosomes yet develop female anatomy. Others have a working SRY gene and internal testes but, due to Complete Androgen Insensitivity Syndrome (CAIS), their cells cannot respond to the testosterone those testes produce. They develop breasts, female external genitalia, and are typically raised as girls—yet an SRY screening would flag them as male.

"Using SRY to establish biological sex is wrong because all it tells you is whether or not the gene is present. It does not tell you how SRY is functioning, whether a testis has formed, whether testosterone is produced and, if so, whether it can be used by the body." — Andrew Sinclair, the gene's discoverer, via the Murdoch Children's Research Institute

The Caster Semenya Era

South African runner Caster Semenya, a two-time Olympic 800-metre champion, became the most prominent athlete caught in the sex-testing crossfire. Born with a difference of sex development (DSD) that produces higher-than-typical testosterone, Semenya was subjected to years of invasive scrutiny. World Athletics required her to take medication to suppress her natural hormone levels—a demand Human Rights Watch called a violation of bodily autonomy. After a seven-year legal battle, Semenya dropped her case, effectively shut out of elite competition.

Why No Test Settles the Debate

The core problem is that sex determination in humans involves dozens of genes, hormones, and developmental pathways. According to the U.S. National Library of Medicine, conditions collectively known as differences of sex development affect roughly 1 in 4,500 births. No single biomarker—chromosomes, hormones, or the SRY gene—captures the full picture.

Sports bodies face a genuine dilemma: protecting fair competition while respecting athletes' dignity and the messy reality of biology. History suggests that every bright line eventually runs into a grey zone—and that the athletes who pay the price are almost always women.