Does Testosterone Really Give Caster Semenya an Edge on the Track?

The Court of Arbitration for Sport in Lausanne, Switzerland, has ruled that women with very high testosterone levels — far above the normal range — cannot compete against other women in races from 400 meters to one mile unless they take drugs to suppress production of the hormone.

The ruling prevents Caster Semenya, 28, an elite runner and Olympic champion from South Africa, from competing in those races because her testosterone levels are naturally very high. She had challenged attempts to disqualify her from racing as a woman.

The science underpinning that decision is complicated, raising difficult questions about biology, fairness and gender identity.

It’s a hormone, an androgen, that has a variety of effects on the body. Women and men produce testosterone, but women don’t make nearly as much.

In men, high levels of testosterone are made by the testes. Much lower levels are produced in the adrenal glands, which rest above the kidneys.

Women also make testosterone in their adrenal glands, and in their ovaries. But testes produce much more: Testosterone levels in men are 295 to 1,150 nanograms per deciliter of blood, while the levels in the women are 12 to 61 nanograms per deciliter of blood.

Testosterone “builds muscle,” said Dr. Benjamin D. Levine, who studies sex differences in athletic performance at the University of Texas Southwestern Medical Center. “It builds skeletal muscle, it builds cardiac muscle. It increases the number of red blood cells.”

The effects are seen whether the hormone is naturally present or introduced with drugs. In one of the most infamous examples, women who represented East Germany at the Olympic Games in the ’70s and ’80s achieved astounding success after they were unknowingly doped with anabolic steroids including testosterone.

“The science is quite clear,” said Dr. Aaron Baggish of Massachusetts General Hospital, who is an expert on testosterone’s effects. “An androgenized body has a performance advantage.”

Most medical experts believe that testosterone is exceptional, not like a long arm span in a swimmer or another natural physical advantage.

In one study, Dr. Levine put sedentary young men and young women through a year of athletic training. At the start, the men and women had similarly sized hearts. A year later, the men’s hearts were much larger, the result of muscle-building directed by the hormone.

The hormone’s effects are amplified among elite athletes, altering the body in ways that can make a huge difference in performance. Male champions in every sport are always much faster and stronger than women who set world records.

[Read more about Caster Semenya’s early life, and what she means to South Africa.]

The gap can be quite wide. Elite female runners would never win races if they competed against elite men, according to Doriane Coleman, a former middle-distance runner who is now a law professor at Duke University.

Ms. Coleman has reviewed the best performances of three female athletes who were the fastest 400-meter runners in history (and were not injecting testosterone).

In 2017 alone, she found, more than 10,000 men and boys running 400-meter races beat the best times these women ever ran.

The rule is consistent with a requirement that it be narrowly tailored to the evidence. Athletes who identify as women but who have testosterone levels in the male range are overrepresented in women’s middle-distance running events, a recent study found.

Ms. Coleman said that in these events, the athletes’ results “show that they have a clear performance advantage.”

These athletes won 30 medals in Olympic and world championship races at distances ranging from 400 to 1,500 meters. Their incidence in the general population is just 1 in 20,000, meaning they were overrepresented by about 1,700-fold on the podium, the study concluded.

Scientists believe that very high testosterone levels yield a competitive advantage to women in every sport. Still, experts like Dr. Baggish and Dr. Levine — who were consulted by the International Association of Athletics Federations — endorsed a limited ruling, because the middle-distance races are where testosterone makes a clearest difference.

The court’s ruling is not about accepting intersex or transgender people in society, Ms. Coleman said. It is about allowing elite female athletes to compete on a level-playing field.

But that raises another interesting question: Why do we segregate athletes according to dated ideas of gender in the first place?

“The issue is why do we have women’s sports at all,” Dr. Levine said. “I think it is critical to protect women, to give them a chance to be successful.”

With an established testosterone cutoff, “some will feel left out,” he acknowledged.

And he has a modest proposal: Athletes could be separated into two groups — a women’s division and an “open” division. Men and anyone with a testosterone level beyond the normal female range would compete in the open division.

That way, he said, “anyone who wants to compete can compete.”

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