This scientist is racing to discover how gender transitions alter athletic performance—including her own

The testosterone that courses through a man's body after puberty triggers and maintains a slew of physical changes: Men, whose levels of the hormone are usually some 10 to 15 times those of women, typically have larger muscles, denser bones, and higher fractions of lean body mass than women. That hormone-fueled transformation confers certain athletic advantages, and men on average run faster, lift more weight, and throw harder and farther than women. Sporting events are therefore usually split into male and female categories to ensure fair competition. But this division of the sexes, which has existed for as long as women have competed as athletes, forces an important question: Who, at least from an athletic standpoint, is female?

Many people believe transgender women such as Harper have athletic advantages over non-transgender women—sometimes called cisgender women—because of their previous exposure to male levels of testosterone. But Harper, a medical physicist at a large medical center in Portland, Oregon, has been challenging that assumption with data. In 2015, she published the first study of transgender athletes' performances, finding that transgender women who received treatment to lower their testosterone levels did no better in a variety of races against female peers than they had previously done against male runners. Although Harper's study included only a few transgender women, Eric Vilain of The George Washington University in Washington, D.C., a geneticist who specializes in gender-based biology, calls it "groundbreaking." 

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