There's no question that South African sprinter Caster Semenya is fast. At the World Athletics Championships in Berlin this week, the 18-year-old streaked to victory in the 800-meter women's event, leaving her competitors a staggering 2.45 seconds behind.
Her time of 1 minute, 55.45 seconds clipped more than a second off the 1:56.72 she ran three weeks ago to win the African Junior Championships, which was more than eight seconds faster than the 2:04.23 Semenya ran last October to win the 2008 Commonwealth Youth Games.
And because of these decisive victories, many people are questioning whether Semenya is actually a woman.
"This is the kind of thing that sullies the reputation of an athlete and a country," said USA Today sports columnist Christine Brennan, who's also an ABC News consultant.
It's not only Semenya's speed that has raised eyebrows. Her muscular physique and deep voice are also causing many observers to take a closer look at her gender.
"When I'm racing, I'm thinking about my own race," Semenya said recently. "I'm not thinking about anybody."
Back home, in the rural South African village of Fairlie, Semenya grew up playing soccer with boys, racing the boys. She was even teased by friends who said she looked like a boy.
"Calling her a boy in public didn't bother her because she was used to being called a boy by her family," said Evelyn Sekgala, a cousin.
Today, her family insists that the 5-foot-7-inch, 140-pound athlete is most definitely a woman. But the International Association of Athletics Federations now wants proof.
"You're talking about someone's life," said Nick Davies of the International Association of Athletics Federations. "She was born, christened, grown up as a woman. So if you're going to say you're now not a woman, you're really a man, you really have to be sure of what you're doing."
So, Semenya is undergoing a battery of tests, including a physical exam by a gynecologist, hormone tests by an endocrinologist and a thorough genetic screening by geneticists.
But even that might not be enough to settle the issue, experts say.
"Sex is actually very complicated," said professor Alice Domurat Dreger of Northwestern University in Evanston, Ill. "It's made up of a whole bunch of different components.
There's no one indicator of what makes you male or female."
On rare occasions, one in roughly every 2,000 children is neither male nor female, but something considered "intersex," meaning they may have male and female biological characteristics.
"Within the categories of man and women, there are natural variations in terms of what's going on in the body," Dreger said.
But some variations are deliberate. In track and field, there are examples, especially from former Soviet-bloc countries, of men changing or hiding their gender, with the use of steroids, so they could compete as women.
Polish athlete Ewa Klobukowska was the first athlete to fail a gender test after winning the gold medal in two track events in 1964. She was diagnosed as having a genetic condition but was banned from competing in professional sports.