Determining a person's gender may sound straightforward. But the controversy over the sex of South African sprinter Caster Semenya has raised questions about exactly where the line is between men and women, and whether an "intersexual" individual should be allowed to compete.
After Semenya, 18, won the 800-meter competition at the World Track and Field Championships in Germany, the International Association of Athletics Federations ordered her to undergo a rigorous series of tests to determine her gender.
She will be examined by a gynecologist (her genitals), an endocrinologist (her hormones) and an internal medicine specialist (her organs).
Semenya has a muscular physique, a deep voice and is incredibly fast, blowing past her competition in Germany. Her family says she has been dealing with this question for years, and insists Semenya is a woman.
"She didn't mind relatives calling her a boy inside the house," said Evelyn Sekgala, Semenya's cousin. "They often made fun of her by calling her a boy. Calling her a boy in public didn't affect her because she was used to being called a boy by her family."
Experts say determining a person's gender is not always easy and can actually be a murky and complex area.
"Sex is actually really complicated, and it's made up of a whole bunch of different components. There's no one indicator of what makes you male or female," said Dr. Alice Domurat Dreger, a professor of clinical medical humanities and bioethics at Northwestern University.
At conception the mother provides the baby with an X chromosome, and the father provides either an X or a Y chromosome. XX embryos are usually female babies, and XY are most often male.
But experts say abnormalities can develop, creating up to 36 known variations of "intersex conditions," which occur in one in 2,000 children.
"We have to get over the fact that within the categories of men and women, there is natural variation in terms of what's going on in the body in terms of hormones, in terms of genetics, and just recognize that that's part of natural variation," Dreger said.
Semenya's case raises challenging questions about whether an intersexual person should be allowed to compete in athletic competitions.
"Can I see a day where there are three categories in the Olympics? No, I can't see that," Christine Brennan, USA Today sports columnist said. "Sometimes that black-and-white world is necessary -- male, female, even if the medical community would say there's a lot of gray in there."
This is not the first time intersex issues have been raised. After Ewa Klobukowska of Poland won two gold medals in track in 1964, she was diagnosed with a genetic condition and was not allowed to compete anymore.
More recently in 2006, Santhi Soundarajan lost her silver medal in the Asian Games because she failed a gender verification test.
But Semenya has said all of the questions about her gender do not distract her.
"When I'm racing, I'm thinking about my own race," Semenya told reporters after a recent race. "I'm not thinking about anybody."