South African runner Caster Semenya has been ordered to undergo a gender verification test, to prove she rightfully won the gold medal in the women's 800 meters at the world championships in Berlin on Wednesday.
Proving one's gender isn't always so easy. Aside from the obvious physical signs, chromosomes usually determine whether a person is male or female. Males are born with XY chromosomes while females have two X chromosomes.
About 1 percent of people are born with some kind of sexual ambiguity, sometimes referred to as intersexuality. These people may have the physical characteristics of both genders, a chromosomal disorder, or simply have ambiguous features. People who have both male and female organs are hermaphrodites.
Until 1999, the International Olympic Committee analyzed chromosomes from saliva samples to confirm the gender of female competitors and prevent men from masquerading as women. Other sports organizations have called the tests unreliable. The tests were scrapped before the 2000 Sydney Games.
The most common cause of sexual ambiguity is congenital adrenal hyperplasia, an endocrine disorder where the adrenal glands produce abnormally high levels of hormones.
In women, this means a masculine appearance. They may have female sexual organs, but the ovaries may be unable to produce estrogen, preventing the growth of breasts or pubic hair.
There are also several rare chromosomal disorders where women may have some male characteristics. Women with Turner syndrome, which affects about 1 in 2,000 babies, typically have broad chests and very small breasts. Their ovaries do not develop normally and they cannot ovulate.
About 1 in 1,000 women are also born with three X chromosomes. They tend to be exceptionally tall, with long legs and slender torsos. They usually have female sexual organs and are fertile.
A handful of athletes have typically dropped out or been thrown out of the Olympics for failing gender tests over the years. But no evidence supports the idea that such competitors have an unfair athletic advantage.