BioEd Online

Caster Semenya's victory in the 800-metre final at the World Athletics Championships in Berlin has been overshadowed by the controversy surrounding her sex. After the race on 19 August, the International Association of Athletics Federations announced that they had asked Semenya to undergo tests to verify that she was female, with IAAF spokesman Nick Davies describing the tests as "extremely complex, difficult". Here, Nature looks at the science behind the latest gender row to hit sport.

What is the background to this controversy?

In the 1960s basic physical examinations were used to determine sex and prevent male athletes competing as women. These exams were not popular with competitors and were replaced by a 'buccal smear' or 'sex chromatin' test. This analyses cells from inside the mouth to determine the presence or absence of two X chromosomes. Male athletes, having only one X chromosome along with a Y chromosome, would be detected by this test.

But buccal smear tests are "notoriously unreliable" says Myron Genel, a paediatrician and expert in sexual development disorders at Yale University who was a leading player behind their abandonment and the development of the guidelines now used in gender verification.

The sex chromatin test was abandoned by the IAAF in 1991; many Olympic federations used an alternative test for Y chromosomes until 2000, although the International Volleyball Federation continued with this test for some years after.

The reason geneticists and doctors, including Genel, were so critical of these tests is that there are medical conditions that lead individuals with two X chromosomes to develop masculine characteristics, and others that mean individuals with one X and one Y chromosome never develop masculine characteristics. Some other individuals also exist outside the usual sexes of XX females and XY males; these may include males who are XXY, further confusing the tests.

What conditions are involved, and do they make you a better athlete?

Many of the conditions relate to levels of androgen hormones, such as testosterone, which lead to male characteristics including greater muscle mass. Some XX individuals have medical conditions that elevate androgen levels.

Other people born XY fail to develop as men due to androgen insensitivity syndrome. Whereas XX individuals with high androgens develop male characteristics, XY individuals who are not sensitive to it may grow up with female characteristics.

What are the current rules for these individuals?

Men attempting to pass themselves off as women would be easily detected by routine tests, such as anti-doping urine tests. Although there is no compulsory testing of sex under current IAAF rules, athletes can be asked to attend a medical evaluation if there is a challenge to their sex.

This involves a medical evaluation performed by a panel comprising a gynaecologist, an endocrinologist, an internal medicine specialist, an expert on gender/transgender issues and a psychologist. This panel must come to a decision about the sex of an athlete and whether they should be allowed to compete.

Men who have undergone sex change operations are allowed to compete as women under certain circumstances. "The crux of the matter is that the athlete should not be enjoying the benefits of natural testosterone predominance normally seen in a male," says the IAAF.

XX women with increased androgen may have some advantage but can also compete under IAAF rules. Those with several of the most common conditions that lead to 'XY females' are also permitted to race as women.

"We don't discriminate against tall women and say they shouldn't compete because they're too tall. We don't discriminate in track events on the basis of whether you have a high prevalence of fast twitch or slow twitch fibres," says Genel.

"If they have been born and raised as women and they think about themselves as women I would say let them compete as women," he adds, "unless, for some reason, it's proven to the contrary."