This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In May of this year, the sports world’s self-appointed judiciary, the international Court of Arbitration for Sport (CAS), upheld a controversial regulation that prevents women with naturally high testosterone (T) from competing in the women’s category in long sprint and middle distance running events. South African middle-distance runner Caster Semenya brought the case, with Athletics South Africa, against the International Association of Athletics Federations (IAAF), arguing that IAAF’s rule is unscientific, unethical and discriminatory.
The CAS panel affirmed that the rule is discriminatory because it only applies to the women’s category, and only to some women within that category. But in a two-to-one decision, CAS deemed the discrimination to be “justified” based on the IAAF’s arguments about sex differences and T. Men are, on average across athletics events, 9–12 percent better than women. The IAAF claims that T is “the main driver” of this difference. By extension, it also claims that women with T levels in the typical male range have an “insuperable advantage” over women with T in the typical female range.
While this case raises important questions of ethics, human rights and medical harm, the IAAF defends the rule with claims about scientific consensus, glossing over profound disagreements about the evidence. Here, we address four myths central to these debates.
Myth 1: T is the “master molecule of athleticism.” T’s effect on athletic performance isn’t always positive, as the IAAF’s own data on elite women athletes well demonstrates. Its initial analysis of data from two world championship competitions showed that women with higher T had significantly better performances in only five of 21 events.
Serious methodological problems with the IAAF paper prompted independent researchers to call for the paper's retraction, and the IAAF issued a correction. But the corrected version still undermines the regulation. In three of 11 running events, the lowest T group did better, and the strongest association across all events was the negative association between T and performance in the 100 meters, where lower T athletes ran 5.4 percent faster than the highest T athletes. In none of the events where high T athletes performed better was the gap greater than 2.9 percent.
One independent group requested and obtained a subset of the IAAF data, concluding: “The results of [the IAAF’s first study] are clearly unreliable, and those of [the second study] are of unknown validity,” making it “impossible” to discern the real relationship, if any, between T and performance. Clearly, though, neither this study nor the broader sports science literature support the IAAF’s claim that targeted athletes “have the same advantages over [other] women as men do over women.”
Many studies across a range of sports show similar mixed relationships between performance and T. Consider a recent analysis of teenage Olympic weightlifters, in which the best predictor of strength was lean body mass, which has a complicated relationship to T. Among girls, body mass was initially the only significant predictor of weightlifting performance, and T was a predictor of body mass. But, counterintuitively, once the investigators controlled for the girls’ size, they unmasked a strong negative relationship between T levels and performance: girls with lower T lifted more weight