Why gender could become an issue at the Rio Olympics
Changes to IOC regulations have caused the debate over transgender competitors to intensify
In 2009, South African runner Caster Semenya was forced to undergo gender testing after widespread incredulity over her improved times in both the 1,500m and 800m and her victory in the latter distance at that year's World Championships.
She was cleared, but the incident shone a spotlight on the sensitive question of gender in athletics.
The question could rear its head once again at this year's Olympics, which will be the first since the IOC relaxed regulations on transgender athletes. Female-to-male athletes can compete "without restriction", while male-to-female athletes must undergo hormone therapy.
Under the previous guidelines, approved in 2003, athletes who transitioned from male to female or vice versa were required to have reassignment surgery followed by at least two years of hormone therapy in order to be eligible to compete.
Former IOC medical commission chairman Arne Ljungqvist, who was among the experts involved in drafting the new guidelines, said the consensus was driven by social and political changes.
"It has become much more of a social issue than in the past," he told Associated Press. "We had to review and look into this from a new angle. We needed to adapt to the modern legislation around the world. We felt we cannot impose a surgery if that is no longer a legal requirement."
"Those cases are very few, but we had to answer the question," he added. “It is an adaptation to a human rights issue. This is an important matter. It's a trend of being more flexible and more liberal."
Some have argued that the new regulations actually disadvantage those who were born female. In an article for the National Review, Paul Crookston says it is "simply inconceivable that athletes who have spent most of their lives as men wouldn't have greater muscle mass, skeletal growth and lung capacity than someone born female even after undergoing the hormone treatments the IOC requires, which halt male development without entirely reversing it."
If Semenya wins the 800m in Rio, "I think we're going to be seeing another public reaction to her accomplishments," Myron Genel, a Yale paediatric endocrinologist, told website FiveThirtyEight.
Semenya is not the first athlete to be questioned over her gender, here are four more examples of athletes who have found themselves under scrutiny.