Dutee Chand is an Indian sprinter who was last week cleared by the Court of Arbitration for Sport to compete as a woman. She was previously deemed ineligible based on an International Association of Athletics Federations rule that females cannot compete if their testosterone levels are significantly elevated.The court's ruling overturned her ineligibility, and also nullified the testosterone exclusion criterion. It is a case that asks some of the most challenging questions facing sport.When is a woman a woman? And how does our desire for neat, binary categorisation of the sexes affect sport?If these questions sound familiar, it's because we had front row seats to the sex verification controversy's most famous case, Caster Semenya. After winning the 800m world title, she found herself the subject of the most intense speculation imaginable. The media swarmed, politicians revealed their non-existent grasp of physiology, and Semenya became the most high-profile case in the long, challenging history of sex verification.Ethically and morally, one cannot fault the court for allowing someone who has lived her entire life as a female to compete as a female. But physiological and performance realities necessitate that the integrity of the gender boundary be defended.Testosterone controls the development of male characteristics, and many of the differences between the sexes. These characteristics include increased muscle mass, denser bones, greater strength, power and endurance - all things that give men a substantial athletic advantage.For example, the best female marathon performance in history - Paula Radcliffe's - has been bettered by 375 men this year alone. Over 5000 men in history have run faster.This is not, by the way, an argument for inequality, but difference - we can be different and remain equal. We should celebrate the women's 100m champion just as we do the men's, because within their own classification, they represent the pinnacle of speed. Blurring the boundary between male and female athletes begins to invite direct comparisons between the two, and that's where things begin to get tricky.There is considerable overlap between the sexes.Many women are taller, heavier, faster and stronger than many men. But at the elite level, this is not true - the best elite men, without exception, outperform the best elite women.Generally, there is no overlap between men's and women's testosterone levels . The highest levels in females are still many-fold lower than the lowest levels in men.Only where medical professionals identify pathology is this separation challenged - when a woman's testosterone reaches that of a man' s.The Court of Arbitration for Sport ruled there is insufficient evidence that women with elevated levels of testosterone have a performance advantage over other women. Whether this is true is questionable - women with higher than average testosterone levels are 140 times more likely to be found in elite sport, which strongly suggests an advantage.Of course, biological advantages are part of human variation. For example. Damian de Allende's size and speed set him apart to play for the Springboks.But, at some point, the testosterone levels could be "too high", and if that leads to performance advantages, then the integrity of women's sport might be affected. As of last week, there is no mechanism in place to respond to this situation, which leaves open the possibility for another major case, like that of Semenya. The next instalment could be very interesting...