Rethinking sex in sport: biology, performance and policy in DSD and transgender athletes

Competitive sport has long been organized around a binary concept of biological sex–male and female–and that concept has served as the foundation for athletic categories, eligibility rules and policy at every level of competition. As research regarding sex determination has grown, the biology behind defining biological sex in humans has revealed that it is not a single variable but rather a complex interacting system of gene expression, hormones, developmental anatomy, and receptor sensitivity that guides development of the multifaceted sexual pathway. This interplay is subject to natural variation from historically typical binary sex and has challenged the adequacy of current policy surrounding the inclusion of transgender athletes and those with biological variations in competitive sport. This Opinion article examines the junction of biology, athletic performance, and policy as it applies to two distinct human physiologies with the conclusion that policies for transgender athletes and athletes with differences in sex development (DSDs) should be considered separately. It is important to note that this discussion focuses exclusively, and separately, on females with DSDs and on transgender women (those who identify as female but were assigned male at birth) regarding their eligibility for women’s sports. The eligibility of transgender men in male sport is far less contested and, therefore, is outside the scope of this consideration.