Trans-Inclusion in Sports: History, Scientific Evidence and Future Directions.

Sport has traditionally been organised into male and female categories, but the increasing visibility of transgender athletes has prompted sporting organisations worldwide to develop relevant eligibility regulations. These policies range from hormone thresholds and open categories to puberty-based exclusions and blanket bans, yet they are rarely based on direct scientific evidence. This review aims to examine the historical evolution of transgender inclusion in sport, map current international regulations and evaluate the evidence underpinning these approaches. First, we outline the temporal evolution of eligibility frameworks. We then review the roles of sex hormones in performance-relevant systems and assess how these mechanisms may result in physiological changes in transgender athletes receiving testosterone- or estradiol-based gender affirming hormone therapy. Studies demonstrate that gender affirming hormone therapy induces physiological changes that may translate to measures of physical fitness, but are limited by cross-sectional designs, small cohorts, short follow-up periods, heterogeneous treatment regimens and outcome measures that are rarely sport specific. Therefore, the certainty of this evidence remains low, and existing data are insufficient to support uniform policies across all disciplines. We therefore recommend that fairness and safety considerations should not be resolved through biology alone and propose a structured, sport-specific framework for policy development and revision under uncertainty.