Who may compete in the female category in sport? Chromosomes, genes, hormones, and psychosocial/cultural aspects.

PURPOSE OF REVIEW: Throughout history, competitive sport has been categorized by sex. Since the 1930s, sporting organizations have implemented various methods of confirming sex for competition in the female category. These policies primarily impact individuals with differences of sex development (DSD), whose biology falls outside the binary of male/female, and individuals who identify as transgender, whose identity differs from their sex at birth. This review will cover the dimensions of biologic sex, the history of sex verification in sport, the role of testosterone, and review data for athletes who have DSD or are transgender.

RECENT FINDINGS: Several recent studies, reviews, and policy statements demonstrate that testosterone and its rise during masculinizing puberty are key determinants of differences in athletic performance between men and women. Recent studies in adult transgender women who went through a masculinizing puberty show that after 2 years of estradiol and antiandrogen therapy, run times are similar to cisgender women, but they still retrain strength advantages. Data are sparse for individuals who had masculinizing puberty blocked and also for individuals who have a DSD.

SUMMARY: Sex is complex and multidimensional, and there are shortcomings with any method of sex verification in sport. Testosterone remains a key determinant of differences in athletic performance by sex.