TO OUR READERS: The following editorial was written in May 2022. Please see an added final paragraph for a brief June update on this rapidly evolving situation.

“I can’t support this” (Cynthia Millen, retired USA Swimming Official).¹ “We need a way forward before the count of fractured young women, whose sporting dreams have been smashed by losing in an unfair competition they cannot win, rises to an unacceptable level” (Nicole Cooke; 2008 Olympic gold medalist).² These statements seem to indicate an issue, although not totally new, currently generating strong reactions in sport communities, politics, and the public.

The issue is competitive sport and the consequences of gender dysphoria (GD). GD is defined as a marked incongruence between an individual’s experienced and/or expressed gender and assigned gender of at least 6 months’ duration. GD is a clear distinction to disorders or differences of sex development (DSD), a category that includes a wide range of conditions where sexual anatomy and/or chromosomal or hormonal patterns do not fit the strictly binary definitions of male and female. Depending on age, the estimated prevalence of GD is 0.2% to 0.6%³; the real figure is probably much higher. GD may be associated with clinical distress and impairment in social, occupational, or other areas of daily life⁴ and comes along with significantly increased risk for suicide.^3,5 Consequently, GD triggers a desire for congruence of physical and sexual characteristics and experienced gender. The median age at which individuals undergo such transition to transgender female or male is ∼42 years.³ The ratio of transgender women to transgender men is ∼3 to 1. The transition process includes options of hormonal intervention partly combined with surgery. Most transgender adults report experience of GD already at the changeover from primary to secondary school. In recent years the number of referrals to child and adolescent gender identity clinics increased substantially.³ Given the number of people concerned, inclusion of transgender people in competitive sport is gathering increasing attention.

Historically, competitive sport, depending on the discipline, is either accessible for both genders or organized for females and males only. The female category has been implemented because in selected events adult males have substantial performance advantages over females due to biological differences. Corresponding rules and attitudes are fluent. For example, in Germany women’s boxing, first promoted in 1911, was officially prohibited in 1921 (amateurs) and 1925 (professionals) before being nationally and internationally reestablished in 1994. In noncontact sports such as, for example, track and field, women had and still have a limited portfolio of events compared with men. It took until 1967 when K.V. Switzer became the first female marathon contender at the Boston Marathon. However, when identified as Kathrine during the run she was confronted with numerous attempts to physically remove her from the race. Up to 1972, when the 1500-m distance was included, the longest Olympic track event for females was 800 m, and it took another 36 years until most Olympic track and field events (some modified) became available for female athletes. Female health had been put forward for selected exclusions—exclusions that didn’t stand the test of time due to lack of evidence.

Biological performance-determinant factors favoring men are higher lean body and muscle mass, bone length, pelvis height-to-width ratio, bone and tendon properties, oxygen transport via respiratory tract, cardiac output blood volume, and transport capacity per liter in males than in females. The extent of such differences may differ depending on performance level and sport. Puberty is associated with ∼20-fold increases in testosterone in boys, but comparably minor changes in girls, and it counts as the decisive period for the above sexual distinction. This is clearly a valid point but surely not the whole story, as also seen in the substantial variance of expressions of DSD. Distinction between female and male attributes begins already in utero. Higher testosterone levels in males than in females age 1 to 6 months, termed minipuberty, may induce higher growth velocity and effects on body mass index and body weight. Six- to nine-year-olds show performance-relevant anthropometric and physiological sex-related differences.⁶ Complete development of sex-specific performance-determining factors may require up to several years into adult high-performance training.

The statements of Cynthia Millen and Nicole Cooke concern transgender women in high-performance sport. On March 17, 2022, the transgender female swimmer Lia Thomas won the 500-yard event of the Division I of the National Collegiate Athletic Association (NCAA) swimming championships. She came in 1.75 seconds ahead of Emma Weyant, silver medalist in the 400-m women’s medley at the 2020 Tokyo Olympics. Up to 2019, Lia (then known as Will Thomas) was a long-distance swimmer on the University of Pennsylvania men’s swimming team but now has ambitions for the 2024 Summer Olympic trials in the women’s category. Emily Bridges, who set a national 25-mile junior men’s record as Zach Bridges and then selected to join the men’s British Cycling senior academy in 2019, was set to compete in the women’s National Omnium event in April 2022. Shortly before the event she was withdrawn because at national championship events international ranking points are allocated. This requires eligibility to race in international competitions, which at the time had not been approved by the Union Cycliste Internationale (UCI).

Both Lia Thomas and Emily Bridges had undergone gender-affirming medical interventions (GAMI) reducing testosterone levels according to currently adopted regulations. The direction of corresponding effects on sex-dependent physiological performance-limiting factors is well known. However, there is still a substantial lack of knowledge with special respect to the extent and dynamics of complex specifics of athletic development.⁶ Cycling and swimming performances are highly dependent on physique. For example, larger cyclists minimize the energy costs to aerodynamic friction per body mass, a significant advantage on level roads and for longer track cycling events. Swimmers impose by heights far beyond the 50% percentile in females and males, respectively. Postpuberty, height surely remains unaffected by GAMI. Emily Bridges (self-reported height: 6 ft 2 in., or 1.88 m)⁷ counts as tall in men’s cycling and strikingly impressive in the women’s category. While average to below average height in men’s swimming, Lia Thomas is among the tallest competitors in women’s swimming.

These rather simple examples of height may emphasize that the current short-cut in terms of the focus solely on testosterone levels combined with given periods of GAMI may fall short in setting fairness for trans- and cis-gender female athletes. Issues like age of transition, pharmacologic delay of puberty pre-GAMI, puberty status and/or athletic age related to maturation at start of GAMI, and possibly numerous other aspects are largely unknown. Therefore, current regulations in transgender sports are surely insufficient, and the potential threat of unfairness in competitive sports is likely to destroy sporting careers of talented cis-gender girls and disrupt funding policies in high-performance development and possibly team strategies in the female category.

Of course, like cis-gender athletes, it is appropriate that trans-gender athletes have the right to participate in competitive sport. However, do we need a transgender women category in competitive sport? Do complex specifics of genetic determination and hormonal modulation of athletic development require even more precise guidelines? Comprehensive research in sport physiology and performance should provide scientific evidence for fair regulation.

UPDATES AS OF LATE JUNE 2022: First decisions likely to trigger evident demand of a transgender women’s category in competitive sport have been made. With the 19th Fédération Internationale de Natation (FINA) World Championships Budapest 2022, FINA established new regulations that male-to-female transgender athletes (transgender women) and athletes with 46 XY DSD whose legal gender and/or gender identity is female are eligible to compete in the women’s category if they have not experienced any part of male puberty beyond Tanner Stage 2 or before age 12, whichever is later, evidenced by having (1) complete androgen insensitivity or (2) suppression of male puberty beginning at Tanner Stage 2 or before age 12, whichever is later, with continuously maintained serum (or plasma) testosterone levels below 2.5 nmol·L^–1. UCI sharpened their regulations, increasing the period of GAMI from 12 to 24 months plus lowering maximum testosterone level from 5 to 2.5 nmol·L^–1. World Rugby decided that transgender women may not currently play women’s rugby. World Athletics (formerly IAAF) President Coe indicated that the governing body will revise its current regulations, possibly in direction of FINA’s policy, and the Fédération Internationale de Football Association (FIFA) reported that they are working on revised regulations.