Mary I. O’Connor MD, Anne Simpson, Carol Brown, Jan Palchikoff, Valerie McClain, Patricia Spratlen Etem MPH
ICONS Rowing
National governing bodies of sports exist to create rules for their sport. These rules should, first and foremost, promote integrity of competition. This article will focus on the biology of sex and its relationship to athletic performance. An understanding of the basic biology of sex is essential to appreciate how policies which permit boys/men (males) who identify as girls/women to compete in the girls/women’s categories are blatantly discriminatory to girls/women (females).
Every cell has a sex and sex dominates how bodies develop
Each human is born with a sex. Sex is genetic. In scientific terms, males have small reproductive cells or gametes (sperm), and females have large gametes (eggs). The presence of large or small reproductive cells is driven by sex chromosomes, XY for males and XX for females, as well as other genes.
Each cell in our body has a sex. While sex hormones such as testosterone and estrogen are well known to strongly influence the body, it is critical to appreciate that there are sex differences completely unrelated to hormones. Male embryos grow faster than female embryos. Some diseases affect males and females differently in ways that cannot be explained by sex hormones. As concluded by the National Institutes of Medicine, “Thus, it is clear that not every difference observed between male and female cells can be attributed to differences in exposure to sex hormones.”1
Sex differences between males and females begin during development in the womb and continue throughout the lifespan. Sex differences which impact athletic performance occur even prior to puberty. Measurement of cardiovascular capacity in pre-puberty school children show that VO2 max is consistently higher in boys than girls, attributed to the ability of a boy’s heart to pump more blood with each heartbeat.2 Physical capabilities studies in elementary children show boys outperform girls in aerobic fitness, strength, speed, and agility; girls outperform boys only in balance and flexibility.3 Boys have a performance advantage in throwing activities compared to girls,4 and girls have poorer hand-eye coordination.5
These physiologic differences that drive athletic performance explode with puberty. In male puberty, circulating testosterone rises to 30 times pre-puberty levels with post-puberty levels being 15-20-fold greater than females of any age.6 Boys undergo significant physiologic changes relative to girls: even greater height, leaner body mass, greater muscle mass, greater muscle strength, larger lungs, bigger airways, and greater cardiac capacity.7
In the absence of pathology (namely disease or abnormal development), testosterone levels never overlap between males and females.7 In healthy males, 95% have circulating testosterone concentrations of 7.7 to 29.4 nmol/l. In healthy premenopausal females, 95% have circulating testosterone concentrations of 0 to 1.7 nmol/l. This is a critical fact in assessing fairness in gender identity sports policies. A study by Healy et al.8 which claimed overlap of testosterone levels between female and male elite athletes has been widely discredited at the highest scientific levels and the Court of Arbitration for Sports for several reasons, one being the lack of genetic testing of the athletes to determine sex.9 This flawed study continues to be pulled into discussions in media, law, and sport to incorrectly suggest there is no clear delineation in testosterone levels between males and females or that testosterone thresholds are meaningless. This study should never be used in the discussion of policy related to transgender athletes.
Rarely, a human is born with a disorder of sexual development (DSD) in which an individual does not have the normal genetics of male or female. It is estimated that DSD occurs in 1 in 5000 live births.10 The DSD most relevant to the issue of fairness for females in sports is complete androgen insensitivity syndrome (CAIS). This individual is genetically male but with cells that do not respond to testosterone and testicles which remain (undescended) in the abdomen. As a newborn this baby appears to be a girl with a vagina and labia, and without a penis or external testes. This disorder is rare, affecting 2-5 per 100,000 live births.11 CAIS is the only DSD in which a genetic male would not have the significant physiologic advantage of male puberty, although non-testosterone related male advantage is retained. Discussion remains ongoing as to the appropriate policy related to competition by CAIS athletes.
Sex is the single most important determinant of athletic performance
Genetic sex is the dominant factor in the development of a body’s capacity for athletic performance.12 The performance gap between males and females becomes most significant at puberty and ranges from 10-50% depending on the sport.7
Testosterone suppression does not level the playing field
After one year of testosterone suppression for males there are some physiologic changes.7 Thigh muscle mass is reduced by 9% but still remains 16% higher compared to females. Reduction in muscle strength is typically 5%. Only hemoglobin levels (the protein that carries oxygen in the blood) equalizes at one year. In the longest follow-up study published, males with 14 years of testosterone suppression remain 20% stronger and have 20% greater heart and lung capacity than females.13,14 Male advantage is not erased even with over a decade of testosterone suppression.
The persistence of male advantage in trans athletes is further supported by research on how exercise reduces the effects of testosterone suppression. Men who have testosterone completely removed from their system for treatment of prostate cancer (androgen deprivation therapy) experience a loss of muscle strength and muscle mass. However, in what is well studied in the medical field, low- to moderate-intensity resistance and aerobic training will increase muscle strength in these men, although no difference was identified in muscle mass.15
Transgender individuals cannot become the opposite sex
Individuals are born with genetics that cannot be changed. Sex is immutable. Medical therapy to suppress testosterone will impact the body but does not make a male into a female or level the athletic playing field. Puberty blocking medications in boys will limit the growth of the penis and scrotum, decrease the growth of facial and body hair, and prevent deepening of the voice.16 However, puberty blocking medications will not change the genetics of that individual or eliminate the physiologic advantages of being male which are unrelated to testosterone. Scientific data is not yet available on the athletic performance capacity for individuals who as young boys were treated with puberty blockers compared to girls and women.
Fairness in competition
The science is brutally clear: no amount or duration of testosterone suppression can erase the advantage of being born male. And while many females can outperform males, at any matched level all males outperform all females.12
While some may argue that the number of male athletes who identify as women (with or without testosterone suppression) is small, that does not negate the fact that fairness in sports is compromised by inclusion of these individuals in girls’ and women’s categories. As concluded by an independent think tank group in the United Kingdom, “governing bodies cannot secure both transgender inclusion and fairness for female athletes within the female category.”17
In January 2023, World Athletics evolved its transgender policy based on the science of athletic performance. Prior policy permitted males who suppressed testosterone over a specified period of time to compete as women. The new policy prohibits athletes who have gone through puberty as a male from participating in female world ranking competitions. Maintaining fairness for female athletes “above all other considerations” was the driving principle behind the policy change.18
Unfortunately many sports still permit males, with or without testosterone suppression, to compete in the girls’ and women’s categories. Without question, this is blatantly discriminatory to females and destroys fairness in competition for them. Inclusion of males in women’s sports tells girls and women, unlike boys and men, that they are not deserving of fair competition. Such policies devalue girls and women compared to boys and men. We urge all National Governing Bodies, who have not already done so, to restore fairness for females by restricting the girls’ and women’s categories based on sex. Males who identify as girls or women can compete in a male/open category.
We want all children and adults to enjoy the benefits of sports. Our approach supports both inclusion for all to participate and fairness in competition for females. Join us as we fight for fairness for girls and women in sport at www.iconswomen.com
References
1. Institute of Medicine (US) Committee on Understanding the Biology of Sex and Gender Differences; Wizemann TM, Pardue ML, editors. Exploring the Biological Contributions to Human Health: Does Sex Matter? Washington (DC): National Academies Press (US); 2001. 2, Every Cell Has a Sex. Available from: https://www.ncbi.nlm.nih.gov/books/NBK222291/
2. Eiberg S, Hasselstrom H, Grønfeldt V, Froberg K, Svensson J, Andersen LB. Maximum oxygen uptake and objectively measured physical activity in Danish children 6-7 years of age: the Copenhagen school child intervention study. Br J Sports Med. 2005 Oct;39(10):725-30. doi: 10.1136/bjsm.2004.015230. PMID: 16183768; PMCID: PMC1725036.
3. Marta CC, Marinho DA, Barbosa TM, Izquierdo M, Marques MC. Physical fitness differences between prepubescent boys and girls. J Strength Cond Res. 2012 Jul;26(7):1756-66. doi: 10.1519/JSC.0b013e31825bb4aa. PMID: 22561975.
4. Thomas JR, French KE. Gender differences across age in motor performance a meta-analysis. Psychol Bull. 1985 Sep;98(2):260-82. PMID: 3901062.
5. Telford RM, Telford RD, Olive LS, Cochrane T, Davey R (2016) Why Are Girls Less Physically Active than Boys? Findings from the LOOK Longitudinal Study. PLoS ONE 11(3): e0150041. https://doi.org/10.1371/journal.pone.0150041
6. Handelsman DJ, Hirschberg AL, Bermon S. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocr Rev. 2018 Oct 1;39(5):803-829. doi: 10.1210/er.2018-00020. PMID: 30010735; PMCID: PMC6391653.
7. Hilton EN, Lundberg TR. Transgender Women in the Female Category of Sport: Perspectives on Testosterone Suppression and Performance Advantage. Sports Med. 2021 Feb;51(2):199-214. doi: 10.1007/s40279-020-01389-3. Erratum in: Sports Med. 2021 Oct;51(10):2235. PMID: 33289906; PMCID: PMC7846503.
8. Healy ML, Gibney J, Pentecost C, Wheeler MJ, Sonksen PH. Endocrine profiles in 693 elite athletes in the postcompetition setting. Clin Endocrinol (Oxf). 2014 Aug;81(2):294-305. doi: 10.1111/cen.12445. Epub 2014 Apr 2. PMID: 24593684.
9. Ritzén M, Ljungqvist A, Budgett R, Garnier PY, Bermon S, Lindén-Hirschberg A, Vilain E, Martínez-Patiño MJ. The regulations about eligibility for women with hyperandrogenism to compete in women’s category are well founded. A rebuttal to the conclusions by Healy et al. Clin Endocrinol (Oxf). 2015 Feb;82(2):307-8. doi: 10.1111/cen.12531. Epub 2014 Jul 25. PMID: 24954211.
10. Walia R, Singla M, Vaiphei K, Kumar S, Bhansali A. Disorders of sex development: a study of 194 cases. Endocr Connect. 2018 Feb;7(2):364-371. doi: 10.1530/EC-18-0022. Epub 2018 Jan 31. PMID: 29386228; PMCID: PMC5825923.
11. https://medlineplus.gov/genetics/condition/androgen-insensitivity-syndrome/
12. Ross Tucker, PhD: https://www.youtube.com/watch?v=69WIe-ENDAg
13. Alvares LAM, Santos MR, Souza FR, Santos LM, Mendonça BB, Costa EMF, Alves MJNN, Domenice S. Cardiopulmonary capacity and muscle strength in transgender women on long-term gender-affirming hormone therapy: a cross-sectional study. Br J Sports Med. 2022 Nov;56(22):1292-1298. doi: 10.1136/bjsports-2021-105400. Epub 2022 Oct 4. PMID: 36195433.
15. Chen Z, Zhang Y, Lu C, Zeng H, Schumann M and Cheng S (2019) Corrigendum: Supervised Physical Training Enhances Muscle Strength but Not Muscle Mass in Prostate Cancer Patients Undergoing Androgen Deprivation Therapy: A Systematic Review and Meta-Analysis. Front. Physiol. 10:1126. doi: 10.3389/fphys.2019.01126