Those who oppose trans people participating in sports as people of their gender rather than whatever gender they were labeled as an infant, have a few typical arguments. One such argument is that a person is forever their so-called “biological sex” regardless of whatever physiological changes may occur or overlaps may exist. This argument is ultimately an ideological one, not open to persuasion unless the notion of “biological sex” itself can be subverted - which it should be.
A second argument is that trans girls and trans women have physical advantages over cisgender girls and cisgender women which persist regardless whatever other changes may come from transitioning.
This is false in three ways:
First, trivially and most often conceded, there is no modal physical difference and no modal performance difference between boys and girls, cis or trans, beyond genital structure itself, prior to Tanner stage 2 of puberty. Following from this trivial point, trans girls who forgo testosterone puberty through the action of puberty blockers and/or hormone therapy develop through puberty indistinguishably from their cisgender girl peers.
The second way is that, that sex associated characteristics are bimodal, most of which have a larger range of overlap than the separation between the modes, and second, that physical changes associated with transition do reduce performance capacity from any so-called male advantage - regardless of any permanent changes arising from a testosterone based puberty prior to adulthood.
The third is that, trans women and trans girls who have passed through some amount of testosterone puberty, and further have undergone hormone therapy for some length of time, accumulate a collection of biological effects which reduce their performance levels to the realm of cisgender women and girls with comparable levels of fitness and training.
Kirsti Miller wrote this well sourced Medium article published November 11, 2021, titled: There Is No Clear List of Features That Allow Us To Even Remotely Cleanly Separate Men From Women.
Following-up. Research from 2014, following hundreds of cis-gender athletes, found that 13.7% of the women's hormone levels were higher than the standard women's range and overlapped the men, while 16.5% of the men had levels below the normal male range, overlapping with the women. M. L. Healy, J. Gibney, C. Pentecost, M. J. Wheeler, P. H. Sonksen, “Endocrine profiles in 693 elite athletes in the postcompetition setting”, Clinical Endocrinology, March 5 2014.
Among youths aged 12 years or older, there was no overlap of the interquartile range of testosterone between male and female youths. After cessation of the age-related increase in testosterone for female youths (at 14 years), there was an intersection of testosterone concentration distributions between the lowest (first) percentile of male youths and the uppermost (99th) percentile of female youths (≥100 ng/dL), which includes 8 of 949 samples (<1%) for female youths. Jonathon W. Senefeld, PhD; Doriane Lambelet Coleman, JD; Patrick W. Johnson; et al “Divergence in Timing and Magnitude of Testosterone Levels Between Male and Female Youths”, JAMA, July 7, 2020.
In 2024, the International Olympic Committee published a preliminary study comparing trans women who train at least three times a week, who had undergone at least one year of hormone therapy, perform less well than their cis women athletic peers across several metrics. This article by Katelyn Burns summarizes the results and the IOC's recommendations.
In late 2022, the Canadian Centre for Ethics in Sport (CCES), published an eighty-six page report on the results of a study titled: Transgender Women Athletes and Elite Sport: A Scientific Review
The study is report is summarized here Trans women athletes have no unfair advantage under current rules, report finds, Cycling Weekly, Anne-Marije Rook, January 25, 2023
Lia Thomas is a trans woman who competed in college, as a woman and attracted undue notoriety for her gender identity when she had some successes.
On the Translucent blog, Paul explores answers to the question: Are Trans Women Competing Fairly In Sport? citing Lia’s case as an example from January 1, 2021.
Space for the next bullet point
Chris Mosier’s site TransAthlete.com, lists the inclusion and exclusion policies across various sports and jurisdictions in The United States and Canada, as well as providing some additional resources on trans athlete performance and advocacy.
Julia Serano's essay Trans People and Sports: Everything You Need to Know
See also: My page on Trans Sports Bans