In the span of five days in late January and early February, two research teams on two continents published findings that undercut the central empirical claim behind every major ban on transgender women in sports: that prior testosterone exposure confers permanent physical advantages hormone therapy cannot erase. The first, a longitudinal brain imaging study out of Germany, tracked structural changes in 64 trans participants over six months of gender-affirming hormone therapy (GAHT) and found receptor-driven cortical remodeling—the brain physically reorganizing in response to hormones. The second, the largest meta-analysis ever conducted on transgender athletes, reviewed 52 studies and 6,485 participants and found no statistically significant differences in strength or aerobic capacity between trans women and cis women after one to three years of hormone therapy.
Both landed the same week the NCAA’s blanket ban on transgender women athletes turned one year old. The ban covers more than 530,000 student-athletes. When the NCAA’s president, Charlie Baker, was asked by Congress in December 2024 how many of those athletes were transgender, he said he was aware of “fewer than 10.” He called the ban “clarity.”
The brain study
The Hüpen team used surface-based and voxel-based morphometry—imaging techniques that measure the thickness, folding, and depth of the brain’s outer layer—to track what happened to cortical architecture over six months of GAHT. They found distinct, divergent changes in cortical thickness: localized thickening in trans men, widespread thinning in trans women, with both groups converging in the occipital cortex. These spatial patterns aligned with maps of where sex hormone receptors are most densely concentrated in the brain, suggesting a receptor-mediated mechanism—meaning the brain was responding to hormone therapy through its own existing receptor infrastructure, in region-specific patterns.
Improved psychological well-being paralleled the neurobiological changes.
There are some caveats to these findings. As of now, the study has not been peer-reviewed. A sample of 64 trans participants is large for neuroimaging—most prior longitudinal studies in this area have worked with far smaller cohorts—but not definitive. Six months is enough to detect cortical changes, not enough to map their full trajectory. The finding is consistent with prior literature on estrogen’s effects on brain morphology, but questions about long-term outcomes remain open.
The sports study
The same week, a different set of researchers was looking at a different question: whether hormone therapy changes what the body can do.
The Sieczkowska meta-analysis in the BJSM reviewed 52 studies covering 2,943 trans women, 2,309 trans men, 568 cis women, and 665 cis men, ages 14 to 41. After one to three years of hormone therapy, trans women showed no significant differences from cis women in upper-body strength, lower-body strength, or maximal oxygen consumption (VO₂ max)—the standard measure of aerobic fitness. Trans women did retain higher absolute lean body mass than cis women. That difference did not translate into superior functional performance.
The evidence has real limitations. The studies reviewed were mostly low-certainty and varied in quality. Only 16 of 52 included any physical activity assessment. Only seven controlled for confounding factors like training history, diet, and baseline fitness. Most participants were not competitive athletes. There is essentially no data on elite-level trans athletes.
Prof. Alun Williams of Manchester Metropolitan University—who has received International Olympic Committee research funding and has published findings reaching different conclusions—offered a critique in the Science Media Centre’s expert roundup: the longitudinal data does not show convincing evidence that hormonal treatment eliminates the known performance gap between men and women. Strength measurements depend on voluntary effort, and none of the reviewed studies assessed effort directly. Hormonal treatment after puberty does not change skeletal dimensions like height, limb length, or shoulder width.
The absence of data on elite trans athletes does not demonstrate that elite trans athletes have an advantage. It demonstrates that the sample barely exists. As the BJSM authors note: “Transgender women remain under-represented in elite athletics, highlighting the lack of dominance by transgender athletes in practice.”
The distinction between “finding no evidence of advantage” and “proving no advantage exists” is real. It is also the distinction that separates “more research” from “blanket ban.”
The policy
The bans preceded both studies by a year. On February 5, 2025, President Trump signed Executive Order 14201, titled “Keeping Men Out of Women’s Sports,” directing federal agencies to withhold funding from educational institutions that allow transgender women to compete on women’s teams. The NCAA changed its participation policy the following day—not after a review period, not after consulting the evidence base. The following day. Baker praised the order for providing “a clear, national standard.”
That standard applied to fewer than 10 known transgender athletes out of 530,000. For context: the NCAA has more student-athletes named “Jake” than it has transgender women competing in women’s sports. Baker called the situation a matter requiring immediate national clarity. He did so within 24 hours of being told to by the president.
Since then, according to the Movement Advancement Project, more than two dozen states have enacted bans on transgender athletes in school sports. In November 2025, USA Hockey quietly approved a new Participant Eligibility Policy—adopted in response to directives from the U.S. Olympic and Paralympic Committee, which had itself aligned with Trump’s executive order—barring trans athletes from all sex-restricted programs. The ban extends to adult recreational leagues. It takes effect April 1, 2026.
Harrison Browne—the first openly transgender professional hockey player, who competed in the National Women’s Hockey League from 2015 to 2018 and came out as a trans man in 2016—told reporters: “USA Hockey is basically telling me I can’t play in a recreational league with friends that I played my entire career with.”
Under the new USA Hockey rules, trans men who have begun testosterone therapy cannot compete in women’s leagues (because they’ve taken testosterone) or in men’s leagues (which Browne described as unsafe for gender non-conforming individuals). The co-ed option remains. The sex-restricted option—every sex-restricted option—does not.
What the evidence says about what the policy says
The blanket bans rest on a specific empirical claim: that transgender women retain inherent physical advantages from prior testosterone exposure, advantages that hormone therapy cannot erase. The largest meta-analysis ever conducted on the question, published in a peer-reviewed journal, found that the data does not support that claim for the functional performance metrics—strength, aerobic capacity—on which the bans rely. The Hüpen brain study, if it survives peer review, further undermines the “experimental treatment” frame by documenting receptor-mediated neurobiological changes consistent with established endocrinology.
Neither study is conclusive. The BJSM evidence is mostly low-certainty. The brain study is a preprint. Williams’s critiques about skeletal dimensions and effort-dependent strength testing deserve engagement, not dismissal. The scientifically honest position is that the question is not fully resolved.
The policy position is that the question is settled. It was settled on February 6, 2025, the day after the executive order, when Baker called it “clarity.” It was settled for a population he estimated at fewer than 10 people.
Bruno Gualano, the lead senior author of the BJSM study, told El País: “Most of these policies are based on the assumption that transgender women retain inherent physical advantages and would therefore dominate women’s competitions. The data does not support this idea.”
On April 1, USA Hockey’s policy takes effect. Harrison Browne will be ineligible for women’s recreational leagues and men’s recreational leagues. Both. For beer hockey.
Great piece, Ari. Especially liked this:
`The distinction between “finding no evidence of advantage” and “proving no advantage exists” is real. It is also the distinction that separates “more research” from “blanket ban.”'
I read the BJSM review because the results were somewhat surprising. After looking at things a bit, I'm really hesitant to say that we're comparable to cis women, athletically, based on this review. The technical phrase `[n]o significant differences' is the term used in the paper that people are pointing to to say that TW and CW perform the same but it's really easy to misinterpret. It just means that, as you point out, from a mathematical standpoint the data does not indicate a difference, not that there is no difference.
So based on a short review of paper...
Bad news: Get rid of an outlier study used in the meta analysis and the review indicates that for athletic pursuits primarily involving strength, TW will have an advantage over CW. Good news: For athletic pursuits that require endurance TW's strength advantage would likely be offset by inability to use that strength (i.e., it would wane during competition). So this issue needs to be studied on a sport-by-sport basis and blanket bans are likely unfair or even unnecessary to protect/ensure fair play for CW. This is more or less inline with what trans sports researcher, and competitive runner, Joanna Harper has found.
The conclusions drawn around the variables we care about (upper-body strength, lower-body strength and VO2 max) for TW vs. CW all have a GRADE of `very low', which means `We have very little confidence in the estimate effect: The true effect is likely to be substantially different from the estimate of effect.' Those same variables for TW vs. CM have a GRADE of `moderate', which means `The true effect is likely to be close to the estimate of the effect, but there is a possibility that is substantially different'. So the data shows we can be pretty sure there's a difference between TW and CM (duh) but not that there isn't a difference between TW and CW (ugh).
From Figure 2 in the review, it seems pretty clear to me that the Alvares et al. (2025) study is an outlier that is pulling the data towards uncertainty over effect; i.e., get rid of that study and it looks like TW do have greater upper- and lower-body strength but CW have slightly better VO2 max.
That Alvares et al. study compares seven TW with eight CW, so a very small pool from which to draw conclusions. When I look at the descriptive data of the participants it's apparent that the TW are at least slightly older, smaller (height/weight), and have significantly higher estradiol levels and significantly lower testosterone than the CW. Unsurprising to most of us then that that study would show CW have greater upper- and lower-body strength compared to TW.
The anti-trans side likes to represent TW like we have the same abilities as CM and the review definitely disproves that idea, but it also seems to indicate that we're not exactly at the same level as CW either.
Really enjoying the posts!
None of this is scientific and treating people encouraging bans as if they are engaging in good-faith is naive. The "scientists" looking for justifications to discriminate are not engaging in science. It's politics looking for justifications to violate human rights. If they could get away with it, these same people would make racially segregated leagues with the exact same justifications.