On February 5, 2025, United States (U.S.) president Donald Trump signed an executive order titled, “Keeping Men Out of Women’s Sports.” The order established, as U.S. policy, that men are not permitted to compete in women’s sports and that government funds will be rescinded from educational institutes that allow men to compete in women’s sports.
Trump’s order has come after years of debate on whether men who identify as women should be permitted to compete in women’s sports. Over this time, organizations like the International Olympic Committee (IOC) refused to set firm sex-based sports categories, culminating in contradictory and unscientific guidelines, including the “no presumption of advantage” clause. Academics in departments of medicine, philosophy, public affairs, and exercise and sports science were also happy to ride along on this sports “inclusivity” wave, and many have yet to come down from it. The debate rages on.
One argument that is sometimes put forward in advocating for prepubertal boys to be allowed to participate in girls’ sports, and for boys who have been administered puberty blocking hormones to be permitted to participate in women’s sports as adults, is that prior to puberty there is no sex difference in physical performance. For example, Joshua Safer of the Mount Sinai Center for Transgender Medicine and the Division of Endocrinology in the School of Medicine at Mount Sinai, posited in a paper published in the Journal of the Endocrine Society that puberty-blocking hormones would prevent males from obtaining their puberty-inducing physical advantages over females, because “prior to puberty there are no measurable athletic differences between boys and girls.” Safer supported this claim by referencing a 2015 Norwegian study of 11-year-old athletes which found a 1-5% difference in sprint and jump performance that favoured boys. Safer deemed these small prepubertal differences in performance inconsequential, though they could mean the difference between finishing positions in close events. Putting aside the serious ethical and medical implications of administering puberty-blocking hormones to children, the claim that there is no sex difference in athletic performance prior to puberty is not well supported by the research literature, and recent studies by Greg Brown of the University of Nebraska at Kearney challenge this claim.
Brown examined finishing scores from boys and girls who competed in the U.S. Track and Field National Youth Outdoor Championships and the National Junior Olympic Championships. Brown found that boys in the 8 and under and 9-10-year-old age groups outperformed the girls in many events. Moreover, the magnitude of the sex difference was event-specific. For the 100-m and 200-m sprints, which rely primarily on lower-limb strength and power, the boys outperformed the girls by 3–6%. However, for the shot put and javelin throw, which rely more heavily on upper-limb strength and power, the boys outperformed the girls by 7–33%.
Results from our two recent meta-analyses on childhood sex differences in muscle strength support Brown’s findings.
In our first study, we examined sex differences in grip strength from birth to age 16. We gathered every study that we could find that compared boys’ and girls’ grip strength. We found 169 studies. They were conducted in 45 countries between 1961 and 2023 and included over 175,000 boys and over 175,000 girls.
We found that boys were stronger than girls at all ages, though the results in children younger than three years old should be interpreted with caution due to the limited and heterogenous data available and because the sex difference at 1-2 years old was not statistically significant. Nevertheless, a clear, statistically significant sex difference in grip strength was present at age 3, and that difference remained constant until age 11, with girls having about 90% of boys’ grip strength. At age 11, boys were still stronger than girls, but the difference in grip strength narrowed for that year, presumably because girls reached puberty earlier than boys. Then, from age 12, the difference in grip strength started to widen considerably. By age 16, girls had about 65% of boys’ grip strength.
Some researchers have posited that the reason that prepubertal boys outperform prepubertal girls on tests of muscle fitness is because boys and girls are socialized differently. Thus, prepubertal sex differences in muscle fitness could “easily be eliminated if girls and boys were treated similarly.” In our study, we indirectly examined this hypothesis by comparing the size of the sex difference in grip strength across decades and countries. If socialization is the key driver behind prepubertal sex differences in muscle fitness, then we might expect the size of that difference to change over time, with increased opportunities for girls’ sport in more recent decades. We might also expect the size of the sex difference in strength to vary between countries because countries will differ in their cultural practices and physical activity and sports policies.
Our results did not support the socialization hypothesis. We found that the size of the sex difference in grip strength in children and adolescents was broadly similar across time and place. Thus, biological factors, such as sex differences in body height, body mass, muscle mass, and fat mass likely explain most of the prepubertal sex difference in grip strength.
In our second study, we examined childhood and adolescent sex differences in all strength tests other than grip strength. We compartmentalized these tests into upper-limb strength tests and lower-limb strength tests. We did this because sex differences in strength in adults are muscle-specific. For example, in lower-limb muscles, women have 60-70% of adult male strength. In upper-limb muscles, women have only 50-60% of adult male strength.
We aggregated data from 34 studies. The studies were conducted in 10 countries and across several decades and included a total of 3,497 boys and 3,137 girls who were 5-17 years old.
Again, we found statistically significant sex differences in muscle strength at all ages. Similar to adults, the size of the sex difference in strength was greater in the upper limbs than the lower limbs. In 5-10-year-olds, the sex difference in lower-limb strength was 8%, whereas it was 17% in upper-limb strength. In 14-17-year-olds, the sex difference in lower-limb strength was 30%, whereas it was 50% in upper-limb strength. These limb-specific sex differences in strength are also consistent with Greg Brown’s studies which showed that the greatest sex differences in athletic performance are observed in events that rely heavily on upper-limb strength.
To conclude, our meta-analyses revealed that muscle strength is greater in boys than girls prior to puberty, that the difference increases markedly with male puberty, that the difference is greater in the upper- than lower-limb muscles, and that the difference has been mostly consistent across time and place. As muscle strength is a key determinant of athletic performance, our results help to explain how prepubertal boys outperform prepubertal girls in some athletic events. Moreover, as the claim that there are no prepubertal sex differences in physical performance has been used to argue that puberty blockers administered to boys will eliminate the future performance advantages of those boys as men, our results also make this argument untenable. Further, our results challenge the hypothesis that prepubertal sex differences in muscle fitness are due to socialization. Instead, our results, considered in light of many other studies in human anatomy and physiology, point to a largely biological origin of prepubertal and postpubertal sex differences in muscle strength and athletic performance.
Finally, our results support policies of two separate sports categories based on sex. The inclusion of men who identify as women in women’s sports is unfair to women and creates unnecessary safety concerns. Moreover, the inclusion of men who identify as women in women’s sports does not have large public support. Polls conducted by Pew, Gallup, and YouGov have all documented that the majority individuals do not support such a policy.
None of this means that transgender individuals should be excluded from sports. Instead, it means that transgender individuals can compete in the category of sport that matches their birth sex, or, sports organizations can create sports where transgender individuals compete in a transgender category, such has been done for athletes who have physical disabilities (i.e., para-athletics) or intellectual disabilities (i.e., Special Olympics).
