What the studies show
This small, heterogeneous collection spans three questions: how sex is defined, whether transgender identity has a measurable basis in brain structure, and how sex differences bear on fairness in sport. On definition, one essay argues that in species with two gamete types there are exactly two sexes, male and female, defined by bodies organized to produce sperm or eggs, and that differences of sex development do not form a third sex (Wright, 2025). It presents an argument rather than new data, and a published critique contends that the definition presupposes its conclusion.
The "brain sex" evidence is thin. The best-known finding, that a small brain region (the BSTc) was female-typical in size in six trans women, came from postmortem tissue of people who had taken estrogen and anti-androgens for years, nearly all after castration (Zhou et al., 1995). A follow-up counted neurons in the same six brains rather than new cases (Kruijver et al., 2000), and the same group later reported that the sex difference in this region emerges only in adulthood. No independent team has replicated the finding. A 2026 reanalysis of published figures from a 2022 imaging study of 24 untreated trans women argues that their reported difference from men may not survive standard statistical corrections, although the difference from women does (Wright et al., 2026). Separately, early typology studies from one Toronto clinic reported sexual arousal to cross-dressing in 66–88% of male-to-female patients not exclusively attracted to men, against 15% of those attracted to men (Blanchard, 1985), and introduced the term autogynephilia (Blanchard, 1989); these ideas remain strongly disputed.
Sport, and the limits of the evidence
On sport, the only entry is a 2022 sports medicine position statement arguing that male-level testosterone is the main driver of sex differences in muscle and performance, and favoring testosterone limits over the IOC's no-presumption-of-advantage framework (Pigozzi et al., 2022). It reviews no direct performance data on trans athletes after testosterone suppression. Across the category, the evidence consists of commentaries, very small postmortem samples confounded by hormone treatment, and single-clinic self-report studies without comparison groups. Several key authors have professional or advocacy ties on one side of the debate, and the category currently contains no empirical study of athletic performance.