IN APRIL HILARY CASS, a British paediatrician, published her review of gender-identity services for children and young people, commissioned by NHS England. It cast doubt on the evidence base for youth gender medicine. This prompted the World Professional Association for Transgender Health (WPATH), the leading professional organisation for the doctors and practitioners who provide services to trans people, to release a blistering rejoinder. WPATH said that its own guidelines were sturdier, in part because they were “based on far more systematic reviews”.

Systematic reviews should evaluate the evidence for a given medical question in a careful, rigorous manner. Such efforts are particularly important at the moment, given the feverish state of the American debate on youth gender medicine, which is soon to culminate in a Supreme Court case challenging a ban in Tennessee. The case turns, in part, on questions of evidence and expert authority.