University of York — Cass systematic reviews
The University of York is a British university. Its Centre for Reviews and Dissemination (CRD) specialises in systematic reviews of healthcare research.
The reviews
For the Cass Review, researchers from York conducted eight systematic reviews and an international survey of gender services (source). They cover the characteristics of referred young people, care pathways, guidelines (two reviews), social transition, psychosocial support, puberty blockers and hormone treatment. The articles were published in 2024 in the journal Archives of Disease in Childhood (source).
Why it is on this list
The reviews form the scientific basis of Cass's final report. The researchers concluded that high-quality research on puberty suppression (source) and on hormone treatment (source) in adolescents is lacking. The methodology also came under criticism, including the way studies were assessed for quality. The link leads to the CRD's page; the articles themselves are published by the journal. See also science and debate.
The Centre for Reviews and Dissemination
The CRD is a research institute that brings together and assesses data from multiple studies, so that decisions about health, public health and social care can be based on research. Its work includes the evaluation of medical technologies, methodological development, children's health, mental health and public health. The institute manages PROSPERO, an international registry in which researchers preregister systematic reviews. According to the website, the registry contains more than 500,000 records.
Design and findings
There were eight reviews and a survey. They were published in April 2024, at the same time as the final report (source). The researchers assessed the quality of studies using adapted versions of the Newcastle-Ottawa Scale and the Mixed Methods Appraisal Tool, and the quality of guidelines using AGREE II. They included only English-language studies and excluded studies on adults (source). In the reviews on puberty blockers and hormones, only studies of moderate and high quality were included in the synthesis. The differences in evidentiary strength between types of research are explained in the evidence pyramid in gender care. For puberty blockers, they found 50 studies: one of high quality, 25 of moderate quality and 24 of low quality (source). For hormone treatment, there were 53 studies: one of high quality, 33 of moderate quality and 19 of low quality (source). In several studies, bone density fell during treatment with puberty blockers. According to the reviewers, no conclusion could be drawn about the effect on gender dysphoria and on mental and psychosocial health (source). A planned study into the later outcomes of approximately 9,000 young people was halted on 30 November 2023 because six of the seven adult clinics were unwilling to cooperate (source).
Criticism and defence
In 2024, a group of researchers around Meredithe McNamara (Yale School of Medicine) and Anne Alstott (Yale Law School) published a critique. They write that the Cass Review repeatedly misuses data and violates its own evidentiary standards, and that many statements in the Review and the conduct of the York reviews reveal 'profound misunderstandings of the evidence base and the clinical issues at hand' (source); see the article on the Yale report against the Cass Review. In May 2025, an article by Noone and colleagues in BMC Medical Research Methodology used the ROBIS tool to identify a high risk of bias in each of the seven reviews they assessed (source). A May 2026 evaluation by the British Medical Association had the reviews on puberty blockers and hormones reassessed by two independent analyses from HEOR. These found no indication that the York reviews had missed or excluded high-quality studies, or that the reviewers were biased; they also concluded that the evidence is limited and uncertain (source). More explanation is provided in systematic reviews.