One of the well-known aspects of many cults, is an insistence on an ideological purity that excludes evidence contrary to their beliefs. This can lead to a kind of conformity that is enforced by wilful blindness to reality. The tweet below struck me as one example of this. The Irish writer Barry Purcell portrays the Cass Review as a weird outlier to otherwise incontrovertible evidence supporting gender-affirming care.
Of course, it is possible to portray even the most widely-verified and repeatedly-validated result as an outlier, if you only compare it to a weird cult’s ideologically constructed set of preferred outcomes. In fact, this is exactly what Barry Purcell has done. In his blog post that includes delusional cult mantras like “biological sex is not binary”, he lists a number of highly compromised studies that have been carefully curated to include only his desired outcomes.
For example, the first research paper linked to in the Barry Purcell blog post, is about “gender-affirming hormone therapy”, and relates to a study conducted by Green et al in 2022. The methodology of the study targeted youths from the age of 13 with an online advertising campaign using platforms like Snapchat. Those who clicked on some Snapchat ads relating to the survey, were asked to complete an online questionnaire. That is, this a survey of self-selected participants. Specifically, whereas youths who have a positive view of gender-affirming care may be keen to respond when they see an ad on this topic, those who have had a negative experience may be less likely to participate.
This kind of self-selecting sample often recruits cohorts that are heavily skewed and unrepresentative of the target population as a whole. Such self-selection is likely to provide us only with the views of those keen to engage with the topic, while excluding by design those who have experienced difficulties with treatments that they are less keen to revisit. Moreover, this study does not include any medical records created by clinicians to describe which dosages of which specific pharmaceuticals were prescribed. Rather, youths from the age of 13 participating in an online questionnaire after clicking on a Snapchat ad, were asked to self-report on their own hormone medication. While the lowest quality of evidence within medical research is mere “expert opinion”, this study is not even based on the opinion of the relevant medical experts about the efficacy of their prescriptions. It instead relies on the opinions of some self-selected teenagers.
In contrast, the gold standard for medical research is a Systematic Review. In such a process, researchers will first define what kind of evidence will be most valuable and reliable on any given topic. The full range of studies on that topic will then be objectively evaluated to determine which data sets are most trustworthy and informative. The widest possible set of useful data from high-quality studies is then synthesised, in order to determine which conclusions are well-supported.
Whereas the writer Barry Purcell has comprised his preferred set of studies by selecting only those that imply his desired conclusions, Dr Hilary Cass FRCP recruited a team of professional medical researchers to conduct a detailed Systematic Review. To determine whether the Cass Review is an outlier, it should be compared to the other Systematic Reviews that have been conducted on this topic.
| 2019: Pasternack et al Medical approaches to the treatment of gender dysphoria: a systematic review. This Systematic Review concludes that evidence for the efficacy of medical treatments for gender dysphoria remains weak to very weak. |
| 2023: Thompson et al A systematic review of adolescent gender dysphoria literature This Systematic Review concludes that there is no clear evidence on adolescent gender dysphoria that can adequately inform clinical decision making, since changes to health parameters from puberty suppression were inconclusive except for an observed decrease in bone density. |
| 2023: Ludvigsson et al A systematic review of hormone treatment for children with gender dysphoria This Systematic Review concludes that evidence to assess the effects of hormone treatment in children with gender dysphoria is insufficient. |
| 2023: Heathcote et al Psychosocial support interventions for children and adolescents experiencing gender dysphoria or incongruence: a systematic review This Systematic Review concludes that there are only a small number of low-quality studies, which limit any conclusions about the efficacy of psychosocial interventions for children experiencing gender dysphoria. |
| 2023: Hall et al Impact of social transition in relation to gender for children and adolescents: a systematic review This Systematic Review concludes that there is only a small volume of low-quality research into the impact of social transition on children, such that there is an absence of robust evidence for the benefit of social transition. |
| 2024: Taylor et al Interventions to suppress puberty in adolescents experiencing gender dysphoria: a systematic review This Systematic Review concludes that there is a lack of high-quality research assessing puberty suppression in adolescents experiencing gender dysphoria, such that no conclusions can be drawn other than bone health and height being compromised. |
| 2024: Zepf et al Updated systematic review of the evidence for puberty blockers and hormone treatment in children with gender dysphoria This Systematic Review concludes that available evidence on the use of puberty blockers in children with gender dysphoria is very limited and doesn’t suggest that mental health significantly improves. |
| 2024: Taylor et al Masculinising and feminising hormone interventions for adolescents experiencing gender dysphoria: a systematic review This Systematic Review concludes that there is a lack of high-quality research assessing the use of hormones in adolescents experiencing gender dysphoria. |
The Cass Review published in 2024 was the world’s largest ever review of gender medicine for children. It concluded that gender medicine is “an area of remarkably weak evidence”. Far from being an outlier, the Cass Review is entirely consistent with the other Systematic Reviews in this area.
The list of Systematic Reviews above has not been cherry-picked, since there are in fact no outliers from these conclusions when all of the Systematic Reviews on this topic are considered. Each of the Systematic Reviews conducted by a wide range professional researchers across many countries, have all arrived at consistent conclusions. In fact, in 2025 the Department of Health and Human Services published an Overview Of Systematic Reviews that demonstrates this. After excluding the Systematic Reviews that involved adult patients, this overview summarised as follows the state of the research with respect to gender affirming care and puberty blockers for gender dysphoric children:
“There is high certainty evidence that puberty blockers exert physiological effects and that they probably lead to infertility when adolescents receiving puberty blockers go on to receive cross sex hormones. Puberty blockers may compromise bone health. A high proportion of youth proceed to cross sex hormones from puberty blockers. The evidence is very uncertain regarding the effect on gender dysphoria, improvement in mental health, and safety.”
Overview of Systematic Reviews, 2025
The Cass Review is in no way an outlier when compared to all of the other high-quality professional research in this area. However, while there is demonstrable agreement among medical researchers about gender medicine, the writer Barry Purcell has indeed demonstrated that 13 year-olds who interrupt their Snapchat consumption to answer a survey, can in fact disagree with the scientific consensus. In fairness, if the Cass Review is only compared to this and the other results in the information bubble that has been defined by the gender cult, then in this context the Cass Review does appear to be something of an outlier.