Background: The hypothesis that mental distress is caused by chemical imbalances in the brain is widespread in public and professional discourse, and has fuelled research into and prescription of antidepressant medication. Nevertheless, this hypothesis has very little supporting evidence, and has been branded as oversimplistic and reductionist, with some even claiming it is nothing but a myth. The continued discussion of this controversial explanatory model by various professions raises the question of how the chemical imbalance or monoamine hypothesis is used by expert witnesses in legal proceedings. Aims: The study examined how expert witnesses have used the chemical imbalance hypothesis as explanations in Court cases involving mental health. Method: The study was a qualitative investigation of secondary data. Two databases of Court transcripts were systematically searched, resulting in 19 transcripts where expert witnesses used/referred to the hypothesis. These transcripts were analysed using a hybrid method of thematic analysis and discourse analysis. Results: The chemical imbalance model was invoked in Court proceedings as an unquestioned scientific “fact” rather than a hypothesis with conflicting evidence, and to support divergent, even contradictory, conclusions. Conclusions: In the context of expert evidence to Courts, this appeared to serve the function of reinforcing the scientific basis of psychology or psychiatry – supporting and rendering unquestionable the subjective opinions of the expert (“trust me, I’m a scientist”). Funding: None to declare. Declaration of Interest: Author A reports no conflicts of interest. Author B is employed by the University of Liverpool and is a clinical psychologist in independent practice. Over the course of his career, Author B has received research grants from the National Institute of Health Research, the Medical Research Council, the Economic and Social Research Council, the Wellcome Trust, the Youth Justice Board for England, various NHS Trusts, the Department of Health, the European Commission, the British Psychological Society, and the Reader, and personal fees from the Department for Constitutional Affairs, legal counsel, the BBC, Smoking Gun Media, GLG Group, True North Productions, and Compass Pathways Ltd. He has also received royalties on published work in the field of mental health from Palgrave Macmillan, and Little, Brown Book Company, and travel and hospitality from a variety of organisations in relation to speaking and other professional activity. Author B has a remunerative secondment contract through his employers to act as a Clinical Advisor for Public Health England. Author B sits on the NICE (National Institute for Health and Care Excellence) Guidelines panel for Adult Depression 2015-2021, is a Member of the Council for Evidence Based Psychiatry, a director of Kyrie Therapeutic Farm Ltd, and a former President of the British Psychological Society. Author B is a member of the British Beekeepers’ Association, and a member of the Giordano Bruno Foundation (Giordano- Bruno-Stiftung). Author B owns shares in Alphabet (the parent company of Google), Twitter, and the Kindling Trust, and is Director and owner of Kinderman Consulting Ltd.