Abstract
The paper explores Lionel Penrose's scientific work. Penrose investigated the causes of mental disorders from clinical and genetic points of view. His investigations on phenylketonuria and Down syndrome helped to demonstrate the heterogenous character of mental disorders, whose causes can range from genetic with high penetrance, to largely environmental. He was specifically selected by JBS Haldane to become University College London's third Galton Chair as a result of his Colchester survey investigations. He became the first Galton Chair who had medical training. He never concealed his distaste for anything related to eugenics. As well as using his scientific work to reject eugenic ideas such as suggestions on sterilisation measures or the existence of a social problem group, he campaigned successfully to rename the Department of Eugenics, Biometry and Genetics to Department of Human Genetics and Biometry. With his work, he discredited prejudiced eugenic ideas on mental disorders and became an advocate for those with mental disabilities.