In January 1843, Daniel McNaughton, a 30 year old Scottish woodturner, travelled to London to assassinate the prime minister. He mistakenly shot Peel’s private secretary, Edward Drummond. At his subsequent murder trial, McNaughton’s motives were revealed to be delusional. He believed that he was being persecuted and kept under surveillance by the ruling political party. In the modern era he would receive a diagnosis of schizophrenia. McNaughton was found not guilty by reason of insanity and was transferred to Bethlem Hospital, then to Broadmoor Hospital, where he remained until his death.

McNaughton’s case established the legal criteria for insanity. These criteria are strict, which is appropriate since the verdict results in an acquittal even though the accused is clearly the agent of the crime. The legal criteria require that those found insane must have a recognised mental disorder that prevented them from knowing the nature of their action and that it was legally wrong, or (in some jurisdictions) that gave …