The McNaughtan Rules

In January 1843 Edward Drummond, secretary to the Conservative British prime minister Robert Peel, was shot and fatally wounded in Westminster. At his trial the gunman’s lawyers pleaded that Daniel McNaughtan was insane because he believed he was being persecuted by the “Tories”. This defence raised the issue of partial insanity. At that time the predominant view among doctors and jurists was that partial insanity did not exist, and even people who suffered from only one form of delusion were considered to be incapable of reasoning.

Asylum, Standish Lawrence Harris, 1824

McNaughtan’s defence assembled some prominent medical opinion to argue that partial insanity did exist, which meant that people like the accused might be able to act reasonably much of the time, but were unable to control their behaviour on some crucial issues.

McNaughtan’s trial was halted, and he was acquitted on the grounds of insanity. He then spent the rest of his life incarcerated in institutions, and died at Broadmoor Asylum in 1865. The case was widely reported and caused considerable concern, which led the House of Lords to seek precise clarification of the grounds on which an insanity plea could be accepted by the courts. A panel of judges ruled that offenders suffering from partial delusions are punishable if they knew they were breaking law; a defence on grounds of insanity must prove that at the time of the offence the accused did not know what they were doing, or they did not know it was wrong.

These tests for criminal insanity were the M’Naughten (or McNaughtan) rules, and were subsequently adopted and adapted in many criminal jurisdictions, including in Australia.

Under these rules, however, it is likely that McNaughtan would have been convicted.