Not Guilty by Reasons Other than Insanity

The M’Naghten Rules of insanity play a dual role in the criminal law, underpinned by dual rationales. One of these roles is plainly exculpatory; they provide a complete defence to defendants whose non-culpability proceeds from mental disorder alone. The other role, however, may be better characterised by its exclusionary or diversionary effect. We identify this where the insanity ‘defence’ is applied despite the defendant satisfying alternative grounds for exculpation, diverting him or her from an unqualified acquittal to the disposal options available via the special verdict of ‘not guilty by reason of insanity’. The rationale for this second role is therefore less a backward-looking assessment of the defendant’s non-culpability, and rather more about a forward-looking assessment of their dangerousness and/or medical need. We make the case for the partial abolition of insanity, maintaining insanity only where it applies as a genuine supervening defence.