Insanity as a Criminal Defence

The M’Naghten Rules

The leading case concerning insanity as a defence to a criminal charge is R v M’Naghten [1843].  M’Naghten had tried to assassinate Robert Peel, then the British Prime Minister.  Instead, he shot Peel’s secretary, who died a few days later.  M’Naghten was charged with murder but acquitted on the grounds that he was insane.  The acquittal caused some controversy, so the House of Lords asked a panel of judges to review the law governing insanity.  The two following rules arose.

  1. Every defendant is presumed sane unless the contrary is proved.

  2. To successfully plead insanity it must be clearly proved that at the time of committing the act the defendant was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong.

The defence is proved on a balance of probabilities.  It is usually a matter for counsel for the defence to raise the issue of insanity, though in R v Dickie [1984] it was held that the trial judge may raise the issue and leave it to the jury, after consideration of all the evidence, to decide whether the accused is insane.  If the defence is successful, then under s2(1) of the Trial of Lunatics Act 1883 the verdict to be returned by the jury is ‘Not guilty by reason of insanity.’  The defence is only available to defendants charged with offences that require mens rea, that is, offences which are not strict liability offences (see DPP v H [1997]).  Under s5 of the Criminal Procedure (Insanity) Act 1964, following a successful defence of insanity, the court can order the accused to be admitted to a hospital, or make a guardianship, supervision or treatment order, or order an absolute discharge.  In practice, the defence is now generally only pleaded when an accused is charged with murder and even then, very rarely.

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For more information on:

  • Elements of the defence
  • Disease of the mind
  • Defect of reason
  • Knowledge that the act is wrong