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.

Elements of the defence

Disease of the mind

It is important to be aware that there is a difference between being legally insane and being medically insane.  For the courts, the question of insanity is decided as a matter of law according to the ordinary rules of interpretation, procedure and evidence.  The judiciary has, over the years, considered various mental states when dealing with the question of a disease of the mind.  In R v Kemp [1956] the defendant suffered from arteriosclerosis.  There was a difference of medical opinion about how much this disease affected his mind.  The court held that, whichever medical opinion was accepted the disease was one capable of affecting the defendant’s mind, and so was a disease of the mind within the M’Naghten Rules.  It was irrelevant whether the disease was one recognised medically as a mental disease.  In R v Hennessy [1989] the defendant was charged with taking a vehicle without consent.  He was a diabetic who, despite needing to take insulin everyday, had not done so for two or three days before the incident.  He was also suffering from stress.  He claimed that as a result of all this he did not know what he was doing, and so he did not have the requisite mens rea.  The judge was of the opinion that this was a disease of the mind within the M’Naghten Rules and that the defendant was insane.  The defendant then changed his plea to guilty and appealed against the trial judge’s ruling on insanity.  The Court of Appeal upheld the trial judge’s ruling.  Hyperglycaemia caused by the lack of insulin was a disease of the mind within the M’Naghten Rules.  The defendant was, at the material time, insane.  The case is an excellent illustration of how the test of insanity is legal and not medical, as a diabetic would not in most circumstances be considered insane.  It is also a good example of how, in law, a person can be temporarily insane.  In other words the law takes the view that it is possible for a person to be sane most of the time but suffer bouts of insanity.  The M’Naghten Rules only require a defendant to be insane at the time of committing the act.

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For more information on:

  • Defect of reason
  • Knowledge that the act is wrong