M'naghten's rules

The m'naghten rules of 1843 established the rules on insanity.

Insanity is a general defence to criminal responsibility most commonly used in assault cases. It is associated with some of the major cases but can be used in the magistrates court. The rules of insanity as we know them are based on M'Naghten's Case.  The standard of proof in criminal cases does not apply to the issue of the defence of insanity. It is for the defendant to prove on a balance of probabilities that he was insane, as defined by the M'Naghten Rules. Incidents of insanity being raised as a defence are unusual, perhaps due to the defendant’s fear of the possibility of long term or even life time detention in hospital. Since the Homicide Act 1957 the plea of diminished responsibility is available to the charge of murder which may have discouraged defendant’s using insanity as a defence or it could be because of the restrictions of the test of insanity under the M'Naghten rules.

The rules came about as a result of concern about the outcome of the trial of M'Naghten for the murder of Sir Robert Peel's secretary. M'Naghten had tried to kill Sir Robert Peel but bungled the attempt and actually killed his secretary.  M'Naghten was found not guilty of murder but, as a result of the case, doubts arose as to what the proper legal definition of insanity was.  The judges sitting in the then House of Lords were given a series of questions touching upon the matter of what amounted to insanity and a series of rules or legal principles were established.

One of these rules founded the principle that 'in all cases every man is presumed to be sane and to possess a sufficient degree of reason to be responsible for his crimes'.

Under the M'Naghten Rules, the defendant must prove that at the time of committing the act, 'he 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'. It is important to note that the 'defect of reason' must be due to 'disease of the mind' and not any other failure of the mind which may lead to the defence of non-insane automatism.

The defendant must prove a defect of reason meaning the defendant cannot reason at all rather than just reasoning imperfectly – in R v Clarke [1972] the defendant, Mrs Clarke, stole some items from a shop and was charged with theft. Her defence was that she had not intended to steal from the shop. She was a diabetic and suffering from depression and was prone to absent-mindedness which may have been as a result of the depression. The trial judge believed the defence was a defence of insanity at which point Mrs Clarke changed her plea to guilty and appealed against the judge's finding. On appeal the Court of Appeal held that the M'Naghten rules did not apply as the defendant was able to reason things through properly but in a moment of confusion had not used her reasoning powers correctly or in full.


It must be caused by a disease of the mind induced by an internal factor.  In R v Kemp (1957) the accused was charged with causing GBH when he struck his wife with a hammer. The defendant suffered from arteriosclerosis which restricted the flow of blood to the brain. Lord Devlin ruled that for the purposes of the defence of insanity, diseases of the mind, and diseases of the body affecting the operation of the mind could not be distinguished apart from each other, and that it made no difference if the condition of mind could be cured or if it was transitory or permanent. The jury returned a verdict of guilty but insane.

The question of 'disease of the mind' was raised again in R v Sullivan when the defendant had an epileptic fit during which he kicked a man and injured him. The trial judge directed the jury on the defence of insanity, but not that of automatism. The House of Lords held that epilepsy was a disease of the mind, it did not matter that this was an organic disease or that it was only intermittent. The defendant's reasoning and mental faculties were impaired.

It must be shown that the defendant does not know the nature and quality of their act or that it is legally wrong. In R v Windle [1952] the defendant gave his wife an overdose of aspirins and she died. When he was arrested he said to the police "I suppose they will hang me for this". This led the trial judge to believe that although he was mentally ill he knew he was committing a crime and the judge refused to allow the defence of insanity. In R v Codere (1916) The appellant was convicted of murder having cut his wife's throat with a bread knife thinking it was a loaf of bread. He was looking to prove the defence of insanity, however, at the time of the killing he knew that it was unlawful to kill. He was not able to rely on the defence of insanity. Lord Reading on the meaning of 'wrong' referred to in the M'Naghten rules:

 "If the accused does know either that his act is morally wrong according to the ordinary standard adopted by reasonable men or that it is legally wrong then it cannot be said that he does not know he was doing what was wrong."

A special verdict is given in the event that the defendant is found guilty.  If the court is satisfied that the individual is insane then the verdict is 'Not guilty by reason of insanity'.

It was thought that the introduction of the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 would mean an increase in insanity pleas as it took away the obligation for judges to hospitalise a person indefinitely, except in the case of murder.


R v Quick & Anor [1973]

R v Hennessy (BAILII: [1989]

M'Naghten's Case (BAILII: [1843]

Bratty v Attorney General of Northern Ireland (BAILII: [1961]



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