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M'Naghten Rules

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The M'Naghten Rules (pronounced, and sometimes spelled, McNaughton) are used to establish insanity as an excuse to potential criminal liability, but the definitional criteria establish insanity in the legal and not the psychological sense. The guidelines were formulated by the House of Lords in M'Naghten's Case (1843) 10 C & F 200.[1] In some countries, when the Rules are satisfied, the accused is subject to a special verdict of "not guilty by reason of insanity", and the sentence is either a mandatory and indeterminate period of treatment in a secure hospital facility or at the discretion of the court depending on the country. The defense is recognized in Australia, Canada, England and Wales, New Zealand, the Republic of Ireland, and most U.S. states with the exception of Montana, Idaho, Utah, and Kansas.


Insanity as a defense operates as an excuse; i.e., it excludes the accused from the process of a trial which makes it consistent with public policies about criminal responsibility and the function of criminal law. At one level, it is a rationale of compassion, accepting that it is morally wrong to treat a person as criminal if, because of severe mental illness or intellectual disability, he or she is temporarily or permanently deprived of the capacity either to choose whether to obey law or to distinguish right from wrong. This may suggest that the insanity provisions operate, at the most fundamental level, to negate the mens rea element in any offence (cf doli incapax in which children are excused from criminal liability due to a presumed incapacity to form a criminal intent). A more utilitarian justification for criminalising any given behaviour is as a deterrent. Yet, if this class of potential offender is genuinely insane, no law or threat of punishment will be effective as a deterrent. Indeed, if the state is seen to punish the mentally ill, this may actually undermine public confidence in the penal system. Thus, the better rationale for the choice of compulsory medical treatment is a general social duty to encourage rehabilitation.

In states where the consequence of satisfying the Rules was indefinite detention, insanity was rarely invoked when the accused was charged with a relatively minor offence. It was better to plead to the charge and receive a determinate prison sentence. Hence, insanity was mainly used in murder cases where the threat of capital punishment offered a genuine incentive to plead insanity as an alternative to death. The fact that murder no longer carries a capital sentence in the majority of countries may remove the incentive to plead it, assuming no change in the dispositional powers of the court after a successful insanity defence.

A practical issue is whether the fact that an accused is labouring under a "mental disability” should be a "necessary" but "not sufficient" condition for negating responsibility i.e. whether the test should also require an incapacity to understand what is being done, to know that what one is doing is wrong, or to control an impulse to do something and so demonstrate a causal link between the disability and the potentially criminal acts and omissions. For example, the Irish insanity defence comprises the M'Naghten Rules and a control test which asks whether the accused was debarred from refraining from committing the act because of a defect of reason due to mental illness (see Doyle v Wicklow County Council (1974) 55 IR 71). But the Butler Committee in England recommended that proof of severe mental disorder should be sufficient to negate responsibility, in effect creating an irrebuttable presumption of irresponsibility arising from proof of a severe mental disorder. This has been criticized as it assumes a lack of criminal responsibility simply because there is evidence of some sort of mental dysfunction, rather than establishing a standard of criminal responsibility. According to this view, the law should be geared to culpability not mere psychiatric diagnosis.

The Rules in English law

… the jurors ought to be told in all cases that every man is presumed to be sane, and to possess a sufficient degree of reason to be responsible for his crimes, until the contrary be proved to their satisfaction; and that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused 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.

Burden of proof

This is a rebuttable presumption and the burden of proof is on the party wishing to rebut. If a jury thinks it is more likely than not that the defendant is insane within the meaning of the Rules, he or she is entitled to a verdict in his or her favour. The common law rule, based on Lord Denning's judgement in Bratty v Attorney-General for Northern Ireland (1963) AC 386, is that wherever the defendant makes an issue of his or her state of mind during a trial, the prosecution can adduce evidence of his insanity. In R v Clarke (1972) 1 All E R 219 a woman charged with a minor s1 theft claimed she had no mens rea because she had absent-mindedly walked out of the shop without paying. When the prosecution began to adduce evidence under the Rules, she changed her plea to guilty. On appeal, the conviction was quashed and the general rule of practice introduced to limit the prosecution's ability to introduce the Rules to cases in which the defect of reason was substantial.

The test

Given that the test examines the accused's knowledge and understanding, the focus is on mens rea issues. In DPP v Harper (1997) it was held that insanity is not generally a defence to strict liability offences. In this instance, the accused was driving with excess alcohol. By definition, the accused is sufficiently aware of the nature of the activity to commit the actus reus of driving and presumably knows that driving while drunk is legally wrong. Any other feature of the accused's knowledge is irrelevant.

At the time

The focus of interest is the state of mind at the time the accused engages in behaviour resulting in a criminal outcome. It is irrelevant that, as a result of treatment, the accused may be entirely sane at the time the matter comes before a court.

Disease of the mind

Whether a particular condition amounts to a disease of the mind within the Rules is not a medical but a legal question to be decided in accordance with the ordinary rules of interpretation. It seems that any disease, which produces a malfunctioning of the mind, is a disease of the mind, and need not be a disease of the brain. It covers any internal as opposed to external disorder, which results in violence and is likely to recur:

  • R v Kemp (1957) 1 QB 399: arteriosclerosis or a hardening of the arteries caused loss of control during which the defendant attacked his wife with a hammer. This was an internal condition and a disease of the mind.
  • R v Sullivan (1984) AC 156 during an epileptic episode, the defendant caused grievous bodily harm: epilepsy was an internal condition and a disease of the mind, and the fact that the state was transitory was irrelevant.
  • R v Quick & Paddison (1973) 3 AER 397 a diabetic committed an assault while in a state of hypoglycaemia caused by the insulin he had taken, the alcohol he had consumed and not eating. Since the immediate condition was caused by external factors (the insulin, alcohol and insufficient food), it was not caused by a disease of the mind. Automatism was not available either because the loss of control was foreseeable.
  • R v Hennessy (1989) 1 WLR 287 a diabetic stole a car and drove it while suffering from a mild attack of hyperglycaemia caused by stress and a failure to take his insulin. Lane LCJ said at 294 that:
In our judgment, stress, anxiety and depression can no doubt be the result of the operation of external factors, but they are not, it seems to us, in themselves separately or together external factors of the kind capable in law of causing or contributing to a state of automatism. They constitute a state of mind which is prone to recur. They lack the feature of novelty or accident, which is the basis of the distinction drawn by Lord Diplock in R v Sullivan (1984) AC 156, 172. It is contrary to the observations of Devlin J., to which we have just referred in Hill v Baxter (1958) 1 QB 277, 285. It does not, in our judgment, come within the scope of the exception of some external physical factor such as a blow on the head or the administration of an anaesthetic.
  • In Bratty, Lord Denning observed in obiter dicta that a crime committed while sleepwalking would appear to him to be one committed as an automaton. However, the ruling in R v Sullivan that diseases of the mind need have no permanence, led many academics to suggest that sleepwalkers might well be found to be suffering from a disease of the mind with internal causes unless there was clear evidence of an external causal factor. In R v Burgess (1991) 2 WLR 1206 the Court of Appeal ruled that the defendant who wounded a woman by hitting her with a video recorder while sleepwalking, was insane under the M'Naghten Rules. Lord Lane said, "We accept that sleep is a normal condition, but the evidence in the instant case indicates that sleepwalking, and particularly violence in sleep, is not normal."
More generally, see automatism (case law).

Defect of reason

The disease of the mind must have given rise to a defect of reason which had one of two consequences: either (a) the defendant did not know the nature and quality of his act, or (b) he did not know his act was wrong. Thus, the requirement is that the defendant's power to reason is impaired in a substantial way. R v Clarke (1972) 1 AER 219 confirms that this must be something more than a state of confusion or absentmindedness. More interesting is whether the Rules should be invoked in cases involving battered woman syndrome.

Nature and quality of the actions

This phrase refers to the physical nature and quality of the act, rather than the moral quality. It covers the situation where the defendant does not know what he is physically doing. Two common examples used are:

  • The defendant cuts a woman's throat under the delusion that they are cutting a loaf of bread,
  • The defendant chops off a sleeping man's head because they have the deluded idea that it would be great fun to see the man looking for it when he wakes up.

Knowledge that the act was wrong

This requirement is a reference to the law rather than the morality. The defendant must not know that he was doing something legally wrong.

Windle - The acknowledgement of the wrong following a man killing his wife. Following her death he phoned the police and said, "I suppose I'll hang for this." This was sufficient for to show that the defendant had been aware the act he had done was wrong and the defense was not allowed.


The judges were asked in M'Naghten's Case if a person could be excused if he committed an offence in consequence of an insane delusion. They replied that if he labours under such partial delusion only, and is not in other respects insane, "he must be considered in the same situation as to responsibility as if the facts with respect to which the delusion exists were real". This rule requires the court to take the facts as the accused believed them to be. If the delusions do not prevent the defendant from having mens rea, there will be no defence. In R v Bell (1984) Crim. LR 685 the defendant used a van to smash through the entrance gates of a holiday camp because, "It was like a secret society in there, I wanted to do my bit against it" as instructed by God. It was held that, as the defendant had been aware of his actions, he could not have been in a state of automatism nor insane, and the fact that he believed that God had told him to do this merely provided an explanation of his motive and did not prevent him from knowing that what he was doing was wrong in the legal sense. Specific conditions may induce the delusions. The court in R v Windle (1952) accepted that folie à deux may cause a defect of reason: over a period of many years, a husband who had acted as a carer for his wife who had a mental illness came to share similar symptoms.

The jury

s1 Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 provides that a jury shall not return a special verdict that "the accused is not guilty by reason of insanity" except on the written or oral evidence of two or more registered medical practitioners of whom at least one has special experience in the field of mental disorder. This may require the jury to decide between conflicting medical evidence which they are not necessarily equipped to do, but the law goes further and allows them to disagree with the experts if there are facts or surrounding circumstances which, in the opinion of the court, justify the jury in coming to that conclusion.


Under s3 Act 1991:

  1. Where the sentence for the offence to which the finding relates is fixed by law (e.g. murder), the court must make a hospital order restricting discharge without limitation of time. Otherwise, if there is adequate medical evidence and the defendant has been convicted of an imprisonable offence, a hospital order requires that the defendant be admitted to and detained in a hospital for treatment for a mental disorder (see ss37-43 Mental Health Act 1983).
  2. In any other case the court may make:
a hospital order and an order restricting discharge either for a limited or unlimited period of time;
or in appropriate circumstances,
a guardianship order;
a supervision and treatment order; or
an order for absolute discharge.

United States

The M'Naghten Rule currently is the standard for legal insanity in nearly half the states.[1]

  • Persons acting under the influence of an insane delusion are punishable if they knew at the time of committing the crime that they were acting contrary to law.
  • Every man is presumed sane and to have sufficient reason to be held responsible for his crimes.
  • A person under a partial delusion is to be considered as if the facts with respect to which the delusion exists were real.
  • To establish a defence on the ground of insanity each element of it must be clearly proved that:
(a) at the time of committing the act,
(b) the accused was labouring under such a defect of reason, from disease of the mind, (c) as not to know the nature and quality of the act he was doing
(d) if he did know the nature and quality of the act, the accused was labouring under such a defect of reason, from disease of the mind (e) that he did not know he was doing what was wrong.
IF (1) the accused was conscious that the act was one that he ought not to do AND (2) if the act was at the same time contrary to the law of the land,
THEN the accused is punishable.

Notes and references

  1. see.e.g.,
  • Boland, F. (1996). "Insanity, the Irish Constitution and the European Convention on Human Rights". 47 ‘’Northern Ireland Legal Quarterly 260.
  • Butler Committee. (1975). The Butler Committee on Mentally Abnormal Offenders, London: HMSO, Cmnd 6244
  • Dalby, J.T. (2006) "The Case of Daniel McNaughton: Let's get the story straight." American Journal of Forensic Psychiatry, 27, 17-32.
  • Ellis, J. W. (1986). "The Consequences of the Insanity Defense: Proposals to reform post-acquittal commitment laws". 35 Catholic University Law Review 961.
  • Gostin, L. (1982). "Human Rights, Judicial Review and the Mentally Disordered Offender". (1982) Crim. LR 779.
  • The Law Reform Commission of Western Australia. The Criminal Process and Persons Suffering from Mental Disorder, Project No. 69, August 1991. [2]

See also

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