The M'Naghten Rules (pronounced, and sometimes spelled, McNaughton) were the first serious attempt to codify and rationalise the attitude of the criminal law towards mentally incompetent defendants. They arise from a set of questions posed to the House of Lords in M'Naghten's Case(1843) 10 C & F 200  following the attempted assassination of the British Prime Minister, Robert Peel, in 1812 by Daniel M'Naghten. The rules have been a standard test for criminal liability in relation to mentally disordered defendants in common law jurisdictions ever since, with some minor adjustments. When the tests set out by the Rules are satisfied, the accused may be adjudged "not guilty by reason of insanity" and the sentence may be a mandatory or discretionary (but usually indeterminate) period of treatment in a secure hospital facility, or otherwise at the discretion of the court (depending on the country and the offence charged) instead of a punitive disposal. 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.
Today, mental incapacity as a defense, when successfully raised, absolves a defendant in a criminal trial from liability, that is to say it applies public policies in relation to criminal responsibility by applying a rationale of compassion, accepting that it is morally wrong to subject a person to punishment if that person is deprived permanently or temporarily of the capacity to form a necessary mental intent that the definition of a crime requires. Indeed, punishment of the obviously mentally ill by the state may act so as to undermine public confidence in the penal system. Thus, in such cases, a utilitarian and humanitarian approach suggests that the interests of society are better served by treatment of the illness rather than punishment of the individual.
Historically, insanity was not seen as a defence in itself but a special circumstance in which there was no acquittal, but the jury could deliver a special verdict and the King would issue a pardon  In R v Arnold (1724) 16 How St. Tr. 765, the test for insanity was expressed in the following terms
whether the accused is totally deprived of his understanding and memory and knew what he was doing "no more than a wild beast or a brute, or an infant".This is clearly, by modern legal, and medical standards, a simplistic test.
The next major advance occurred in Hadfield's Trial (1800), 27 How St. Tr. 765 in which the court decided that a crime committed under some delusion would only be excused if it would have been excusable had the delusion been true. This would deal with the situation, for example, when the accused imagines he is cutting through a loaf of bread, whereas in fact he is cutting through a person's neck.
The M'Naghten Rules
The House Of Lords, having deliberated, delivered the following exposition of the Rules:
- "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"
The central issue of this definition may be stated as "did the defendant know what he was doing?", and the issues raised have been analysed in subsequent appellate decisions:
Presumption of Sanity and Burden of Proof
Sanity is a rebuttable presumption and the burden of proof is on the party relying upon it; the standard of proof is on a balance of probabilities, that is to say that mental incapacity is more likely than unlikely. If this burden is successfully discharged, the party relying upon it is entitled to succeed. In Lord Denning's judgement in Bratty v Attorney-General for Northern Ireland (1963) AC 386, whenever the defendant makes an issue of his state of mind, the prosecution can adduce evidence of insanity. However, this will normally only arise to negate the defence case when automatism or diminished responsibility is in issue. In practical terms, the defence will be more likely to raise the issue of mental incapacity to negate or minimise criminal liability. In R v Clarke (1972) 1 All E R 219 a defendant charged with a minor theft (shoplifting) claimed she had no mens rea because she had absent-mindedly walked out of the shop without paying because she suffered from depression. When the prosecution attempted to adduce evidence that this constituted insanity within the Rules, she changed her plea to guilty but on appeal, the Court ruled that she had been merely denying mens rea rather than raising a defence under the Rules and her conviction was quashed. The general rule was stated that the Rules only apply to cases in which the defect of reason is substantial.
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 itself. The term has been held to cover numerous conditions:
- 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, 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
- 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 obiter 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."
The courts have clearly drawn a distinction between internal and external factors affecting a defendant's mental condition; this distinction would appear to depend largely upon elements of voluntariness and awareness in the circumstances surrounding the defendant's actions; this is clear from Hennessy, above. In Quick & Paddison, above, for example, the distinction was made that although the defendant was a diabetic, his mental state was influenced less by his diabetes than by alcohol consumption and not eating, and it is implicit in the judgement that this had been, or could have been, within the defendant's control. It would appear that if a defendant is aware of a medical condition which may affect his mental capacity and acts in a manner inimical to that condition, a defence of insanity under the Rules will not be available.
There is occasionally a blurred interface between insanity and automatism, as the comment in Burgess above shows. Actions committed while sleepwalking would normally be considered as "non-insane automatism", whereas in that case, violent assault was considered to support a finding of insanity within the Rules. See also automatism (case law).
Nature and Quality of the Act
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.
The judges were specifically asked 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 and follows Hadfield's Trial, above. 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 smashed a van 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 neither 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.
Knowledge that the act was wrong
"Wrong" here means legally rather than morally wrong. The defendant must be functionally unaware that his actions are legally wrong at the time of the offence to satisfy this requirerment. In Windle ( 2QB 826;  2 All ER 1 246), the defendant killed his wife with an overdose of aspirin; he telephoned the police and said, "I suppose I'll hang for this." It was held that this was sufficient to show that although the defendant was suffering from a mental illness, he was aware that his act was wrong, and the defense was not allowed.
Crimes Without Specific Intent
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.
The Function of 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:
- 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).
- 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.
There have been five major criticisms of the law as it currently stands:
- Medical Irrelevance
- The legal definition of insanity has not advanced significantly since 1843; in 1953 evidence was given to the Royal Commission on Capital Punishment that doctors even then regarded the legal definition to be obsolete and misleading.
- This distinction has led to absurdities such as
- (a) even though a legal definition suffices, mandatory hospitalisation can be ordered in cases of murder; if the defendant is not medically insane, there is little point in requiring medical treatment.
- (b) diabetes has been held to facilitate a defence of insanity when it causes hyperglycemia, but not when it causes hypoglycemia.
- (c) Article 5 of the European Convention on Human Rights, imported into English law by the Human Rights Act 1998 provides that a person of unsound mind may only be detained where proper account of objective medical expertise has been taken. As yet, no cases have occurred in which this point has been argued.
- Burden of Proof
- The shift of burden of proof from the prosecution to the defence in cases where insanity may be in issue conflicts with the Woolmington v. DPP principle that the burden is always on the prosecution.
- The rules currently do not distinguish between defendants who represent a public danger and those who do not. Illnesses such as diabetes and epilepsy can be controlled by medication such that sufferers are less likely to have temporary aberrations of mental capacity, but the law does not recognise this.
- Sentencing for Murder
- A finding of insanity may well result in indefinite confinement in a hospital, whereas a conviction for murder may well result in a determinate sentence of between ten and fifteen years; faced with this choice, it may be that most defendants would prefer the certainty of the latter option.
- 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 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.
Notes and references
- Boland, F. (1996). "Insanity, the Irish Constitution and the European Convention on Human Rights". 47 ‘’Northern Ireland Legal Quarterly 260.
- Bucknill, J. C. (1881). The Late Lord Chief Justice of England on Lunacy. Brain 4: 1-26.
- 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. 
- Elliott, Catherine; Quinn, Frances (2000). Criminal Law, 241 - 248, Harlow, Essex: Pearson Education Ltd.