Insanity — M'Naghten's Case

In January 1843, Daniel M'Naghten shot and fatally wounded Edward Drummond, the private secretary to Prime Minister Sir Robert Peel, on Whitehall in London. M'Naghten had been labouring under a system of paranoid delusions, believing that he was being persecuted by the Tory government and its agents. Acting on those delusions, he had intended to kill the Prime Minister himself but mistook Drummond for Peel and shot him instead. Drummond died five days after the shooting from his wounds.

At his trial before the Central Criminal Court, medical evidence was adduced demonstrating that M'Naghten was suffering from a severe form of mental disorder characterised by fixed and persistent delusions of persecution. Nine medical witnesses gave evidence supporting the conclusion that M'Naghten was insane at the time of the killing. The jury, following the trial judge's direction, returned a verdict of not guilty on the ground of insanity, and M'Naghten was ordered to be detained indefinitely.

The acquittal provoked immediate and widespread public outrage, including concern expressed by Queen Victoria herself, who had survived multiple assassination attempts and was alarmed by the apparent ease with which a plea of insanity could secure an acquittal for a deliberate killing. The House of Lords, then sitting as the ultimate judicial authority of the land, was moved to convene a special session to address the state of the law relating to the criminal responsibility of those suffering from mental disease.

Rather than deciding an appeal in the conventional sense — no formal appeal had been brought — the House of Lords took the extraordinary procedural step of summoning the judges of England to answer a series of questions concerning the proper legal standard to be applied when a defendant raises insanity as a defence to criminal charges. Five questions were put to the fifteen judges who attended before the House. The answers given by the judges, delivered collectively, came to constitute what are universally known as the M'Naghten Rules.

The proceeding was thus consultative and declaratory rather than appellate in the ordinary sense. Nonetheless, the principles articulated by the judges in answer to the Lords' questions acquired immediate authority and have been treated as binding on subsequent courts for nearly two centuries. The case is accordingly reported at (1843) 10 Cl & Fin 200, and the answers given form the foundational statement of the English law of insanity.

The central question posed by the House of Lords was: what is the proper legal test to determine whether a person suffering from mental disease or disorder is exempt from criminal responsibility for an act which would otherwise constitute a criminal offence? More specifically, the Lords asked whether, and on what basis, a delusion operating on the mind of the accused could affect his criminal liability, and who bore the burden of establishing insanity before the jury.

An ancillary but practically significant question concerned the position of a defendant who acts under a partial delusion — that is, who is not wholly insane but who, by reason of an insane delusion, perceives the facts of his situation in a distorted way. The Lords sought to know whether such a defendant should be judged as if the facts he deludedly believed were true, or whether the mere existence of delusion precluded any moral or legal evaluation of the act.

A further question concerned the role of medical witnesses in insanity cases: whether it was permissible for such witnesses to give an opinion on the state of the defendant's mind at the time of the act, or whether their evidence was confined to describing the symptoms and character of the disease from which the defendant suffered, leaving the ultimate legal and moral conclusion to the jury.

The judges answering the questions posed by the House of Lords began from the foundational premise of English criminal law that moral responsibility, and hence criminal liability, presupposes the capacity of the actor to understand what he is doing and to distinguish right from wrong. This principle, deeply embedded in the common law, furnished the conceptual starting point from which the specific rules were derived. A person who entirely lacks the cognitive faculty necessary to appreciate the nature or moral quality of his act cannot be held to the same standard of responsibility as one who acts with full understanding.

The judges formulated the core test in the following terms: to establish a defence on the ground of insanity, it must be clearly proved that, at the time of committing 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. This formulation identifies two distinct limbs, either of which, if established, will afford a complete defence.

The first limb addresses cases where the defendant, by reason of a disease of the mind causing a defect of reason, did not know the nature and quality of the act he was doing. This limb concerns a fundamental cognitive incapacity: the defendant does not appreciate what he is physically doing. The classic illustration offered in subsequent commentary is the person who, due to mental disease, strangles another believing he is squeezing a lemon — he has no appreciation of the physical reality of his conduct. The incapacity must be attributable to a disease of the mind, not merely to voluntary intoxication or simple ignorance.

The second, and in practice more frequently invoked, limb addresses the defendant who does know the nature and quality of his act but does not know that it is wrong. The judges made clear that "wrong" in this context means legally wrong — contrary to the law of the land — though subsequent judicial decisions, notably in R v Windle [1952] 2 QB 826, have affirmed that the relevant standard is knowledge that the act is contrary to law rather than a broader moral sense of wrongfulness. The defendant who knows that what he is doing is prohibited by law cannot rely on the second limb, however sincere his belief in the moral or divine justification for his conduct.

A critical and express requirement in both limbs is that the defect of reason must arise from a "disease of the mind." The judges did not define disease of the mind in physiological or psychiatric terms; the concept is a legal one, elaborated by subsequent case law. The width of this phrase was later confirmed to be considerable, encompassing not only endogenous psychotic illness but also conditions such as arteriosclerosis, epilepsy, and hyperglycaemia, provided they manifest internally and affect the mental faculties, as determined in cases including R v Kemp [1957] 1 QB 399, Bratty v Attorney-General for Northern Ireland [1963] AC 386, and R v Sullivan [1984] AC 156.

The judges also addressed the question of burden of proof. They held that every person is presumed sane and to possess a sufficient degree of reason to be responsible for his crimes, until the contrary is proved to the satisfaction of the jury. The burden of proving insanity thus lies on the defendant. The standard of proof required of the defendant is proof on the balance of probabilities, a position confirmed by subsequent authority. This allocation of the burden was significant: it departs from the ordinary rule in criminal proceedings that the prosecution bears the burden of proof, and has been the subject of ongoing academic debate concerning its compatibility with the presumption of innocence.

On the question of partial delusions, the judges offered important guidance in terms that have been described as obiter but which have been consistently applied. Where a person is labouring under a partial delusion only, and is not in other respects insane, he is to be considered in the same situation as to responsibility as if the facts with respect to which the delusion exists were real. The judges gave the illustrative example: if the defendant, under a delusion, supposes another man to be in the act of attempting to take his life and kills that man, he would be exempt from punishment; but if his delusion was that the deceased had done him a great injury, and he killed him in revenge, he would be liable to punishment. The test is therefore whether, on the assumed facts of the delusion, the act would have been legally justifiable.

Regarding the role of medical witnesses, the judges held that questions of the kind posed in insanity trials — whether the defendant's conduct was the product of an insane impulse, whether he was conscious of the nature and quality of his act, and whether he knew that what he was doing was wrong — are questions of fact for the jury. Medical witnesses may give evidence of the history and nature of the disease from which the defendant suffers. They may also, where they observed the defendant and can speak to his state of mind from direct knowledge, express an opinion. However, where a medical witness has not observed the defendant personally but is asked to form a hypothetical opinion based on facts proved in evidence, the ultimate decision remains with the jury.

The judges were careful to cabin the defence within cognitive rather than volitional terms. A defendant who knew the nature and quality of his act and knew it to be wrong, but who acted because of an irresistible impulse arising from mental disease, was afforded no defence under the rules as stated. This rejection of a volitional limb — one that would exculpate a defendant who knew right from wrong but was unable to control his conduct — has been one of the most persistently criticised features of the M'Naghten Rules. Proposals to introduce a control test have been made by the Royal Commission on Capital Punishment in 1953 and by the Law Commission, but have not been enacted in English law, leaving the purely cognitive formulation intact.

The verdict consequent upon a successful insanity plea, as it operated at the time of M'Naghten's Case, was one of acquittal on the ground of insanity, resulting in indefinite detention at Her Majesty's pleasure. The Trial of Lunatics Act 1883 subsequently introduced the special verdict of "guilty but insane," which was itself later superseded by the Criminal Procedure (Insanity) Act 1964, which provides for a verdict of "not guilty by reason of insanity." The disposals available to the court following such a verdict were significantly expanded by the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, removing the former mandatory order of indefinite hospitalisation and permitting supervision orders, absolute discharge, and other measures.

The House of Lords, through the answers provided by the judges, established that the defence of insanity in English criminal law requires clear proof that, at the time of committing the act, the accused was labouring under such a defect of reason, from disease of the mind, as either (a) not to know the nature and quality of the act he was doing, or (b) if he did know it, not to know that what he was doing was wrong. Every person is presumed sane, and the burden of establishing insanity falls on the defendant on the balance of probabilities.

A defendant suffering from a partial delusion, but otherwise sane, is to be judged as if the delusional belief were true. The defence does not extend to a defendant who, despite suffering from mental disease, retained knowledge of the nature of his act and knowledge that it was wrong. The rules as formulated are cognitive in nature and do not admit of a volitional or irresistible impulse limb.

On the procedural question, medical witnesses are competent to give opinion evidence as to the disease and its symptoms, and may, where they have personally observed the defendant, express a view on his state of mind. The ultimate question of whether the defendant satisfies the legal test of insanity is, however, one of fact for the jury to determine in light of all the evidence.

The M'Naghten Rules constitute the foundational statement of the law of insanity in England and Wales and remain the governing test today, notwithstanding persistent calls for reform. They have been applied and interpreted in a substantial body of subsequent case law that has elaborated each of their constituent elements. The meaning of "disease of the mind" has been expanded significantly beyond psychiatric illness to encompass any internal condition that affects the functioning of the mind, with the courts drawing the critical distinction — established in cases such as R v Quick [1973] QB 910 — between insanity arising from an internal cause and automatism arising from an external cause such as a blow to the head or the administration of insulin.

The meaning of "wrong" in the second limb was definitively resolved in favour of legal rather than moral wrongness by the Court of Appeal in R v Windle [1952] 2 QB 826, a decision that has been criticised for its narrowness but which remains authoritative in England and Wales, in contrast to the approach taken in other common law jurisdictions such as Australia, where the High Court in R v Stapleton (1952) 86 CLR 358 adopted a broader moral wrongness test. The English approach means that a defendant who kills in the genuine but deluded belief that he is carrying out a divine command will not satisfy the second limb if he appreciates that the law prohibits his conduct.

The M'Naghten Rules have been criticised on multiple grounds. The absence of a volitional limb, the arcane legal concept of disease of the mind divorced from contemporary psychiatric understanding, the procedural consequences of a successful insanity plea which historically carried mandatory indefinite hospitalisation and which continue to carry significant stigma, and the restrictive cognitive threshold — all have been identified as deficiencies by academic commentators, the Butler Committee (1975), and successive Law Commission reports. Despite the enactment of the Homicide Act