The Insanity Defence: M'Naghten, Durham, MPC and BNS § 22

Daniel M'Naghten shot Edward Drummond, secretary to Prime Minister Robert Peel, in January 1843, believing Drummond to be Peel and believing that Peel was the source of a persecutory conspiracy against him. M'Naghten was acquitted on grounds of insanity, provoking outrage including from Queen Victoria, who had survived two assassination attempts of her own. The House of Lords convened a panel of fifteen judges to establish a clear rule. The resulting M'Naghten Rules, articulated in their answer to the House of Lords in 1843, remain the operative insanity standard in England and Wales and form the baseline for most common-law jurisdictions.

The Rules establish two alternative cognitive grounds for a defence. First, the defendant may show that at the time of the offence they were "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." Second, if the defendant did know the nature and quality of the act, they may show that they "did not know he was doing what was wrong." The key word in both limbs is "know." The M'Naghten test is purely cognitive. It asks nothing about the defendant's ability to control their conduct once they understood it.

"Disease of the mind" in English case law. The judges in M'Naghten did not define "disease of the mind." Courts have since struggled with the boundary between conditions that qualify as disease of the mind and those that are ordinary physical disorders. In Bratty v. Attorney-General for Northern Ireland (1963), the House of Lords held that disease of the mind must be a disorder "due to some pathological state of the brain." Epilepsy and arteriosclerosis were treated as diseases of the mind because the mental incapacity was internally caused. Hypoglycaemia resulting from insulin injection was treated differently in R v. Quick (1973) as an external physical cause not amounting to a disease of the mind. The resulting distinction, criticised as medically arbitrary, has nevertheless been consistently applied.

The "wrong" prong and moral wrongness. The second limb asks whether the defendant knew their act was "wrong." In English law, "wrong" means legally wrong (R v. Windle 1952). In Australian law after Stapleton v. R (1952), "wrong" includes moral wrongness even where the defendant knew the act was illegal. In Canada, the Criminal Code § 16 uses the phrase "knowing that the act or omission was wrong" and Canadian courts have adopted the broader moral-wrongness interpretation. This single-word difference has produced divergent outcomes in cases where a deluded defendant commits a killing they know is legally prohibited but which they believe to be morally required by a divine command.

In Durham v. United States (1954), Judge David Bazelon of the US Court of Appeals for the District of Columbia Circuit rejected the M'Naghten test as too narrow. Monte Durham had a history of severe mental illness; the M'Naghten test would have found him sane because he understood what he was doing and knew it was wrong. Judge Bazelon announced what became known as the Durham or product rule: "an accused is not criminally responsible if his unlawful act was the product of mental disease or defect."

The product test had an appealing breadth. It freed juries and experts from the cognitive straightjacket of M'Naghten and allowed the full picture of psychiatric impairment to reach the fact-finder. In the eighteen years between Durham and its effective supersession, DC courts heard psychiatric testimony ranging across the full spectrum of diagnostic categories. That freedom proved the test's undoing. Psychiatrists testified to causation they could not scientifically demonstrate. The test provided no boundary on what counted as a "mental disease or defect" or what "product" required. In McDonald v. United States (1962), the DC Circuit tried to limit the test by defining mental disease as "any abnormal condition of the mind which substantially affects mental or emotional processes and substantially impairs behaviour controls." In Washington v. United States (1967), the court criticised the psychiatric dominance of the insanity question, noting that ultimate-issue testimony from psychiatric experts was displacing jury determination.

In United States v. Brawner (1972), the DC Circuit formally abandoned the Durham test and replaced it with the Model Penal Code formulation. The product rule is not operative in any US federal or state court today. It remains an important waypoint in the doctrinal development of the insanity defence and illustrates the risks of delegating legal responsibility determinations too completely to clinical expertise.

The American Law Institute's Model Penal Code, adopted in 1962, proposed a new insanity test at § 4.01: "A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of law."

The MPC test differs from M'Naghten in three important ways. First, it uses "appreciate" rather than "know." Appreciation implies a deeper, affective-cognitive understanding rather than bare factual knowledge. A defendant who "knows" abstractly that killing is wrong but whose psychosis renders that knowledge emotionally meaningless may satisfy M'Naghten but fail the MPC test. Second, the MPC adds a volitional prong: the inability to conform conduct to the requirements of law. This directly captures the irresistible-impulse cases that M'Naghten fails to address. Third, the MPC uses "substantial capacity" rather than a complete incapacity, acknowledging that mental disorder rarely produces total destruction of cognitive or volitional faculties.

Volitional prong controversy. The volitional prong was always controversial, but the controversy intensified after John Hinckley's 1981 shooting of President Reagan. Hinckley was charged under the DC Circuit's MPC-derived test. He was acquitted by reason of insanity in 1982. Public and legislative reaction was intense. The core objection was that the volitional prong was unverifiable: no psychiatrist can demonstrate that a defendant's will was truly irresistible rather than merely unresisted. Congress passed the Insanity Defense Reform Act of 1984 (IDRA), which for federal courts: (a) eliminated the volitional prong entirely, (b) raised the standard to "severe mental disease or defect," (c) shifted the burden of proof to the defendant by clear and convincing evidence, and (d) eliminated the affirmative insanity defence as a vehicle for arguing anything less than complete cognitive incapacity.

Post-Hinckley landscape. The IDRA resurrected a M'Naghten-like cognitive standard at the federal level. By the late 1980s, most states had either tightened their MPC-derived standards or reverted to M'Naghten. As of 2026, approximately twenty US states use a M'Naghten or similar cognitive-only standard, approximately fifteen use a version of the MPC substantial-capacity test, and a small number use hybrid tests. Four states (Kansas, Montana, Idaho, and Utah) have effectively abolished the affirmative insanity defence entirely, permitting the defendant only to present mental-illness evidence to negate the mens rea element of the offence.

India's insanity defence was housed in § 84 of the Indian Penal Code 1860 until the Bharatiya Nyaya Sanhita 2023 came into force on 1 July 2024. The BNS § 22 carries forward the substance of IPC § 84 with revisions to the statutory language. The provision reads: "Nothing is an offence which is done by a person who, at the time of doing it, by reason of mental illness, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law."

Several features of BNS § 22 merit attention.

"Mental illness" replaces "unsoundness of mind." IPC § 84 used the phrase "unsoundness of mind." The BNS 2023 replaces this with "mental illness," which aligns with the terminology of the Mental Healthcare Act 2017. The MHA 2017 § 2(s) defines "mental illness" as a substantial disorder of thinking, mood, perception, orientation, or memory that grossly impairs judgment, behaviour, capacity to recognise reality, or ability to meet the ordinary demands of life. This definitional linkage is significant: it connects the insanity defence to a statute designed for therapeutic rather than punitive purposes, and it potentially expands the range of conditions that can ground a defence beyond the traditional psychosis-centred interpretation.

Cognitive prong only. Like M'Naghten, BNS § 22 is purely cognitive. It requires incapacity to know the nature of the act or that the act is wrong or contrary to law. There is no volitional prong. The Supreme Court has consistently held, from Ratan Lal v. State of MP (1970) to Hari Singh Gond v. State of MP (2008), that partial insanity or diminished responsibility without complete cognitive incapacity does not support a BNS § 22 / IPC § 84 defence. The threshold is high: the defendant must show, on the preponderance of evidence, total inability to know, not merely impaired or distorted knowledge.

The Hari Singh Gond standard. In Hari Singh Gond v. State of MP (2008), the Supreme Court reviewed the elements required to establish the IPC § 84 defence (now BNS § 22). The court held that (1) the accused must have been suffering from mental illness at the time of the commission of the act; (2) that mental illness must have rendered the accused incapable of knowing the nature of the act or knowing that the act was wrong; (3) there must be a direct causal connection between the mental illness and the incapacity; and (4) the burden of proving this is on the accused, on the balance of probabilities. The court emphasised that every person is presumed to be of sound mind and that psychiatric evidence must specifically address the mental state at the time of the offence, not the defendant's general history of mental illness.

BSA § 39 and expert evidence. The admissibility of the expert psychiatric opinion that supports a BNS § 22 defence is governed by the Bharatiya Sakshya Adhiniyam 2023 § 39, which replaces IEA § 45 and provides that "when the court has to form an opinion upon a point of foreign law or of science or art, or as to identity of handwriting or finger impressions, the opinions upon that point of persons specially skilled in such foreign law, science, or art, or in questions as to identity of handwriting or finger impressions are relevant facts." The CFSL and government forensic psychiatry units (NIMHANS, IHBAS) provide the primary forensic psychiatric assessment service for courts.

Canada: the not-criminally-responsible-on-account-of-mental-disorder verdict. The Criminal Code of Canada § 16 provides: "No person is criminally responsible for an act committed or an omission made while suffering from a mental disorder that rendered the person incapable of appreciating the nature and quality of the act or omission or of knowing that it was wrong." The Canadian test uses "appreciating" rather than "knowing," importing the MPC's deeper-understanding standard into a M'Naghten-type cognitive formulation. A verdict of not criminally responsible on account of mental disorder (NCRMD) does not result in automatic release; the defendant is reviewed by a provincial review board which may order detention, conditional release, or absolute discharge based on public safety and treatment needs.

Australia: the various state and territory formulations. Australia has no uniform federal insanity standard. New South Wales uses a M'Naghten-derived test under the Mental Health and Cognitive Impairment Forensic Provisions Act 2020. Victoria's Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 adopts a broader formulation covering cognitive impairment due to any mental impairment, including neurological conditions and intellectual disability. The High Court in R v. Porter (1933) established that the defence requires the defendant to show a "defect of reason" resulting in incapacity to appreciate the nature or wrongfulness of the act, and that appreciation includes the affective dimension. This Porter-appreciation test is broader than the bare M'Naghten knowledge test.

Germany: the Schuldfähigkeit provisions. Germany's approach under StGB § 20 (criminal code) provides: "Any person who at the time of the commission of the offence is incapable of appreciating the unlawfulness of their actions or of acting in accordance with any such appreciation due to a pathological mental disorder, a profound consciousness disorder, debility or any other serious mental abnormality shall be deemed to act without guilt." The German standard notably includes a volitional prong (acting in accordance with any such appreciation) and covers a broader range of mental conditions including "profound consciousness disorders" and "serious mental abnormalities." § 21 provides for diminished responsibility (verminderte Schuldfähigkeit) where the threshold of § 20 is not fully met, allowing a mandatory reduction in sentence. The German approach is influential in comparative criminal law, demonstrating that the cognitive-volitional debate is not unique to common-law systems.

Burden of proof. In England and Wales, the defendant bears the burden of establishing the M'Naghten defence on the balance of probabilities. This was confirmed in R v. Podola (1960). In the US, Leland v. Oregon (1952) held that placing the burden on the defendant to prove insanity beyond a reasonable doubt did not violate due process, though most states require proof only by a preponderance of the evidence or by clear and convincing evidence. After the IDRA 1984, the federal standard is clear and convincing evidence. In India, the Supreme Court's interpretation places the burden on the defendant on the balance of probabilities.

The "guilty but mentally ill" alternative. Several US states, beginning with Michigan in 1975, introduced the "guilty but mentally ill" (GBMI) verdict as an alternative to not-guilty-by-reason-of-insanity. A GBMI verdict means the defendant is found guilty (and receives the applicable criminal sentence) but is entitled to psychiatric treatment while incarcerated. Critics argue that GBMI has depressed the use of the insanity defence by giving juries a middle-ground option that seems to address mental illness without acquittal, but it provides no guarantee of meaningful treatment and results in criminal punishment for people who might have succeeded under a strict insanity test.

Post-acquittal consequences. An insanity acquittal does not produce unconditional freedom in any jurisdiction. In England and Wales, the trial judge may impose a hospital order (MHA 1983 § 37), a supervision order, or an absolute discharge following an insanity verdict. In India, a defendant acquitted under BNS § 22 is typically subject to a magistrate's order for custody or care under the Mental Healthcare Act 2017, with the BNSS § 367 procedure governing subsequent proceedings if the issue of mental state arises again. In the United States, Jones v. United States (1983) held that a defendant acquitted by reason of insanity may be committed to a psychiatric institution without the same procedural protections required for civil commitment, because the insanity verdict itself constitutes proof of both mental illness and dangerousness sufficient to justify commitment. In Canada, the review board system ensures ongoing supervision and treatment.

The Hinckley release and legislative recalibration. John Hinckley Jr.'s unconditional release in 2022, after forty years under court supervision, prompted renewed US legislative debate about the balance between the criminal sanction and therapeutic response in insanity cases. His case illustrates the entire arc: the shooting in 1981, the acquittal in 1982 that triggered the IDRA 1984, decades of supervised release, and eventual full release. The IDRA was a congressional response to a single high-profile case; Hinckley's eventual release confirms that the underlying reform impulse was punitive as much as doctrinal.

The states that have abolished the affirmative insanity defence (Kansas, Montana, Idaho, and Utah) permit a defendant with mental illness to argue only that the illness negated the mens rea required by the offence. This is a significantly narrower door. For example, a charge of first-degree murder requires proof of premeditation. A defendant with a delusional disorder that caused them to believe they were killing an evil spirit rather than a human being might negate the premeditation element under the mens rea approach. But a defendant who understood they were killing a human being and intended to do so, but whose actions were driven by a psychosis that made the killing seem necessary or morally required, has no defence under the mens rea-only approach.

The US Supreme Court addressed whether abolishing the affirmative insanity defence violates due process in Kahler v. Kansas (2020). The Court held, 6-3, that the Constitution does not require states to provide an insanity defence beyond the right to negate mens rea, and that Kansas's approach was therefore constitutional. The dissent argued that the insanity defence is a fundamental principle of justice predating the constitution and that its total abolition is constitutionally impermissible. As of 2026, the four abolition states remain outside the mainstream, and no other state has moved to full abolition since the Kahler decision.