1. Introduction

Criminal liability ordinarily assumes a voluntary act accompanied by the mental element required for the offence. The insanity defence qualifies that assumption where a mental condition fundamentally impaired legally relevant capacities at the time of the act. The difficult question is how much impairment is sufficient. Mental illness does not itself establish legal insanity: an accused may have a psychiatric diagnosis and still understand and regulate conduct. The central issue is the gap between knowing that conduct is wrong and being substantially able to control it.

2. The M’Naghten rule: origin and limits

In M’Naghten’s Case (1843) 8 ER 718, the judges stated that an accused is presumed sane unless, at the time of the act, a defect of reason arising from disease of the mind meant that the person did not know the nature and quality of the act, or did not know that it was wrong. The rule provides a relatively clear threshold and prevents every emotional disturbance from becoming an excuse. Its weakness is its cognitive focus. It does not expressly recognise a person who understands the act and its wrongfulness but suffers a severe disease-related impairment in the ability to inhibit it. Cognition and behavioural regulation are related but distinct. A person may identify a rule and anticipate consequences, yet have impaired executive functioning, emotional regulation or inhibitory control. Conversely, ordinary anger, jealousy, poor judgment, intoxication or a claim of irresistible desire cannot by themselves amount to insanity. Any broader test must distinguish severe mental incapacity from everyday difficulty resisting an impulse.

3. Indian statutory position

Section 22 of the Bharatiya Nyaya Sanhita, 2023 (BNS), substantially carries forward Section 84 of the Indian Penal Code, 1860. It excuses an act where, at the time of commission, unsoundness of mind made the person incapable of knowing the nature of the act, or that it was wrong or contrary to law. The statutory test remains cognitive and contains no independent volitional limb. The Mental Healthcare Act, 2017 modernises the framework for mental healthcare, but a clinical diagnosis under that Act is not equivalent to legal insanity under Section 22 BNS.

The distinction between medical and legal insanity is therefore central. Courts must assess functional incapacity at the time of the offence, not simply ask whether a disorder exists. The Bharatiya Sakshya Adhiniyam, 2023 also preserves the general evidentiary principle placing the burden of proving circumstances bringing a case within a general exception upon the accused, subject to the prosecution’s continuing burden to prove the offence.

4. Indian case law: principles and evidentiary burden

State of Madhya Pradesh v Ahmadulla (1961) 3 SCR 583 recognised that the relevant inquiry is the accused’s mental condition at the time of the act. In Dahyabhai Thakker v State of Gujarat (1964) 7 SCR 361, the Supreme Court explained that the accused’s burden to establish the general exception is on a civil standard, while the prosecution must still prove guilt beyond reasonable doubt. Even where the exception is not fully established, the evidence may create reasonable doubt about mens rea.

Bhikari v State of Uttar Pradesh (1966) 3 SCR 194 illustrates that a mere assertion of insanity or irrational conduct is insufficient; the evidence must support the statutory incapacity. In Ratan Lal v State of Madhya Pradesh (1970) 3 SCC 533, the Court emphasised that insanity must be assessed at the time of the crime and that oral, documentary and circumstantial evidence may be considered. It also cautioned against automatically disbelieving relatives or insisting that an accused must always produce expert testimony.

15. Indian case law: applying the test

In Bapu v State of Rajasthan (2007) 8 SCC 66, the Supreme Court reiterated that medical insanity and legal insanity are not interchangeable. The accused’s conduct before, during and after the occurrence may assist in determining the mental state at the material time. Hari Singh Gond v State of Madhya Pradesh (2008) 16 SCC 109 similarly held that mere abnormality of mind, partial delusion or unusual behaviour does not automatically satisfy the legal test.

In Srikant Anandrao Bhosale v State of Maharashtra (2002) 7 SCC 748, the Court considered a history of paranoid schizophrenia, prior treatment, family history, absence of a clear motive and conduct surrounding the killing. Taken together, the circumstances supported the inference that the accused was under a delusion at the relevant time. The case shows that circumstantial and medical evidence must be assessed cumulatively, rather than demanding direct proof of the accused’s internal state.

Surendra Mishra v State of Jharkhand (2011) 11 SCC 495 reaffirmed that the defence requires legal, not merely medical, insanity and that the incapacity must exist when the offence was committed. State of Rajasthan v Shera Ram (2012) 1 SCC 602 also discusses the legal standard and the relevance of evidence of mental disorder. In Devidas Loka Rathod v State of Maharashtra (2018) 7 SCC 718, the Court reiterated that the accused’s burden is not as onerous as the prosecution’s burden beyond reasonable doubt.

Most recently, Prakash Nayi alias Sen v State of Goa, Criminal Appeal No. 177 of 2018, judgment dated 12 January 2023, restated that the crucial time is the commission of the offence and that conduct preceding, attending and following the incident, read with medical evidence, can assist the court. The judgment also addresses the procedural duties concerning an accused person of unsound mind. Collectively, these cases provide a coherent evidentiary approach, but do not create an independent defence for impaired impulse control.

6. Forensic psychology and impulse-control disorders

Forensic psychology examines functioning at the time of the offence, not merely the label attached to a condition. Relevant domains include executive functioning, inhibition, planning, emotional regulation, cognitive flexibility, attention and the ability to anticipate consequences. Conditions classified among disruptive, impulse-control and conduct disorders include intermittent explosive disorder, kleptomania and pyromania. Their clinical features differ. None automatically establishes insanity, and the presence of an impulse or urge does not prove that it was irresistible.

The proper inquiry is functional and causal: did a qualifying mental disease or defect produce a substantial impairment in a capacity relevant to the offence? A person with kleptomania may understand that theft is unlawful; the diagnosis alone does not decide whether a particular theft meets the legal test. Similarly, an aggressive outburst associated with intermittent explosive disorder does not automatically excuse assault. The court must examine severity, context, available control, and the connection between the disorder and the specific act.

Retrospective assessment is difficult because examination often occurs after the event. Experts should consider contemporaneous behaviour, prior clinical records, treatment history, witness accounts, the events before and after the act, and alternative explanations. Planning, concealment or an apparent motive may be relevant but should not be treated as conclusive proof of sanity.

7. Neuroscience: assistance, not automatic exculpation

The prefrontal cortex and its networks contribute to planning, decision-making, inhibition and emotional regulation. Structural MRI, functional MRI, PET and neuropsychological testing may identify abnormalities or deficits relevant to these functions. Such evidence may assist an assessment of executive dysfunction, but it cannot by itself establish legal insanity or reveal precisely what the accused understood at the moment of an earlier offence.

Three cautions follow. First, brain scans show structure or activity under particular conditions, not a direct measurement of legal capacity. Second, group-level associations do not prove that a particular abnormality caused a particular offence. Third, people with similar findings may behave differently. Neuroscientific evidence should be admitted only where the method is reliable, the expert explains its limitations, and the evidence is linked to the legal issue. The ultimate decision remains judicial.

28. Comparative case law and alternative tests

England. R v Kemp [1957] 1 QB 399 treated arteriosclerosis as capable of constituting a disease of the mind. R v Sullivan [1984] AC 156 recognised epilepsy as capable of falling within that concept. R v Quick [1973] QB 910 distinguished hypoglycaemia caused by insulin as an external factor, while R v Hennessy (1989) 89 Cr App R 10 treated hyperglycaemia resulting from failure to take insulin as internal. R v Burgess [1991] 2 QB 92 concerned sleepwalking. These cases show how legal categories respond to medical causes, although the cognitive structure remains. R v Windle [1952] 2 QB 826 adopted a narrow legal interpretation of “wrong”.

United States. The “irresistible impulse” approach developed to address cases where mental disease impaired control despite awareness of wrongfulness. Parsons v State, 81 Ala 577 (1887), is an influential example of a volitional formulation. Durham v United States, 214 F.2d 862 (D.C. Cir. 1954), adopted a product test asking whether the unlawful act was the product of mental disease or defect; it was later abandoned because it gave insufficiently precise guidance and risked transferring the legal decision to experts. United States v Brawner, 471 F.2d 969 (D.C. Cir. 1972), adopted a substantial-capacity approach.

Section 4.01 of the American Law Institute’s Model Penal Code excuses a person who, because of mental disease or defect, lacks substantial capacity either to appreciate the criminality or wrongfulness of conduct, or to conform conduct to law. The second limb expressly recognises volitional impairment without requiring proof that resistance was literally impossible. Clark v Arizona, 548 U.S. 735 (2006), and Kahler v Kansas, 589 U.S. 271 (2020), confirm that American jurisdictions may adopt different insanity standards; the Constitution does not mandate one universal test.

9. Critical evaluation and proposed reform

The strongest argument for retaining M’Naghten is certainty: cognitive incapacity is a familiar and relatively manageable legal question. The strongest argument for reform is that knowledge and control are not identical. A legal system that ignores severe, disease-related impairment of behavioural regulation risks treating all persons who understand wrongfulness as equally capable of conforming their conduct. Yet an unrestricted volitional defence could excuse ordinary impulsiveness, invite exaggerated claims and make outcomes dependent on conflicting expert opinions.

India should therefore consider a carefully limited additional limb to Section 22 BNS. A person could be exempted where, at the time of the act, a severe and clinically established mental disease or defect caused the person to lack substantial capacity either to appreciate the nature or wrongfulness of the conduct, or to conform conduct to the requirements of law. This is a proposal for legislative consideration, not the present law. The threshold should exclude ordinary anger, poor judgment, antisocial conduct, criminal propensity and voluntary intoxication. A diagnosis or brain scan alone must never suffice.

A structured forensic report should address diagnosis, severity, functioning at the material time, causal connection, contemporaneous evidence and alternative explanations. Independent assessment may be appropriate in serious cases. Experts should explain the limits of their methods; the court must retain the final decision. A separate diminished-responsibility provision could be considered for substantial but non-exculpatory impairment, with clear statutory boundaries and proportionate sentencing consequences.

10. Conclusion

The M’Naghten rule remains valuable because it requires a serious connection between mental disease and the inability to understand an act or its wrongfulness. As an exclusive test, however, it gives inadequate attention to severe disorders that may impair behavioural regulation while leaving intellectual awareness intact.

Forensic psychology and neuroscience do not prove that an accused lacked free will, and must not become automatic excuses. They can provide disciplined evidence about capacities that the law should assess. The sound course is neither wholesale abandonment nor rigid preservation: retain M’Naghten’s cognitive foundation, add a narrowly defined substantial-capacity limb, and insist on reliable evidence and judicial control. Criminal responsibility should reflect both what a person understood and, in exceptional cases, what a serious mental disorder left the person substantially capable of doing.

Selected authorities and references

1. M’Naghten’s Case (1843) 8 ER 718; Parsons v State, 81 Ala 577 (1887).

2. BNS 2023, s 22; Bharatiya Sakshya Adhiniyam 2023, s 108; Mental Healthcare Act 2017; Indian Penal Code 1860, s 84 (historical).

3. State of MP v Ahmadulla (1961) 3 SCR 583; Dahyabhai Chhaganbhai Thakker v State of Gujarat (1964) 7 SCR 361.

4. Bhikari v State of UP (1966) 3 SCR 194; Ratan Lal v State of MP (1970) 3 SCC 533.

5. Srikant Anandrao Bhosale v State of Maharashtra (2002) 7 SCC 748; Bapu v State of Rajasthan (2007) 8 SCC 66.

6. Hari Singh Gond v State of MP (2008) 16 SCC 109; Surendra Mishra v State of Jharkhand (2011) 11 SCC 495.

7. State of Rajasthan v Shera Ram (2012) 1 SCC 602; Devidas Loka Rathod v State of Maharashtra (2018) 7 SCC 718.

8. Prakash Nayi alias Sen v State of Goa, Criminal Appeal No. 177 of 2018, judgment dated 12 January 2023.

9. R v Kemp [1957] 1 QB 399; R v Quick [1973] QB 910; R v Sullivan [1984] AC 156.

10. R v Hennessy (1989) 89 Cr App R 10; R v Burgess [1991] 2 QB 92; R v Windle [1952] 2 QB 826.

11. Durham v United States, 214 F.2d 862 (D.C. Cir. 1954); United States v Brawner, 471 F.2d 969 (D.C. Cir. 1972).

12. Clark v Arizona, 548 U.S. 735 (2006); Kahler v Kansas, 589 U.S. 271 (2020); Model Penal Code § 4.01 (1962).

13. National Research Council, Reference Manual on Scientific Evidence, 3rd ed (National Academies Press, 2011), ‘Reference Guide on Neuroscience’.