Forensic Psychiatry: Insanity Defence and Capacity

Daniel M'Naghten (the spelling in the case report is M'Naghten; the correct spelling of his name is M'Naghten or McNaughtan; subsequent legal usage standardised to M'Naghten) was tried at the Old Bailey in 1843 for the murder of Edward Drummond, private secretary to Prime Minister Sir Robert Peel. M'Naghten had been labouring under a delusion that the Tory party was persecuting him and shot Drummond believing him to be Peel. He was acquitted on grounds of insanity. The acquittal provoked debate in Parliament and questions to the House of Lords, which responded by posing five questions to the fifteen common-law judges. The answers to those questions, delivered by Lord Chief Justice Tindal on behalf of fourteen of the fifteen judges, constitute the M'Naghten Rules: (1843) 8 Eng Rep 718.

The core formulation is: 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 test has two limbs. The first limb (not knowing the nature and quality of the act) covers the defendant who was so disordered that they did not appreciate the physical nature of what they were doing, the classic example being a person in a psychotic state who believed they were chopping wood when they were in fact striking a person. The second limb (not knowing the act was wrong) covers the defendant who understood the physical nature of the act but, due to a disease of the mind, did not appreciate that it was legally or morally wrong.

"Disease of the mind" has been interpreted broadly in English case law to include epilepsy (R v. Sullivan 1984 House of Lords), sleepwalking (R v. Burgess 1991 Court of Appeal), and severe arteriosclerosis causing a confusional state (Kemp 1957). This breadth means that the insanity defence, if successful, leads to an absolute discharge order in theory but historically resulted in indefinite detention in a special hospital (Broadmoor, Rampton, Ashworth in the UK), which deterred defendants from raising it. The Criminal Procedure (Insanity) Act 1964 and the subsequent Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 reformed the disposal options and now allow for supervised discharge and community treatment orders as alternatives to hospital detention.

In England and Wales, the M'Naghten test applies in criminal proceedings. Scotland has a separate statutory framework under the Criminal Procedure (Scotland) Act 1995, with its own "not guilty by reason of insanity" verdict. Where the forensic psychiatrist is asked to give an opinion for a court, the obligations and admissibility standards governing that evidence are covered in the medico-legal expert in court.

India's IPC § 84 adopted a nearly identical formulation: "Nothing is an offence which is done by a person who, at the time of doing it, by reason of unsoundness of mind, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law." BNS 2023 § 22 reproduces this formulation. The Indian courts have consistently interpreted § 84 (and now § 22) through the lens of the M'Naghten Rules, and the extensive Supreme Court and High Court case law on § 84 maps closely to the English M'Naghten jurisprudence.

The Durham Rule emerged from Durham v. United States (1954) 214 F.2d 862 (DC Circuit Court of Appeals), decided by Judge David Bazelon. Monte Durham had a long psychiatric history and had been convicted of housebreaking. Judge Bazelon held that the M'Naghten cognitive test was too narrow because it excluded defendants who were severely mentally ill but who could formally "know" the nature and quality of their acts or that they were wrong in an abstract cognitive sense. He proposed a new rule: an accused is not criminally responsible if their unlawful act was the product of a mental disease or mental defect.

The Durham Rule was intended to open the insanity defence to a broader range of mental disorders and to give psychiatric evidence a more central role in the determination. In practice, it created two problems. First, "product" was undefined: it was difficult to determine when an act was "produced" by a mental condition versus when the mental condition was present but the act was causally independent of it. Second, the rule left the determination almost entirely to psychiatric experts, effectively transferring what is a normative legal judgment (should this person be excused?) into an empirical psychiatric determination (was this act the product of illness?). The DC Circuit recognised the problems and ultimately abandoned Durham in US v. Brawner (1972), adopting the Model Penal Code test instead.

The Model Penal Code (MPC), published by the American Law Institute in 1962, proposed what it called the substantial-capacity test: a person is not responsible for criminal conduct if, 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 has two significant improvements over M'Naghten. First, it uses "appreciate" rather than "know", which acknowledges that a defendant may abstractly know something is wrong while failing to genuinely appreciate its significance, a distinction psychiatrists consider clinically meaningful. Second, it adds a volitional limb: the inability to conform one's conduct to legal requirements covers impulse-control disorders where the defendant knows the act is wrong but cannot resist the impulse. The volitional limb is controversial and was abandoned federally after the Hinckley verdict.

John Hinckley Jr shot President Ronald Reagan in 1981 in Washington DC, inflicting serious wounds on Reagan and his press secretary James Brady. He was acquitted by reason of insanity in 1982 under the federal jurisdiction's version of the substantial-capacity test. The acquittal prompted Congress to pass the Insanity Defense Reform Act 1984, which adopted a more restrictive standard for federal courts: a defendant must prove by clear and convincing evidence that, as a result of severe mental disease or defect, they were unable to appreciate the nature and quality or the wrongfulness of their acts. The volitional limb was eliminated from federal law. At the time of writing, the federal standard is more restrictive than the MPC test, and several US states have eliminated the insanity defence entirely, substituting a guilty-but-mentally-ill verdict that does not result in acquittal.

Section 22 of the Bharatiya Nyaya Sanhita 2023 provides: "Nothing is an offence which is done by a person who, at the time of doing it, by reason of unsoundness of mind, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law."

This is IPC § 84 reproduced with minimal drafting change. The courts' interpretation of § 84 under the IPC therefore governs § 22. Three Supreme Court decisions anchor the Indian understanding.

In Surendra Pal Singh v. State of Uttar Pradesh (2005) SC, the Supreme Court reiterated that the burden of proving unsoundness of mind at the time of the offence lies on the accused, on the balance of probabilities. The presumption of sanity is the default position: section 22 / § 84 is an exception that the accused must establish. Evidence of a psychiatric history, even a severe one, is not sufficient alone: the crucial requirement is proof that the mental disorder was such, at the moment of the act, that the accused could not know the nature of the act or that it was wrong. Conduct around the act is highly relevant: planning, concealment, flight, and disposal of evidence are all inconsistent with the degree of disorientation required by the standard. The parallel forensic-psychology assessment tools used in risk and competency evaluations are covered in structured risk assessment.

In Anandi v. State of Uttar Pradesh (2008) SC, the Court emphasised that the mere existence of a mental illness does not establish the § 84 defence. The illness must produce the specific incapacity: not knowing the nature of the act, or not knowing it was wrong. A person with schizophrenia who kills in a planned manner, disposes of the weapon, and conceals the death does not satisfy § 84, because those actions are inconsistent with not knowing the nature of the act.

The Bachchan Singh v. State of Punjab (1980) SC judgment, decided in the capital-punishment context, established the principle that severe mental illness at the time of sentencing is a mitigating factor even where the § 84 insanity defence was not established. This is the Indian equivalent of the principle in Atkins v. Virginia (2002) US Supreme Court (intellectual disability bars execution) and Roper v. Simmons (2005) US (juvenile offending bars execution): mental-health status affects not only acquittal but also the proportionality of the ultimate sentence.

In practice, the § 84 defence in India is rarely successfully invoked. The courts have consistently emphasised the difficulty of establishing the required state of mind at the moment of the offence, particularly when the only post-facto evidence is psychiatric diagnosis rather than contemporaneous observations. The defence is more frequently raised in cases involving severe psychotic disorders such as paranoid schizophrenia and less frequently in cases involving personality disorders, substance use, or affective disorders, reflecting both the legal standard (unsoundness of mind at the time of the act) and the forensic-psychiatric literature on what conditions produce the relevant degree of cognitive disorganisation.

Selvi v. State of Karnataka (2010) 7 SCC 263 is the leading Indian authority on the right of a suspect or accused to refuse narco-analysis, polygraph testing, and brain electrical oscillations profile (BEOP, known as brain mapping or P300) testing. The three-judge bench, led by Chief Justice K.G. Balakrishnan, held unanimously that:

Compelled administration of narco-analysis, polygraph testing, or brain-mapping on an unwilling subject is a violation of Article 20(3) of the Indian Constitution (right against self-incrimination) and of the right to personal liberty and mental privacy under Article 21.

The reasoning extended beyond the textual right against self-incrimination. The court drew on the principle that the mind of an individual is the last private domain, and that compelling the disclosure of mental contents through chemical, physiological, or neurological techniques is a violation of that domain regardless of whether the disclosure happens to be truthful. This is a distinct constitutional principle that goes further than the US Fifth Amendment, which does not protect against compelled physical acts that reveal incriminating information (though it does protect testimonial communications).

The court also addressed the reliability question: the scientific validity of polygraph testing as a means of detecting deception is contested internationally, including by the American Psychological Association (APA) and the UK Polygraph Association, which have both noted that polygraph error rates in field conditions are substantially higher than in laboratory conditions, and that sophisticated subjects can learn to defeat the test. Brain-mapping presents distinct reliability issues because the P300 response that the technique interprets as recognition of crime-scene details can be produced by prior exposure to crime-scene photographs through news coverage rather than through participation in the crime.

The Selvi ruling specifies that these procedures may be conducted voluntarily with informed written consent, but any evidence obtained from an unwilling subject is inadmissible. The broader framework for expert psychiatric evidence reaching a court, including the Daubert and Frye admissibility tests, is examined in the medico-legal expert in court topic. The ruling also specifies that even where consent is given, the evidence may not be used as a direct statement of fact (as testimony) but only as a lead to independently corroborable evidence. Confessional statements made during narco-analysis are inadmissible as confessions; they may be used only to discover material objects that are independently seized and whose discovery is documented on a panchnama.

Internationally, the polygraph occupies different legal positions. In the United States, federal law generally bars the admission of polygraph evidence in criminal proceedings, though some states admit it by stipulation of both parties (Connecticut, Massachusetts). The UK permitted limited polygraph use for sex offenders on probation under the Offender Management Act 2007, but the polygraph is not used as evidence in criminal proceedings. Germany, France, and most EU member states exclude polygraph evidence.

Forensic psychiatry is engaged in civil law wherever a person's legal capacity to perform an act is in question. The three most frequently litigated capacities are testamentary capacity (the capacity to make a valid will), capacity to consent to medical treatment, and capacity to marry. In each case, the assessment is specific to the act in question and to the moment of the act, not a global assessment of the person's mental health.

Testamentary capacity in English law has been defined since Banks v. Goodfellow (1870) LR 5 QB 549: the testator must understand the nature of making a will and its effects; they must understand the extent of the property being disposed; they must understand the claims of those who might reasonably be expected to benefit (the "natural objects of their bounty"); and they must not be suffering from a disorder of the mind that poisons their affections, perverts their sense of right, or prevents the exercise of their natural faculties in disposing of their property. The Banks v. Goodfellow test was not superseded by the Mental Capacity Act 2005 (England and Wales) for testamentary capacity, though the MCA 2005 defines capacity for other purposes. In India, testamentary capacity is governed by the Indian Succession Act 1925, which requires that a testator be of sound mind, and the courts have interpreted this by reference to the Banks v. Goodfellow criteria. In the US, testamentary capacity is similarly a four-part test varying slightly by state jurisdiction.

Capacity to consent to medical treatment under the Mental Capacity Act 2005 (England and Wales) requires: the ability to understand the relevant information; the ability to retain it long enough to use it; the ability to use and weigh the information; and the ability to communicate the decision. The MCA 2005 creates a presumption of capacity, and any assessment of incapacity must be on the balance of probabilities with evidence specific to the decision in question. In India, the Mental Healthcare Act 2017 governs capacity to consent to mental health treatment, with specific provisions on advance directives and the rights of persons with mental illness in treatment decisions. In the US, the standards for medical decision-making capacity are governed by state law and by common-law principles informed by the President's Commission for the Study of Ethical Problems in Medicine (1982) report on capacity.

Capacity to marry is a distinct test. The common-law position in England and Wales, formulated in Re: Park (1954) P 112 and affirmed in Sheffield City Council v. E (2004) EWHC, requires that the person understand the nature of the marriage contract and the duties and responsibilities it creates. It is not necessary that the person understand the identity of their intended spouse in a sophisticated way; it is necessary that they understand they are entering a legally binding relationship with another person that affects property rights, next-of-kin status, and sexual relations. In India, the Hindu Marriage Act 1955 and the Special Marriage Act 1954 both provide that a marriage is void if either party is of unsound mind, incapable of giving valid consent by reason of mental disorder, or subject to recurrent attacks of insanity. The Indian Divorce Act 1869 includes similar provisions for Christians.

The forensic-psychiatric assessment for an insanity defence or a capacity determination follows a broadly similar structure across jurisdictions, though the legal test applied differs. The assessment involves: a clinical interview with the defendant or subject; review of all available psychiatric, medical, and social history; review of the circumstances and conduct surrounding the relevant act; administration of validated tools where relevant (the MacArthur Competence Assessment Tool for Treatment, MacCAT-T, for treatment-consent capacity; the Competency to Stand Trial Assessment Instrument, CAI, for fitness-to-plead assessments); and formulation of an opinion that is explicit about the diagnosis, the functional deficits arising from the diagnosis, and whether those deficits satisfy the legal standard.

The Royal College of Psychiatrists (RCPsych) in the UK, the American Academy of Psychiatry and the Law (AAPL) in the US, and the Indian Psychiatric Society (IPS) in India all publish ethical guidelines for forensic-psychiatric practice. The AAPL's practice guidelines (2015 edition) specify that a forensic psychiatrist asked to provide a court report should not be a treating clinician for the same person, because the roles are in tension: the treating clinician's duty is to the patient's wellbeing, while the forensic assessor's duty is to provide an honest and objective opinion to the court. This dual-role problem is more common in India, where the number of specialists in forensic psychiatry is limited, and it is more likely that the only available psychiatric expert will have prior clinical contact with the subject.

The DSM-5-TR (American Psychiatric Association, 2022) and ICD-11 Chapter 6 (WHO, 2022) are the diagnostic classification systems against which the forensic-psychiatric diagnosis is made. However, neither DSM-5-TR nor ICD-11 includes insanity criteria: the classification systems describe mental disorders; the legal standard for insanity, capacity, or culpability is determined by the court, not by the diagnostic manual. The forensic psychiatrist's role is to assess and describe the mental state and the functional consequences of any disorder, and then to apply those findings to the legal test the court is using.