What is Settled Insanity?


Settled insanity is defined as a permanent or “settled” condition caused by long-term substance abuse and differs from the temporary state of intoxication.

In some United States jurisdictions “settled insanity” can be used as a basis for an insanity defence, even though voluntary intoxication cannot, if the “settled insanity” negates one of the required elements of the crime such as malice aforethought. However, US federal and state courts have differed in their interpretations of when the use of “settled insanity” is acceptable as an insanity defence and also over what is included in the concept of “settled insanity”.

Brief History

Early English common law recognized “settled insanity” as a complete defence for a person who is a habitual drunk but is not intoxicated at the time of the offense. A complete defence exonerates the accused and is a verdict of not guilty. Thus a person meeting the criteria of “settled insanity” is not considered responsible for his actions. Under the M’Naghten Rules, the first attempt in criminal law to address the issue of a mentally ill defendant, mental illness (or insanity) can be used as a defence if the defendant was unable to understand the criminal nature of his act or was unable to distinguish right from wrong at that time of the offense. The standard for an insanity defence developed by the American Law Institute requires a showing that the defendant’s mental illness prevented him from abiding by the law. Traditionally, under English common law intoxication, no matter to what degree, was not considered grounds for excusing the defendant’s criminal behaviour. However, over the last half century, there has been a movement toward allowing intoxication as evidence admissible in court to help the jury understand the criminal act and perhaps use it as an excuse or a mitigating factor.

Although voluntary intoxication is not considered an excuse for a criminal act, if it can be shown that the defendant was too intoxicated to deliberate or premeditate the wrongful act, (lacking malice aforethought), a defence of diminished responsibility/capacity, while not excusing the defendant from responsibility for the act, can serve to reduce the charges. Similarly, the plea of temporary insanity (applicable only to charges of murder) can serve to reduce the charges from first degree murder to assault or lessen the sentence if it can be shown that the defendant, due to intoxication, acted without deliberation or reflection (lacking malice aforethought), thus negating specific intent. However, ten states have rejected that specific intent can be negated by voluntary intoxication. Some jurisdictions allow voluntary intoxication in the context of a pre-existing mental disorder to qualify for an insanity defence.

Settled Insanity

Over time, as United States court ruling have been refining the insanity defence, the concept of “settled insanity” has been evolving. Originally, any form of insanity caused by the voluntary use of drugs was not an eligible defence for a criminal offense. The rationale was that any act that results from voluntary behaviour, including the voluntary intake of drugs, is choosing to increase the risk of breaking the law. Most United States jurisdictions now recognise that the long-term voluntary use of an intoxicating substance can cause a stable or “settled insanity” that can serve as a defence to a criminal act, especially if the long-term use exacerbated a pre-existing mental condition. For example, the concept of “settled insanity” includes the delirium tremens experienced by alcoholic during alcohol withdrawal, but it excludes temporary insanity of intoxication.

California law recognises “settled insanity” in the case of long-term use, but it does not recognise the temporary mental state caused by the recent consumption of an intoxicant as a sufficient defence. Moreover, recent rulings have upheld that the insanity need not be permanent to qualify as a defence of “settled insanity”. For instance, in a case where a woman with a substance-induced psychosis murdered her mother expert witnesses testified that the defendant had “personality defects” that predisposed her to psychosis, and that the psychosis was triggered by chronic substance abuse and the resulting nine months of hospitalisation. The defendant was found guilty because the court ruled that her insanity was temporary; however, the Supreme Court of California overturned the lower court’s guilty finding, ruling not guilty by reason of insanity, and stating that temporary psychosis not caused by an episode of intoxication constitutes settled insanity and qualifies as a complete defence.

In People v. Skinner (1985), the California Supreme Court further specified the criteria for “settled insanity”. The person must have a mental illness that is relatively stable over time, not caused solely by the length of time the substance was abused, and it must also meet the legal definition of insanity in that jurisdiction. Therefore, it appears that the court is stating that a threshold condition for the insanity defence exists when there is a permanent impairment caused by chronic substance abuse in a person with a pre-existing mental illness unrelated to substance abuse, but aggravated or set off by voluntary intoxication.

However, a 2007 decision by the Colorado Court of Appeals in People v. Grant upheld a lower court ruling that did not allow expert testimony on the defendant’s state of mind due to voluntary intoxication, thus ruling out any possibility that the issue of “settled insanity” might be raised.

Case Example

In Jervon Lamont Herbin v. Commonwealth of Virginia (1998), Herbin appealed his convictions of malicious wounding, two counts of forcible sodomy, abduction, and attempted rape. At the time of the offenses, Herbin was a long-term guest of the victim’s parents and was on crutches due to a gunshot wound received when he tried to enter his mother’s house while on crack cocaine a week before. After the victim’s parents had left the home, Herbin asked the victim to help him put on his socks. Then he threatened her with a knife and ordered her to take off her clothes. He slashed each of her breasts and stabbed her in the abdomen. Then he committed the sex offenses. The defendant asked the victim about whether the cars had manual or automatic transmission. When the victim told him that the car had a manual transmission, the victim was able to convince Herbin to call paramedics. Herbin pretended to call for assistance and eventually did call paramedics. He first told authorities that she was injured in the kitchen, and then he told the authorities that he was trying to commit suicide and she was injured trying to save him.

At trial, Herbin testified that he felt disturbed that day and had no memory of what happened except seeing the victim sitting in a pool of blood. He also testified to numerous stressors, including the gunshot wound, breaking up with his girlfriend, and recent attempts at suicide. He introduced extensive evidence of a history of physical and sexual abuse, drug abuse, and suicide attempts as well as a lengthy hospitalisation. Further, he had attended a sex offender treatment programme. On the other hand, Herbin called paramedics and properly recited the address, which had many numbers, and his voice on the 911 tape was controlled and calm. He gave paramedics a version of the events that was inconsistent with the facts and which exculpated him (show or declare that (someone) is not guilty of wrongdoing).

Virginia does allow for a drug induced “settled insanity” as a defence to crime. However, Virginia draws a distinction between intoxication and organic brain damage resulting from long term-substance abuse. In order to qualify for this defence, Herbin was required to provide substantial evidence of the presence of a mental disorder and the connection between it and the substance abuse. The term settled insanity refers to a degree of organic brain damage that would cause permanent, or settled, impairment. Herbin provided evidence of recent drug abuse and the victim’s mother testified that, the day before, she had provided him with prescription drug, Halcion. No lay witnesses testified that Herbin appeared to be under the influence of any substance that day.

To meet the standard for any kind of insanity, the degree of impairment must be severe:

The first portion of M’Naghten relates to an accused who is psychotic to an extreme degree. It assumes an accused who, because of mental disease, did not know the nature and quality of his act; he simply did not know what he was doing. For example, in crushing the skull of a human being with an iron bar, he believed that he was smashing a glass jar. The latter portion of M’Naghten relates to an accused who knew the nature and quality of his act. He knew what he was doing; he knew that he was crushing the skull of a human being with an iron bar. However, because of mental disease, he did not know that what he was doing was wrong. He believed, for example, that he was carrying out a command from God. White v. Com., 636 S.E.2d 353, 356–7, 272 Va. 619, __ (2006).

The appeals court held that a “settled insanity” defence requires substantial evidence of not only long-term and heavy substance abuse, but convincing evidence of a mental disorder that is caused by long-term substance abuse. In Herbin, the court found that:

The weight of authority in this country recognizes an insanity defense that is based on a mental disease or defect produced by long-term substance abuse.”  Commonwealth v. Herd, 413 Mass. 834, 604 N.E.2d 1294, 1299 (1992).   At the same time, “evidence of mere narcotics addiction, standing alone and without other physiological or psychological involvement, raises no issue of such a mental defect or disease as can serve as a basis for the insanity defense.” United States v. Lyons, 731 F.2d 243, 245 (5th Cir.1984) (citing cases).

Although appellant produced evidence of long-term and severe drug abuse, he did not present any evidence that he was suffering from any mental disease as a result of this drug abuse. See Hooks v. State, 534 So.2d 329, 353 (Ala.Crim.App.1987), aff’d sub nom. Ex parte Hooks, 534 So.2d 371 (Ala.1988). – See more at: http://caselaw.findlaw.com/va-court-of-appeals/1349937.html#sthash.YduwJJE7.dpuf

Although Herbin did provide evidence of substantial drug abuse in the past, he was unable to provide expert witness testimony of any mental disorder. The court held that the substance abuse did not serve as evidence for a “settled insanity” defence alone without the link to a mental disorder. No lay witnesses testified as to any behaviour consistent with insanity, even if lay testimony were admissible to establish the insanity defence in Virginia. Also, although Herbin did provide an extensive history of drug and sexual abuse, the court said no evidence showed either of these issues were causes or results of a mental disorder. Therefore, the appeals court upheld his conviction.


In those states allowing a “settled insanity” defence, the expert witness must first determine whether any symptoms of a mental disorder were present at the time of the offense, and if there were, determine if those symptoms were the result of a lasting impairment rather than caused by intoxication no matter how acute. If it can be shown that any existing the mental disorder is lasting or relatively enduring, then the expert must be able to show how the mental illness interfered with the defendant’s ability to know the nature and consequences of their behaviour and know that their behaviour was wrong, or if it impaired their ability to control their behaviour.

Aggressiveness, memory lapses and other common symptoms resulting from acute intoxication are not sufficient in themselves to excuse criminal acts. Further, not all psychotic reactions caused by substance abuse result in behaviour that can be related to the criminal act in a way that can support an insanity defence. The presence of psychosis does not mean that the criminal act was caused by the psychosis. A relationship must be shown to exist between the psychosis and the behaviour of the defendant.

What is Diminished Responsibility?


In criminal law, diminished responsibility (or diminished capacity) is a potential defence by excuse by which defendants argue that although they broke the law, they should not be held fully criminally liable for doing so, as their mental functions were “diminished” or impaired.

Refer to Chronology of UK Mental Health Legislation, Diminished Responsibility (in English Law) and Insanity Defence.

Diminished capacity is a partial defence to charges that require that the defendant act with a particular state of mind. For example, if the felony murder rule does not apply, first degree murder requires that the state prove beyond a reasonable doubt that the defendant acted with premeditation, deliberation, and the specific intent to kill – all three are necessary elements of the state’s case. If evidence exists, sufficient to create a reasonable doubt as to whether the defendant because of mental illness or “defect” possessed the capacity to premeditate, deliberate or form the specific intent to kill then the state cannot convict the defendant of first degree murder. This does not mean that the defendant is entitled to an acquittal. The defendant still might be convicted of second degree murder which only requires that the defendant act with general malice.

The defence’s acceptance in American jurisdictions varies considerably. The majority of states have adopted it by statute or case decision, and a minority even recognise broader defences such as “irresistible impulse”. Some US states restrict the defence to the charge of murder only where a successful defence will result in a manslaughter conviction instead of murder. Until recently, the Republic of Ireland did not accept the partial defence. The Irish Supreme Court had rejected the existence of the defence in DPP v O’Mahony. The case was recently abrogated, however, by enactment of the Criminal Law (Insanity) Act 2006, effective 01 June 2006. The act, in pertinent part, specifically adopted the partial defence for the charge of murder where a successful defence will result in a manslaughter conviction instead of murder.

The defence is to be contrasted with insanity which is a complete but affirmative defence. In most jurisdictions a defendant would be acquitted on the grounds of insanity if the defendant established to the satisfaction of the jury that he suffered from such a mental disease or defect that he was unable to appreciate the consequences of his actions or did not know what he was doing was wrong. As noted a successful insanity defence will result in acquittal although a number of jurisdictions have adopted the guilty but insane verdict. The defence of insanity and diminished capacity although clearly distinct are not inconsistent defences and both may be at issue in the same case. The critical distinctions are that diminished capacity is a partial, negating defence (negates an element of the state’s case) with the burden on the state to show that the defendant acted with the requisite state of mind while insanity is a complete but affirmative defence – the defendant bearing the burden of proving that he was legally insane.

This is an aspect of a more general insanity defence (see the M’Naghten rules). The defence “was first recognized by Scottish common law to reduce the punishment of the ‘partially insane’.” It developed from the practice of juries in the 19th century of returning verdicts of guilty with a recommendation as to mercy or mitigation of sentence to reflect any extenuating circumstances. In a series of decisions, given mainly by Lord Deas, a doctrine grew that various types of mental weakness could have the effect of reducing what would otherwise be a conviction for murder (which attracted capital punishment) to one for culpable homicide (where the courts had greater discretion in sentencing). An example of a “diminished capacity” might be extremely low intelligence. In the English case of R v Raven, a man who had a physical age of 22 years but a mental age of only 9 years felt provoked by homosexual advances and killed his perceived attacker. His mental deficiency was not in dispute and, since a child of 9 years would not have been criminally responsible (see s50 Children and Young Persons Act 1933), and his mental responsibility for his acts was substantially impaired, manslaughter was the only realistic verdict. The rationale of the defence is that, as a precondition to punishment, the criminal law requires conduct to be voluntary. If something interferes with the capacity of the individual to choose to break the law, this should be reflected by an excuse or exculpation. The law should balance the need to be fair to the individual wrongdoer, but equally offer some protection to society from a person who may not have complete control over their behaviour.

The effect of the defence varies between the jurisdictions. In some, it will result in full excuse and therefore produce a verdict of “not guilty”. In others, it offers only exculpation to a degree, resulting in the substitution of a lesser offence (e.g. manslaughter instead of murder) or a mitigated sentence.

English Law

Refer to Diminished Responsibility in English Law.

Section 2 of the Homicide Act 1957 states: (1) Where a person kills or is party to a killing of another, he shall not be convicted of murder if he was suffering from an abnormality of mental functioning which –

(a) arose from a medical condition

(b) substantially impaired D’s ability to do one or more of the things mentioned in subsection (1A), and

(c) provides an explanation for D’s acts and omissions in doing or being a party to the killing.

(1A) Those things are –

(a) to understand the nature of D’s conduct;

(b) to form a rational judgment;

(c) to exercise self-control.

(1B) For the purposes of subsection (1)(c), an abnormality of mental functioning provides and explanation of D’s conduct if it causes, or is a significant contributory factor in causing, D to carry out that conduct.

Scottish Law

Although the term is not used during the proceedings, the 1795 trial of Sir Archibald Gordon Kinloch for the murder of his brother Sir Francis Kinloch, 6th baronet of Gilmerton under Robert McQueen, Lord Braxfield is one of the earliest clear examples of recognition of diminished responsibility. Whilst found guilty, and usually expecting a death sentence, not only was Kinloch sentenced to life imprisonment instead, but two days after the judgement (17 July 1795) the accused was released into the care of a doctor (William Farquharson) on the understanding that Kinloch be kept in a secure environment (the doctor’s own house).

During the course of the 20th century the courts began to limit the mental conditions falling within diminished responsibility. In HM Advocate v Savage Lord Alness addressed the jury (at 51):

It is very difficult to put it in a phrase, but it has been put in this way: that there must be aberration or weakness of mind; that there must be some form of mental unsoundness; that there must be a state of mind which is bordering on, though not amounting to, insanity; that there must be a mind so affected that responsibility is diminished from full responsibility to partial responsibility. In other words, the prisoner in question must be only partially accountable for his actions. And I think one can see running through the cases that there is implied … that there must be some form of mental disease.

This statement became the authoritative version of the test for diminished responsibility and the various factors mentioned by Lord Alness were regarded as being cumulative in nature. The effect was that the test became difficult to satisfy, and the courts adopted the position that the scope of the plea was not to be further widened (e.g. Carraher v HM Advocate) held that the plea was not available to a person suffering from psychopathic personality. But in Galbraith v HM Advocate it was held that the formula in Savage was not to be read in a narrow sense, and it was not necessary that all the criteria in that formula had to be present. Furthermore, although the plea had to be based on some form of mental abnormality, that condition need not be one bordering on insanity. Instead the court ruled that diminished responsibility required the existence of an abnormality of mind which had the effect that the accused’s ability to determine or control his actings was substantially impaired. However, the Court excluded from the scope of the plea:

  • Any condition brought on by the consumption of drink or drugs; and
  • Psychopathic personality disorder.

The Scottish Law Commission reported in 2004 proposing changes to the law on insanity and diminished responsibility.


At present, diminished responsibility exists as a statutory partial defence in most Australian jurisdictions. The defence is only available in cases of murder and serves to reduce the offence to manslaughter. In Australia it has been the subject of sentencing concerns specifically in relation to the weight attributed to protection of the community when sentencing offenders found guilty of manslaughter on the grounds of diminished responsibility

In NSW, the partial defence of ‘diminished responsibility’ was replaced by the partial defence of “substantial impairment” in 1998. The burden is on the defendant to prove the defence, on the balance of probabilities. There are three conditions that the defendant must prove.

  • The first is the defendant must be suffering from an abnormality of the mind at the time of the acts/omissions causing death; see also the case of Byrne for the definition of ‘abnormality of the mind’.
  • Second, the abnormality must be the result of an underlying condition.
  • Third, the impairment must be so substantial as to warrant liability for murder being reduced to manslaughter.


Supreme Court of India bench headed by Justice Gogoi in a review petition upheld the principle of Diminished responsibility in the 2000 Dharmapuri bus burning and commuted to life imprisonment the death penalty given by the Salem district court and upheld by the Madras High Court and by another Supreme Court bench to three AIADMK party activists who had a set on fire a fully occupied bus with 44 girls and 2 lecturers of the Tamil Nadu Agricultural University on an educational tour to protest Jayalalithaa’s conviction in Pleasant Stay hotel case in this three college girls were burnt alive and 16 college girls suffered burn injuries were acting on mob frenzy and setting a legal precedent.

United States

Federal Law

The US Sentencing Guidelines provide, “A downward departure may be warranted if (1) the defendant committed the offense while suffering from a significantly reduced mental capacity; and (2) the significantly reduced mental capacity contributed substantially to the commission of the offense.”

State Law

California was the first state in the US to adopt the diminished capacity defence, beginning with People v. Wells and People v. Gorshen. The doctrine would soon be abolished by ballot initiative in 1982 following the negative publicity surrounding the case of Dan White, who had killed George Moscone and Harvey Milk. While White’s defence team did argue successfully for a ruling of diminished capacity, resulting in a verdict of voluntary manslaughter rather than murder, an urban legend that the defence had blamed White’s actions on the ingestion of sugar and junk food (the so-called “Twinkie defence”) sprang up out of inaccurate media coverage. One participant in the debate over diminished capacity rulings waved a Twinkie in the air to make his point. Currently, the California Penal Code states (2002), “The defense of diminished capacity is hereby abolished … there shall be no defense of diminished capacity, diminished responsibility, or irresistible impulse…”

What is Insanity Defence?


The insanity defence, also known as the mental disorder defence, is an affirmative defence by excuse in a criminal case, arguing that the defendant is not responsible for their actions due to an episodic or persistent psychiatric disease at the time of the criminal act.

This is contrasted with an excuse of provocation, in which the defendant is responsible, but the responsibility is lessened due to a temporary mental state. It is also contrasted with a finding that a defendant cannot stand trial in a criminal case because a mental disease prevents them from effectively assisting counsel, from a civil finding in trusts and estates where a will is nullified because it was made when a mental disorder prevented a testator from recognising the natural objects of their bounty, and from involuntary civil commitment to a mental institution, when anyone is found to be gravely disabled or to be a danger to themselves or to others.

Exemption from full criminal punishment on such grounds dates back to at least the Code of Hammurabi. Legal definitions of insanity or mental disorder are varied, and include the M’Naghten Rule, the Durham rule, the 1953 British Royal Commission on Capital Punishment report, the ALI rule (American Legal Institute Model Penal Code rule), and other provisions, often relating to a lack of mens rea (“guilty mind”). In the criminal laws of Australia and Canada, statutory legislation enshrines the M’Naghten Rules, with the terms defence of mental disorder, defence of mental illness or not criminally responsible by reason of mental disorder employed. Being incapable of distinguishing right from wrong is one basis for being found to be legally insane as a criminal defence. It originated in the M’Naghten Rule, and has been reinterpreted and moderniced through more recent cases, such as People v. Serravo.

In the United Kingdom, Ireland, and the United States, use of the defence is rare; however, since the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, insanity pleas have steadily increased in the UK. Mitigating factors, including things not eligible for the insanity defence such as intoxication (or, more frequently, diminished capacity), may lead to reduced charges or reduced sentences.

The defence is based on evaluations by forensic mental health professionals with the appropriate test according to the jurisdiction. Their testimony guides the jury, but they are not allowed to testify to the accused’s criminal responsibility, as this is a matter for the jury to decide. Similarly, mental health practitioners are restrained from making a judgment on the “ultimate issue” – whether the defendant is insane.

Some jurisdictions require the evaluation to address the defendant’s ability to control their behaviour at the time of the offense (the volitional limb). A defendant claiming the defence is pleading “not guilty by reason of insanity” (NGRI) or “guilty but insane or mentally ill” in some jurisdictions which, if successful, may result in the defendant being committed to a psychiatric facility for an indeterminate period.

Refer to Chronology of UK Mental Health Legislation, Diminished Responsibility (in English Law), and Diminished Responsibility (General).

Brief History

The concept of defence by insanity has existed since ancient Greece and Rome.[citation needed] However, in colonial America a delusional Dorothy Talbye was hanged in 1638 for murdering her daughter, as at the time Massachusetts’s common law made no distinction between insanity (or mental illness) and criminal behaviour. Edward II, under English Common law, declared that a person was insane if their mental capacity was no more than that of a “wild beast” (in the sense of a dumb animal, rather than being frenzied). The first complete transcript of an insanity trial dates to 1724. It is likely that the insane, like those under 14, were spared trial by ordeal. When trial by jury replaced this, the jury members were expected to find the insane guilty but then refer the case to the King for a Royal Pardon. From 1500 onwards, juries could acquit the insane, and detention required a separate civil procedure (Walker, 1985). The Criminal Lunatics Act 1800, passed with retrospective effect following the acquittal of James Hadfield, mandated detention at the regent’s pleasure (indefinitely) even for those who, although insane at the time of the offence, were now sane.

The M’Naghten Rules of 1843 were not a codification or definition of insanity but rather the responses of a panel of judges to hypothetical questions posed by Parliament in the wake of Daniel M’Naghten’s acquittal for the homicide of Edward Drummond, whom he mistook for British Prime Minister Robert Peel. The rules define the defence as “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 as not to know that what he was doing was wrong.” The key is that the defendant could not appreciate the nature of his actions during the commission of the crime.

In Ford v. Wainwright 477 U.S. 399 (1986), the US Supreme Court upheld the common law rule that the insane cannot be executed. It further stated that a person under the death penalty is entitled to a competency evaluation and to an evidentiary hearing in court on the question of his competency to be executed. In Wainwright v. Greenfield, the Court ruled that it was fundamentally unfair for the prosecutor to comment during the court proceedings on the petitioner’s silence invoked as a result of a Miranda warning. The prosecutor had argued that the respondent’s silence after receiving Miranda warnings was evidence of his sanity.

Mitigating Factors and Diminished Capacity

The United States Supreme Court (in Penry v. Lynaugh) and the United States Court of Appeals for the Fifth Circuit (in Bigby v. Dretke) have been clear in their decisions that jury instructions in death penalty cases that do not ask about mitigating factors regarding the defendant’s mental health violate the defendant’s Eighth Amendment rights, saying that the jury is to be instructed to consider mitigating factors when answering unrelated questions. This ruling suggests specific explanations to the jury are necessary to weigh mitigating factors.

Diminished responsibility or diminished capacity can be employed as a mitigating factor or partial defence to crimes and, in the United States, is applicable to more circumstances than the insanity defence. The Homicide Act 1957 is the statutory basis for the defence of diminished responsibility in England and Wales, whereas in Scotland it is a product of case law. The number of findings of diminished responsibility has been matched by a fall in unfitness to plead and insanity findings (Walker, 1968). A plea of diminished capacity is different from a plea of insanity in that “reason of insanity” is a full defence while “diminished capacity” is merely a plea to a lesser crime.

Non Compos Mentis

Non compos mentis (Latin) is a legal term meaning “not of sound mind”. Non compos mentis derives from the Latin non meaning “not”, compos meaning “having command” or “composed”, and mentis (genitive singular of mens), meaning “of mind”. It is the direct opposite of Compos mentis (of a sound mind).

Although typically used in law, this term can also be used metaphorically or figuratively; e.g. when one is in a confused state, intoxicated, or not of sound mind. The term may be applied when a determination of competency needs to be made by a physician for purposes of obtaining informed consent for treatments and, if necessary, assigning a surrogate to make health care decisions. While the proper sphere for this determination is in a court of law, this is practically, and most frequently, made by physicians in the clinical setting.

In English law, the rule of non compos mentis was most commonly used when the defendant invoked religious or magical explanations for behaviour.

Withdrawal or Refusal of Defence

Several cases have ruled that persons found not guilty by reason of insanity may not withdraw the defense in a habeas petition to pursue an alternative, although there have been exceptions in other rulings. In Colorado v. Connelly, 700 A.2d 694 (Conn. App. Ct. 1997), the petitioner who had originally been found not guilty by reason of insanity and committed for ten years to the jurisdiction of a Psychiatric Security Review Board, filed a pro se writ of habeas corpus and the court vacated his insanity acquittal. He was granted a new trial and found guilty of the original charges, receiving a prison sentence of 40 years.

In the landmark case of Frendak v. United States in 1979, the court ruled that the insanity defence cannot be imposed upon an unwilling defendant if an intelligent defendant voluntarily wishes to forgo the defence.

Psychiatric Treatments

Those found to have been not guilty by reason of mental disorder or insanity are generally then required to undergo psychiatric treatment in a mental institution[citation needed], except in the case of temporary insanity (see below). In England and Wales, under the Criminal Procedure (Insanity and Unfitness to Plead) Act of 1991 (amended by the Domestic Violence, Crime and Victims Act, 2004 to remove the option of a guardianship order), the court can mandate a hospital order, a restriction order (where release from hospital requires the permission of the Home Secretary), a “supervision and treatment” order, or an absolute discharge. Unlike defendants who are found guilty of a crime, they are not institutionalized for a fixed period, but rather held in the institution until they are determined not to be a threat. Authorities making this decision tend to be cautious, and as a result, defendants can often be institutionalised for longer than they would have been incarcerated in prison.

In Foucha v. Louisiana (1992) the Supreme Court of the United States ruled that a person could not be held “indefinitely”.

So far, in the United States, those acquitted of a federal offense by reason of insanity have not been able to challenge their psychiatric confinement through a writ of habeas corpus or other remedies. In Archuleta v. Hedrick, 365 F.3d 644 (8th Cir. 2004), the U.S. Court of Appeals for the Eighth Circuit the court ruled persons found not guilty by reason of insanity and later want to challenge their confinement may not attack their initial successful insanity defense:

The appellate court affirmed the lower court’s judgment: “Having thus elected to make himself a member of that ‘exceptional class’ of persons who seek verdicts of not guilty by reason of insanity…he cannot now be heard to complain of the statutory consequences of his election.” The court held that no direct attack upon the final judgment of acquittal by reason of insanity was possible. It also held that the collateral attack that he was not informed that a possible alternative to his commitment was to ask for a new trial was not a meaningful alternative.

Incompetency and Mental Illness

An important distinction to be made is the difference between competency and criminal responsibility.

  • The issue of competency is whether a defendant is able to adequately assist his attorney in preparing a defence, make informed decisions about trial strategy and whether to plead guilty, accept a plea agreement or plead not guilty.
  • This issue is dealt with in UK law as “fitness to plead”.

Competency largely deals with the defendant’s present condition, while criminal responsibility addresses the condition at the time the crime was committed.

In the United States, a trial in which the insanity defence is invoked typically involves the testimony of psychiatrists or psychologists who will, as expert witnesses, present opinions on the defendant’s state of mind at the time of the offense.

Therefore, a person whose mental disorder is not in dispute is determined to be sane if the court decides that despite a “mental illness” the defendant was responsible for the acts committed and will be treated in court as a normal defendant. If the person has a mental illness and it is determined that the mental illness interfered with the person’s ability to determine right from wrong (and other associated criteria a jurisdiction may have) and if the person is willing to plead guilty or is proven guilty in a court of law, some jurisdictions have an alternative option known as either a Guilty but Mentally Ill (GBMI) or a Guilty but Insane verdict. The GBMI verdict is available as an alternative to, rather than in lieu of, a “not guilty by reason of insanity” verdict. Michigan (1975) was the first state to create a GBMI verdict, after two prisoners released after being found NGRI committed violent crimes within a year of release, one raping two women and the other killing his wife.

Temporary Insanity

The notion of temporary insanity argues that a defendant was insane during the commission of a crime, but they later regained their sanity after the criminal act was carried out. This legal defence is commonly used to defend individuals that have committed crimes of passion. The defence was first successfully used by US Congressman Daniel Sickles of New York in 1859 after he had killed his wife’s lover, Philip Barton Key.

United States Law

In the United States, variances in the insanity defence between states, and in the federal court system, are attributable to differences with respect to three key issues:

  • Availability: whether the jurisdiction allows a defendant to raise the insanity defence;
  • Definition: when the defence is available, what facts will support a finding of insanity; and
  • Burden of proof: whether the defendant has the duty of proving insanity or the prosecutor has the duty of disproving insanity, and by what standard of proof.


In the United States, a criminal defendant may plead insanity in federal court, and in the state courts of every state except for Idaho, Kansas, Montana, and Utah. However, defendants in states that disallow the insanity defence may still be able to demonstrate that a defendant was not capable of forming intent to commit a crime as a result of mental illness.

In Kahler v. Kansas (2020), the US Supreme Court held, in a 6-3 ruling, that a state does not violate the Due Process Clause by abolishing an insanity defence based on a defendant’s incapacity to distinguish right from wrong. The Court emphasized that state governments have broad discretion to choose laws defining “the precise relationship between criminal culpability and mental illness.”


Each state and the federal court system currently uses one of the following “tests” to define insanity for purposes of the insanity defence. Over its decades of use the definition of insanity has been modified by statute, with changes to the availability of the insanity defence, what constitutes legal insanity whether the prosecutor or defendant has the burden of proof, the standard of proof required at trial, trial procedures, and to commitment and release procedures for defendants who have been acquitted based on a finding of insanity.

M’Naghten Test

he guidelines for the M’Naghten Rules, state, among other things, and evaluating the criminal responsibility for defendants claiming to be insane were settled in the British courts in the case of Daniel M’Naghten in 1843. M’Naghten was a Scottish woodcutter who killed the secretary to the prime minister, Edward Drummond, in a botched attempt to assassinate the prime minister himself. M’Naghten apparently believed that the prime minister was the architect of the myriad of personal and financial misfortunes that had befallen him. During his trial, nine witnesses testified to the fact that he was insane, and the jury acquitted him, finding him “not guilty by reason of insanity.”

The House of Lords asked the judges of the common law courts to answer five questions on insanity as a criminal defence, and the formulation that emerged from their review – that a defendant should not be held responsible for his actions only if, as a result of his mental disease or defect, he (i) did not know that his act would be wrong; or (ii) did not understand the nature and quality of his actions – became the basis of the law governing legal responsibility in cases of insanity in England. Under the rules, loss of control because of mental illness was no defence. The M’Naghten rule was embraced with almost no modification by American courts and legislatures for more than 100 years, until the mid-20th century.

Durham/New Hampshire Test

The strict M’Naghten standard for the insanity defence was widely used until the 1950s and the case of Durham v. United States case. In the Durham case, the court ruled that a defendant is entitled to acquittal if the crime was the product of his mental illness (i.e. crime would not have been committed but for the disease). The test, also called the Product Test, is broader than either the M’Naghten test or the irresistible impulse test. The test has more lenient guidelines for the insanity defence, but it addressed the issue of convicting mentally ill defendants, which was allowed under the M’Naghten Rule. However, the Durham standard drew much criticism because of its expansive definition of legal insanity.

Model Penal Code Test

The Model Penal Code, published by the American Law Institute, provides a standard for legal insanity that serves as a compromise between the strict M’Naghten Rule, the lenient Durham ruling, and the irresistible impulse test. Under the MPC standard, which represents the modern trend, a defendant 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 of his conduct or to conform his conduct to the requirements of the law.” The test thus takes into account both the cognitive and volitional capacity of insanity.

Federal Courts

After the perpetrator of President Reagan’s assassination attempt was found not guilty by reason of insanity, Congress passed the Insanity Defence Reform Act of 1984. Under this act, the burden of proof was shifted from the prosecution to the defence and the standard of evidence in federal trials was increased from a preponderance of evidence to clear and convincing evidence. The ALI test was discarded in favour of a new test that more closely resembled M’Naghten’s. Under this new test only perpetrators suffering from severe mental illnesses at the time of the crime could successfully employ the insanity defence. The defendant’s ability to control himself or herself was no longer a consideration.

The Act also curbed the scope of expert psychiatric testimony and adopted stricter procedures regarding the hospitalisation and release of those found not guilty by reason of insanity.

Guilty but Mentally Ill

As an alternative to the insanity defence, some jurisdictions permit a defendant to plead guilty but mentally ill. A defendant who is found guilty but mentally ill may be sentenced to mental health treatment, at the conclusion of which the defendant will serve the remainder of their sentence in the same manner as any other defendant.

Burden of Proof

In a majority of states, the burden of proving insanity is placed on the defendant, who must prove insanity by a preponderance of the evidence.

In a minority of states, the burden is placed on the prosecution, who must prove sanity beyond reasonable doubt.

In federal court, and in Arizona, the burden is placed on the defendant, who must prove insanity by clear and convincing evidence. See 18 U.S.C.S. Sec. 17(b); see also A.R.S. Sec. 13-502(C).


The insanity plea is used in the US Criminal Justice System in less than 1% of all criminal cases. Little is known about the criminal justice system and the mentally ill:

[T]here is no definitive study regarding the percentage of people with mental illness who come into contact with police, appear as criminal defendants, are incarcerated, or are under community supervision. Furthermore, the scope of this issue varies across jurisdictions. Accordingly, advocates should rely as much as possible on statistics collected by local and state government agencies.

Some US. states have begun to ban the use of the insanity defence, and in 1994 the Supreme Court denied a petition of certiorari seeking review of a Montana Supreme Court case that upheld Montana’s abolition of the defence. Idaho, Kansas, and Utah have also banned the defence. However, a mentally ill defendant/patient can be found unfit to stand trial in these states. In 2001, the Nevada Supreme Court found that their state’s abolition of the defence was unconstitutional as a violation of Federal due process. In 2006, the Supreme Court decided Clark v. Arizona upholding Arizona’s limitations on the insanity defence. In that same ruling, the Court noted “We have never held that the Constitution mandates an insanity defence, nor have we held that the Constitution does not so require.” In 2020, the Supreme Court decided Kahler v. Kansas upholding Kansas’ abolition of the insanity defence, stating that the Constitution does not require Kansas to adopt an insanity test that turns on a defendant’s ability to recognise that his crime was morally wrong.

The insanity defence is also complicated because of the underlying differences in philosophy between psychiatrists/psychologists and legal professionals. In the United States, a psychiatrist, psychologist or other mental health professional is often consulted as an expert witness in insanity cases, but the ultimate legal judgement of the defendant’s sanity is determined by a jury, not by a mental health professional. In other words, mental health professionals provide testimony and professional opinion but are not ultimately responsible for answering legal questions.

Australian Law

In Australia there are nine law units. All may have varying rules. In South Australia, the Criminal Law Consolidation Act 1935 (SA) provides that:

269C – Mental competence

A person is mentally incompetent to commit an offence if, at the time of the conduct alleged to give rise to the offence, the person is suffering from a mental impairment and, in consequence of the mental impairment:

(a) does not know the nature and quality of the conduct; or
(b) does not know that the conduct is wrong; or
(c) is unable to control the conduct.

269H – Mental unfitness to stand trial

A person is mentally unfit to stand trial on a charge of an offence if the person’s mental processes are so disordered or impaired that the person is:

(a) unable to understand, or to respond rationally to, the charge or the allegations on which the charge is based; or
(b) unable to exercise (or to give rational instructions about the exercise of) procedural rights (such as, for example, the right to challenge jurors); or
(c) unable to understand the nature of the proceedings, or to follow the evidence or the course of the proceedings.
In Victoria the current defence of mental impairment was introduced in the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 which replaced the common law defence of insanity and indefinite detention at the governor’s pleasure with the following:

  • the accused was suffering from a mental impairment; and
  • the mental impairment affected the accused so they either did not understand the nature and quality of the conduct, or did not know that it was wrong.

These requirements are almost identical to the M’Naghten Rules, substituting “mental impairment” for “disease of the mind”.

In New South Wales, the defence has been renamed the ‘Defence of Mental Illness’ in Part 4 of the Mental Health (Forensic Provisions) Act 1990. However, definitions of the defence are derived from M’Naghten’s case and have not been codified. Whether a particular condition amounts to a disease of the mind is not a medical but a legal question to be decided in accordance with the ordinary rules of interpretation. This defence is an exception to the Woolmington v DPP (1935) ‘golden thread’, as the party raising the issue of the defence of mental illness bears the burden of proving this defence on the balance of probabilities. Generally, the defence will raise the issue of insanity. However, the prosecution can raise it in exceptional circumstances: R v Ayoub (1984).

Australian cases have further qualified and explained the M’Naghten Rules. The NSW Supreme Court has held there are two limbs to the M’Naghten Rules, that the accused did not know what he was doing, or that the accused did not appreciate that what he was doing was morally wrong, in both cases the accused must be operating under a ‘defect of reason, from a disease of the mind’. The High Court in R v Porter stated that the condition of the accused’s mind is relevant only at the time of the actus reus. In Woodbridge v The Queen the court stated that a symptom indicating a disease of the mind must be prone to recur and be the result of an underlying pathological infirmity. A ‘defect of reason’ is the inability to think rationally and pertains to incapacity to reason, rather than having unsound ideas or difficulty with such a task. Examples of disease of the mind include Arteriosclerosis (considered so because the hardening of the arteries affects the mind.

Canadian Law

Criminal Code Provisions

The defence of mental disorder is codified in section 16 of the Criminal Code which states, in part:

16. (1) 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.

To establish a claim of mental disorder the party raising the issue must show on a balance of probabilities first that the person who committed the act was suffering from a “disease of the mind”, and second, that at the time of the offence they were either:

  1. Unable to appreciate the “nature and quality” of the act; or
  2. Did not know it was “wrong”.

The meaning of the word “wrong” was determined in the Supreme Court case of R. v. Chaulk [1990] 3 S.C.R. which held that “wrong” was NOT restricted to “legally wrong” but to “morally wrong” as well.

Post-Verdict Conditions

The current legislative scheme was created by the Parliament of Canada after the previous scheme was found unconstitutional by the Supreme Court of Canada in R. v. Swain. The new provisions also replaced the old insanity defence with the current mental disorder defence.

Once a person is found not criminally responsible (“NCR”), they will have a hearing by a Review Board within 45 days (90 days if the court extends the delay). A Review Board is established under Part XX.1 of the Criminal Code and is composed of at least three members, a person who is a judge or eligible to be a judge, a psychiatrist and another expert in a relevant field, such as social work, criminology or psychology. Parties at a Review Board hearing are usually the accused, the Crown and the hospital responsible for the supervision or assessment of the accused. A Review Board is responsible for both accused persons found NCR or accused persons found unfit to stand trial on account of mental disorder. A Review Board dealing with an NCR offender must consider two questions: whether the accused is a “significant threat to the safety of the public” and, if so, what the “least onerous and least restrictive” restrictions on the liberty of the accused should be in order to mitigate such a threat. Proceedings before a Review Board are inquisitorial rather than adversarial. Often the Review Board will be active in conducting an inquiry. Where the Review Board is unable to conclude that the accused is a significant threat to the safety of the public, the review board must grant the accused an absolute discharge, an order essentially terminating the jurisdiction of the criminal law over the accused. Otherwise, the Review Board must order that the accused be either discharged subject to conditions or detained in a hospital, both subject to conditions. The conditions imposed must be the least onerous and least restrictive necessary to mitigate any danger the accused may pose to others.

Since the Review Board is empowered under criminal law powers under s. 91(27) of the Constitution Act, 1867 the sole justification for its jurisdiction is public safety. Therefore, the nature of the inquiry is the danger the accused may pose to public safety rather than whether the accused is “cured.” For instance, many “sick” accused persons are discharged absolutely on the basis that they are not a danger to the public while many “sane” accused are detained on the basis that they are dangerous. Moreover, the notion of “significant threat to the safety of the public” is a “criminal threat.” This means that the Review Board must find that the threat posed by the accused is of a criminal nature.

While proceedings before a Review Board are less formal than in court, there are many procedural safeguards available to the accused given the potential indefinite nature of Part XX.1. Any party may appeal against the decision of a Review Board.

In 1992 when the new mental disorder provisions were enacted, Parliament included “capping” provisions which were to be enacted at a later date. These capping provisions limited the jurisdiction of a Review Board over an accused based on the maximum potential sentence had the accused been convicted (e.g. there would be a cap of 5 years if the maximum penalty for the index offence is 5 years). However, these provisions were never proclaimed into force and were subsequently repealed.

A Review Board must hold a hearing every 12 months (unless extended to 24 months) until the accused is discharged absolutely.

Accused Unfit to Stand Trial

The issue of mental disorder may also come into play before a trial even begins if the accused’s mental state prevents the accused from being able to appreciate the nature of a trial and to conduct a defence.

An accused who is found to be unfit to stand trial is subject to the jurisdiction a Review Board. While the considerations are essentially the same, there are a few provisions which apply only to unfit accused. A Review Board must determine whether the accused is fit to stand trial. Regardless of the determination, the Review Board must then determine what conditions should be imposed on the accused, considering both the protection of the public and the maintenance of the fitness of the accused (or conditions which would render the accused fit). Previously an absolute discharge was unavailable to an unfit accused. However, in R. v. Demers, the Supreme Court of Canada struck down the provision restricting the availability of an absolute discharge to an accused person who is deemed both “permanently unfit” and not a significant threat to the safety of the public. Presently a Review Board may recommend a judicial stay of proceedings in the event that it finds the accused both “permanently unfit” and non-dangerous. The decision is left to the court having jurisdiction over the accused.

An additional requirement for an unfit accused is the holding of a “prima facie case” hearing every two years. The Crown must demonstrate to the court having jurisdiction over the accused that it still has sufficient evidence to try the accused. If the Crown fails to meet this burden then the accused is discharged and proceedings are terminated. The nature of the hearing is virtually identical to that of a preliminary hearing.

German Law

According to section 20 of the German criminal code, those who commit an illegal act because a mental disorder makes them unable to see the wrong of the act or to act on this insight is considered not guilty.

Japanese Law

If the ability to recognise the right or wrong of action or the ability to act accordingly is lost due to a mental disorder, then the defendant cannot be pursued under Japanese criminal law so if this is recognised during a trial then an innocent judgement will be given. This is, however, rare, happening in only around 1 in 500,000 cases.

Polish Law

Insanity is determined through a judicial decision issued on the basis of expert opinions of psychiatrists and psychologists.

Russian Law

A forensic psychiatric examination is used to establish insanity. The result of the forensic examination is then subjected to a legal assessment, taking into account other circumstances of the case, from which a conclusion is drawn about the defendants sanity or insanity. The Criminal Code of Russia establishes that a person who during the commission of an illegal act was in a state of insanity, that is, could not be aware of the actual nature and social danger of their actions or was unable to control their actions due to a chronic mental disorder, a temporary mental disorder, or dementia is not subject to criminal liability.

Scottish Law

The Scottish Law Commission, in its Discussion Paper No 122 on Insanity and Diminished Responsibility (2003, pp.16-18), confirms that the law has not substantially changed from the position stated in Hume’s Commentaries:

We may next attend to the case of those unfortunate persons, who have plead the miserable defense of idiocy or insanity. Which condition, if it is not an assumed or imperfect, but a genuine and thorough insanity, and is proved by the testimony of intelligent witnesses, makes the act like that of an infant, and equally bestows the privilege of an entire exemption from any manner of pain; Cum alterum innocentia concilii tuetur, alterum fati infelicitas excusat. I say, where the insanity is absolute, and is duly proved: For if reason and humanity enforce the plea in these circumstances, it is no less necessary to observe a caution and reserve in applying the law, as shall hinder it from being understood, that there is any privilege in a case of mere weakness of intellect, or a strange and moody humor, or a crazy and capricious or irritable temper. In none of these situations does or can the law excuse the offender. Because such constitutions are not exclusive of a competent understanding of the true state of the circumstances in which the deed is done, nor of the subsistence of some steady and evil passion, grounded in those circumstances, and directed to a certain object. To serve the purpose of a defense in law, the disorder must therefore amount to an absolute alienation of reason, ut continua mentis alienatione, omni intellectu careat – such a disease as deprives the patient of the knowledge of the true aspect and position of things about him – hinders him from distinguishing friend from foe – and gives him up to the impulse of his own distempered fancy.

The phrase “absolute alienation of reason” is still regarded as at the core of the defence in the modern law (see HM Advocate v Kidd (1960) JC 61 and Brennan v HM Advocate (1977).

Nordic Countries

In the Nordic countries, insanity is not a defence; instead, it is the responsibility of the court system as such to consider whether the accused may have been psychotic or suffering from other severe mental defects when perpetrating the criminal act. This explains why, in Norway, the court considered the sanity of Anders Behring Breivik, even if he himself declared to be sane.

Rules differ between Nordic countries.

  • In Sweden, psychotic perpetrators are seen as accountable, but the sanction is, if they are psychotic at the time of the trial, forensic mental care.
  • In Denmark and Norway, psychotic perpetrators are declared guilty, but not punished. Instead of prison, they are sentenced to mandatory treatment. Still, important differences exist between Norway and Denmark.
  • In Norway, §44 of the penal code states specifically that “a person who at the time of the crime was insane or unconscious is not punished”.
  • In Denmark, §16 of the penal code states that “Persons, who, at the time of the act, were irresponsible owing to mental illness or similar conditions or to a pronounced mental deficiency, are not punishable”.
    • This means that in Denmark, ‘insanity’ is a legal term rather than a medical term and that the court retains the authority to decide whether an accused person is irresponsible.
  • In Finland, punishments can only be administered if the accused is compos mentis, of sound mind; not if the accused is insane (syyntakeeton, literally “unable to guarantee [shoulder the responsibility of] guilt”).
    • Thus, an insane defendant may be found guilty based on the facts and his actions just as a sane defendant, but the insanity will only affect the punishment.
    • The definition of insanity is similar to the M’Naught criterion above: “the accused is insane, if during the act, due to a mental illness, profound mental retardation or a severe disruption of mental health or consciousness, he cannot understand the actual nature of his act or its illegality, or that his ability to control his behavior is critically weakened”.
    • If an accused is suspected to be insane, the court must consult the National Institute for Health and Welfare (THL), which is obliged to place the accused in involuntary commitment if he is found insane.
    • The offender receives no judicial punishment; he becomes a patient under the jurisdiction of THL, and must be released immediately once the conditions of involuntary commitment are no longer fulfilled.
    • Diminished responsibility is also available, resulting in lighter sentences.

Usage and Success Rate

This increased coverage gives the impression that the defence is widely used, but this is not the case. According to an eight-state study, the insanity defence is used in less than 1% of all court cases and, when used, has only a 26% success rate. Of those cases that were successful, 90% of the defendants had been previously diagnosed with mental illness.

What are the M’Naghten Rules?


The M’Naghten rule (pronounced, and sometimes spelled, McNaughton) is any variant of the 1840s jury instruction in a criminal case when there is a defence of insanity:

that every man is to be presumed to be sane, 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 rule was formulated as a reaction to the acquittal in 1843 of Daniel M’Naghten on the charge of murdering Edward Drummond. M’Naghten had shot Drummond after mistakenly identifying him as UK Prime Minister Robert Peel, who was the intended target. The House of Lords asked a panel of judges, presided over by Sir Nicolas Conyngham Tindal, Chief Justice of the Common Pleas, a series of hypothetical questions about the defence of insanity. The principles expounded by this panel have come to be known as the “M’Naghten Rules”, though they have gained any status only by usage in the common law and M’Naghten himself would have been found guilty if they had been applied at his trial.

The rules so formulated as M’Naghten’s Case 1843 10 C & F 200 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” or “guilty but insane” 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 insanity defence is recognised in Australia, Canada, England and Wales, Hong Kong, India, the Republic of Ireland, New Zealand, Norway and most US states with the exception of Idaho, Kansas, Montana, Utah, and Vermont but not all of these jurisdictions still use the M’Naghten Rules. States that disallow the insanity defence still allow defendants to demonstrate that they are not capable of forming intent to commit a crime as a result of mental illness.

Refer to Chronology of UK Mental Health Legislation, Diminished Responsibility (in English Law), Diminished Responsibility (General), and Insanity Defence.

Historical Development

There are various justifications for the exemption of the insane from criminal responsibility. When mental incapacity is successfully raised as a defence in a criminal trial it absolves a defendant from liability: it applies public policies in relation to criminal responsibility by applying a rationale of compassion, accepting that it is morally wrong to punish a person if that person is deprived permanently or temporarily of the capacity to form a necessary mental intent that the definition of a crime requires. Punishment of the obviously mentally ill by the state may undermine public confidence in the penal system. 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 seen as grounds for leniency. In pre-Norman times in England there was no distinct criminal code – a murderer could pay compensation to the victim’s family under the principle of “buy off the spear or bear it”. The insane person’s family were expected to pay any compensation for the crime. In Norman times insanity was not seen as a defence in itself but a special circumstance in which the jury would deliver a guilty verdict and refer the defendant to the King for a pardon.

eo quod sensu carent et ratione, non-magis quam brutum animal iniuriam facere possunt nec feloniam, cum non-multum distent a brutis, secundum quod videri poterit in minore, qui si alium interficeret in minori ætate, iudicium non-sustineret. [Translation:] since they are without sense and reason and can no more commit a tort or a felony than a brute animal, since they are not far removed from brutes, as is evident in the case of a minor, for if he should kill another while under age he would not suffer judgment.

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”.

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 be excused only 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.

Each jurisdiction may have its own standards of the insanity defence. More than one standard can be applied to any case based on multiple jurisdictions.

The M’Naghten Rules

The House of Lords delivered the following exposition of the Rules:

the jurors ought to be told in all cases that every man is to be 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, or, if so, that it was wrong?”, 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 denying it; the standard of proof is on a balance of probabilities, that is to say that mental incapacity is more likely than not. 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 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 apply only 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:

  • In 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.
  • In 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.
  • In R v Quick; R v Paddison [1973] QB 910 the defendant committed an assault while in a state of hypoglycaemia caused by the insulin he had taken, the alcohol he had consumed and not eating properly. It was ruled that the judge should have left the defence of automatism open to him, so his conviction was quashed (he had pleaded guilty rather than not guilty by reason of insanity). This was where the internal/external divide doctrine was first expressed, probably due to judicial reluctance to hospitalise someone for a condition that could be cured by a sugar lump. It is doubtful that a jury would have accepted a defence of automatism, but nonetheless the issue should have been left to them.
  • In 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 v Attorney-General for Northern Ireland [1963] AC 386 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 QB 92 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 is partly based on risk of recurrence, whereby the High Court of Australia has expressed that the defence of automatism is unable to be considered when the mental disorder has been proved transient and as such not likely to recur. However, the distinction between insanity and automatism is difficult because the distinction between internal and external divide is difficult. Many diseases consist of a predisposition, considered an internal cause, combined with a precipitant, which would be considered an external cause. Actions committed while sleepwalking would normally be considered as “non-insane automatism”, but often alcohol and stress trigger bouts of sleepwalking and make them more likely to be violent. The diabetic who takes insulin but does not eat properly – is that an internal or external cause?

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 he is cutting a loaf of bread,
  • The defendant chops off a sleeping man’s head because he has 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

The interpretation of this clause is a subject of controversy among legal authorities, and different standards may apply in different jurisdictions.

“Wrong” was interpreted to mean legally wrong, rather than morally wrong, in the case of Windle 1952 2QB 826; 1952 2 All ER 1 246, where the defendant killed his wife with an overdose of aspirin; he telephoned the police and said, “I suppose they will hang me 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 defence was not allowed. Under this interpretation, there may be cases where the mentally ill know that their conduct is legally prohibited, but it is arguable that their mental condition prevents them making the connection between an act being legally prohibited and the societal requirement to conform their conduct to the requirements of the criminal law.

As an example of a contrasting interpretation in which defendant lacking knowledge that the act was morally wrong meets the M’Naghten standards, there are the instructions the judge is required to provide to the jury in cases in New York State when the defendant has raised an insanity plea as a defence:

… with respect to the term “wrong”, a person lacks substantial capacity to know or appreciate that conduct is wrong if that person, as a result of mental disease or defect, lacked substantial capacity to know or appreciate either that the conduct was against the law or that it was against commonly held moral principles, or both.

There is other support in the authorities for this interpretation of the standards enunciated in the findings presented to the House of Lords regarding M’Naghten’s case:

If it be accepted, as can hardly be denied, that the answers of the judges to the questions asked by the House of Lords in 1843 are to be read in the light of the then existing case-law and not as novel pronouncements of a legislative character, then the [Australian] High Court’s analysis in Stapleton’s Case is compelling. Their exhaustive examination of the extensive case-law concerning the defence of insanity prior to and at the time of the trial of M’Naughten establishes convincingly that it was morality and not legality which lay as a concept behind the judges’ use of “wrong” in the M’Naghten rules.

Offences of Strict Liability

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

Section 1 of the 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 section 5 of the Criminal Procedure (Insanity) Act 1964 (as amended):

  • 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 (see section 37 Mental Health Act 1983) with a restriction order limiting discharge and other rights (see section 41 Mental Health Act 1983).
  • In any other case the court may make:
    • A hospital order (with or without a restriction order);
    • A supervision order; or
    • An order for absolute discharge.


There have been four 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:
    • 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.
    • Diabetes has been held to facilitate a defence of insanity when it causes hyperglycaemia, but not when it causes hypoglycaemia.
    • 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 be detained only where proper account of objective medical expertise has been taken. As yet, no cases have occurred in which this point has been argued.
  • Ineffectiveness: 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 15 years; faced with this choice, it may be that defendants would prefer the certainty of the latter option. The defence of diminished responsibility in section 2(1) of the Homicide Act would reduce the conviction to voluntary manslaughter with more discretion on the part of the judge in regards to sentencing.
  • Scope: 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 that 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. 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.

Alternative Rules

The insanity defence article has a number of alternative tests that have been used at different times and places. As one example, the ALI test replaced the M’Naughten rule in many parts of the United States for many years until the 1980s; when in the aftermath of John Hinckley shooting President Ronald Reagan many ALI states returned to a variation of M’Naughten.

In Popular Culture

The M’Naghten rules are at the focus of John Grisham’s legal thriller A Time to Kill. The M’Naghten rules apply in the US State of Mississippi, where the plot is set, and using them is the only way for the lawyer protagonist to save his client.

What is Diminished Responsibility in English Law?


In English law, diminished responsibility is one of the partial defences that reduce the offence from murder to manslaughter if successful (termed “voluntary” manslaughter for these purposes).

This allows the judge sentencing discretion, e.g. to impose a hospital order under section 37 of the Mental Health Act 1983 to ensure treatment rather than punishment in appropriate cases. Thus, when the actus reus (Latin for “guilty act”) of death is accompanied by an objective or constructive version of mens rea (Latin for “guilty mind”), the subjective evidence that the defendant did intend to kill or cause grievous bodily harm because of a mental incapacity will partially excuse his conduct. Under s.2(2) of the Homicide Act 1957 the burden of proof is on the defendant to the balance of probabilities. The M’Naghten Rules lack a volitional limb of “irresistible impulse”; diminished responsibility is the volitional mental condition defence in English criminal law.

Refer to Chronology of UK Mental Health Legislation, Insanity Defence, and Diminished Responsibility (General).

The Statutory Provision

Section 2 of the Homicide Act 1957 states: (1) Where a person kills or is party to a killing of another, he shall not be convicted of murder if he was suffering from an abnormality of mental functioning which:

(a) arose from a medical condition;
(b) substantially impaired D’s ability to do one or more of the things mentioned in subsection (1A); and
(c) provides an explanation for D’s acts and omissions in doing or being a party to the killing.

(1A) Those things are:

(a) to understand the nature of D’s conduct;
(b) to form a rational judgment; and
(c) to exercise self-control.

(1B) For the purposes of subsection (1)(c), an abnormality of mental functioning provides and explanation of D’s conduct if it causes, or is a significant contributory factor in causing, D to carry out that conduct.

The defence has recently been amended by s. 52 of the Coroners and Justice Act 2009, which came into force on 4 October 2010.

How Substantial must the Impairment Be?

R v Golds1 provides a recent authority from the Court of Appeal Criminal Division on how the courts will interpret the term ‘substantial’. At paragraph [55] of Elias LJ’s judgement (following the paragraphing from the neutral citation given below) two senses of the word ‘substantial’ are identified: (i) something substantial is more than something which is merely trivial or minimal owing to the fact that it has “substance”, or (ii) something substantial is big or large (e.g. in the sense that a substantial salary is a large one). At paragraph [72] Elias LJ concludes by opining that the court should (i) leave interpretation of the word ‘substantial’ to the jury, but if asked for further help should (ii) direct them under the second meaning of the term (i.e. substantial meaning big).

Diminished Responsibility and Voluntary Intoxication

Voluntary intoxication will not satisfy the criterion that there must be an abnormality of mental functioning arising from a recognised medical condition (s.2(1)(a) Homicide Act 1957) and therefore cannot be relied upon as grounds for the partial defence2. However a person suffering from alcoholism that has led to an abnormality of mental function may have access to the partial defence3. In R v Gittens4 a defendant who suffered from depression killed his wife and stepdaughter after drinking and taking drugs for medication. The direction to a jury facing both diminished responsibility and drunkenness should be:

  • Would the defendant have killed as he did if he had not been drunk?
    • and if the answer to that is yes,
  • Was he suffering from diminished responsibility when he did so?

The more chronic forms of alcoholism and the long-term use of heroin and cocaine (see R v Sanderson5) can become a relevant factor where a craving for drink or drugs causes an abnormality of mind. This must be distinguished from the situation in which the abnormality of mind causes a craving for drink or drugs . R v Tandy6 held that where a defendant could show that she was suffering from an abnormality of the mind, that it was induced by disease (namely alcoholism), and that it substantially impaired her responsibility for her actions, then the defence of diminished responsibility would be made out. In the actual case, the craving for alcohol did not render the use of alcohol involuntary. The defendant was in control when she began drinking, and the state of mind in which she killed her daughter was merely induced by the alcohol. In R v Dietschmann,7 the House of Lords held that where a defendant suffers from an abnormality of mind within s2(1) also consumes alcohol before the killing, the jury should find him or her guilty of manslaughter if they are satisfied that, notwithstanding the alcohol consumed and its effect, the abnormality of mind substantially impaired the mental responsibility for the fatal acts. The sub-section does not require the abnormality of mind to be the sole cause of the defendant’s acts; even if the defendant would not have killed if he had not consumed alcohol, the causative effect of the alcohol does not prevent an abnormality of mind suffered by the defendant from substantially impairing his mental responsibility for the fatal acts. Dietschmann was later applied by the Court of Appeal in R v Hendy8.


  1. R v Golds[2014] EWCA Crim 748.
  2. R v Fenton (1975) 61 Cr App Rep 261.
  3. R v Tandy [1989] 1 WLR 350.
  4. R v Gittens (1984) QB 698.
  5. R v Sanderson (1994) 98 Cr. App. R. 325.
  6. R v Tandy (1989) 1 AER 267.
  7. R v Dietschmann [2003] UKHL 10.
  8. R v Hendy [2006] EWCA Crim 819.