Introduction:

The insanity defence is one of the most complicated and debated topics in criminal law. It asks a basic but difficult question: when should mental illness excuse someone from criminal responsibility? Two major approaches have shaped how courts answer this question: the m’naghten rule, established in 1843, and the durham rule, introduced in 1954. M’naghten asks whether the defendant knew right from wrong, while durham asks whether mental illness caused the crime. The difference between these two tests reveals deeper debates about free will, when punishment is fair, how much power psychiatric experts should have in court, and whether society should punish or treat mentally ill offenders. Understanding these competing frameworks is important for anyone studying criminal law, as they continue to influence insanity standards today and shape discussions about mental health and criminal justice.

Definition and Legal Standards:

The M’Naghten Rule:

The M’Naghten Rule originates from an 1843 English case known as M'Naghten's Case [M’Naghten’s Case (1843) 10 Cl & Fin 200, 8 ER 718 (HL)]. Under this standard, a defendant is not criminally responsible if, at the time of the crime, they had such a severe mental defect that they did not know what they were doing, or if they did know what they were doing, they did not know it was wrong. This test has two parts: either the defendant did not understand what they were physically doing, or they did not understand that their actions were morally or legally wrong.

The M’Naghten standard focuses on knowledge and reasoning. It assumes that criminal responsibility requires the ability to think morally, to tell right from wrong. Courts using M’Naghten generally interpret “wrong” to mean legally wrong, though some courts have used a broader interpretation that includes moral wrong according to society’s standards. The rule makes it hard for defendants to win, as they must prove not just that they had a mental illness, but that this illness specifically prevented them from understanding their actions at the exact moment of the crime.

The Durham Rule:

The Durham Rule, also called the “product test,” came from a 1954 case called Durham v. United States in the D.C. Circuit Court [214 F.2d 862 (D.C. Cir. 1954)]. Judge (A) Bazelon wrote the opinion stating that “an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect.” This standard made the insanity defence much broader by shifting focus from knowledge to causation. Instead of requiring proof that the defendant could not tell right from wrong, Durham only asked whether a mental disease or defect caused the criminal conduct.

The Durham Rule was specifically designed to bring modern psychiatric understanding into legal proceedings. Judge Bazelon believed the M’Naghten framework was too restrictive and failed to account for mental illnesses that affect willpower rather than knowledge. Under Durham, defendants could potentially establish insanity even if they understood their actions were wrong, as long as mental illness was the cause of their behaviour. The rule gave significant authority to psychiatric experts to define “mental disease” and to establish connections between illness and conduct.

Illustration and Application:

To understand the practical differences between these standards, consider this example: (A) suffers from paranoid schizophrenia and believes his neighbour (B) is an alien planning to invade Earth. One evening, driven by intense delusions and voices in his head commanding him to “save humanity,” (A) breaks into (B)’s home and kills him. (A)’s psychotic episode is well-documented, and he has a history of severe mental illness.

Under the M’Naghten Rule, (A)’s defence would focus on whether he knew right from wrong when he killed (B). The prosecution might argue that (A) understood killing was legally wrong; he broke into the house at night, brought a weapon, and tried to hide his actions afterward. These facts suggest he knew what he was doing was wrong. However, (A)’s defence could argue that his delusions were so extreme that he genuinely believed he was saving humanity from an alien threat, meaning he did not know his actions were morally wrong within his delusional world. The outcome would depend on whether the jury accepts that (A)’s delusions completely destroyed his ability to make moral judgments. If (A) took steps to avoid getting caught, prosecutors might successfully argue he knew society viewed his actions as wrong, defeating the insanity defence under M’Naghten.

Under the Durham Rule, the analysis is completely different. (A)’s attorneys would not need to prove he lacked knowledge of right and wrong. Instead, they would simply need to show that:

(1) (A) suffered from a mental disease (paranoid schizophrenia)

(2) The killing was a “product” of that disease.

Given (A)’s documented psychotic delusions and hallucinations directly commanding the act, establishing this connection would be much easier. Even if (A) intellectually understood that killing was wrong, his defence could succeed by showing that his mental illness caused the criminal conduct. Durham’s broader standard would likely result in a successful insanity defence in this scenario, where M’Naghten might not.

Case Law Analysis:

M’Naghten’s Case (1843)

The case that created the M’Naghten Rule involved Daniel M’Naghten, a Scottish woodworker who had paranoid delusions that the Tory Party was persecuting him. In 1843, M’Naghten tried to kill Prime Minister Robert Peel but mistakenly killed Peel’s secretary, Edward Drummond, instead. At trial, medical testimony showed M’Naghten’s severe mental illness, and the jury found him not guilty by reason of insanity.

The not guilty verdict caused public outrage, which prompted the House of Lords to ask judges to clarify the insanity defence standards. The resulting guidelines established what became known as the M’Naghten Rules, emphasising that insanity required proof that the defendant could not understand what they were doing or know it was wrong. This case reflects Victorian-era beliefs about reasoning, moral responsibility and mental illness. The judges prioritised cognitive capacity (the ability to reason and understand moral boundaries) as the essential requirement for criminal responsibility.

The M’Naghten case established that criminal liability requires rational understanding. The rule assumes that punishment is only fair when the defendant had the mental ability to understand that their conduct was wrong. However, critics note that M’Naghten’s narrow focus on knowledge fails to account for mental illnesses that preserve intellectual understanding while destroying self-control; the person knows right from wrong but cannot control their behaviour.

Durham v. United States (1954)

Monte Durham had a long history of mental illness and multiple psychiatric hospitalizations. In 1951, he was charged with breaking into a house, and his defence attorney argued that Durham’s mental illness should excuse his criminal conduct. The trial court applied the M’Naghten Rule and rejected the insanity defence. On appeal, the D.C. Circuit Court, led by Judge A. Bazelon reversed the conviction and established a new standard.

Judge Bazelon criticized M’Naghten as outdated and inconsistent with modern psychiatric understanding of mental illness. He argued that focusing only on cognitive capacity ignored other types of mental disease, particularly those affecting emotional control and willpower. The Durham Rule was explicitly designed to give psychiatrists greater influence in determining criminal responsibility, allowing them to testify about causation rather than being limited by legal definitions focused on cognitive capacity.

The Durham Rule proved controversial and did not last long. Courts struggled with defining “mental disease” and “product,” leading to inconsistent results. Critics argued that Durham essentially gave the ultimate legal question of criminal responsibility to psychiatric experts, taking away the jury’s role. By 1972, even the D.C. Circuit abandoned Durham in United States v. Brawner [471 F.2d 969 (D.C. Cir. 1972)], adopting instead the Model Penal Code’s substantial capacity test, which combined cognitive and volitional elements while maintaining clearer legal standards.

The Durham case represents an ambitious but ultimately unsuccessful attempt to modernize the insanity defence by including broader psychiatric perspectives. While its intentions were progressive, the rule’s lack of clear boundaries and heavy reliance on expert testimony created practical difficulties. Durham’s brief use highlighted the challenges of translating medical concepts into workable legal standards and showed the legal system’s reluctance to give ultimate questions of guilt entirely to psychiatric authority.

Practical Application:

Application Under M’Naghten:

In modern American jurisdictions applying M’Naghten (about half of U.S. states), defendants face major obstacles in establishing insanity. The cognitive focus requires clear evidence that mental illness completely destroyed the defendant’s ability to understand their actions or recognise wrongfulness. Prosecutors routinely defeat insanity defences by showing evidence of rational planning, attempts to hide crimes, or efforts to avoid getting caught- all suggesting the person’s thinking was still working.

For example, in cases involving schizophrenia, prosecutors emphasise that people with psychosis often retain enough cognitive capacity to understand legal prohibitions, even if their delusions motivate criminal conduct. A defendant who hears voices commanding violence might still know that killing violates the law, defeating the M’Naghten defence. Similarly, defendants with severe depression who understood their actions were wrong typically cannot use M’Naghten, even if their illness made them unable to control their impulses.

The practical effect is that M’Naghten limits successful insanity defences to cases of profound cognitive impairment, severe intellectual disabilities, extreme psychotic episodes that completely destroy reality testing, or deteriorated organic brain conditions that destroy comprehension. The standard generally excludes defendants whose mental illness primarily affects emotional regulation, impulse control, or willpower rather than rational understanding.

Application Under Durham:

During Durham’s brief period of use in the D.C. Circuit (1954-1972), the standard dramatically expanded insanity defences. Defendants could establish a lack of criminal responsibility through psychiatric testimony establishing diagnosis and causation, without the strict cognitive requirements of M’Naghten. This broader approach helped defendants with conditions like severe personality disorders, impulse control disorders, or emotional disturbances that might not destroy cognitive capacity but substantially influenced criminal behaviour.

However, Durham created significant practical problems. Courts struggled to define “mental disease,” with some psychiatrists testifying that conditions like pathological gambling or kleptomania qualified. The causal “product” requirement proved equally unclear- determining whether mental illness caused criminal conduct or merely provided context for voluntary wrongdoing lacked clear standards. These unclear definitions led to inconsistent outcomes and concerns that too many defendants were escaping criminal responsibility.

Furthermore, Durham shifted enormous power to psychiatric experts, whose testimony essentially determined legal outcomes. Juries had minimal independent basis for evaluating insanity when expert witnesses disagreed about whether conduct was the “product” of mental disease. This delegation of the ultimate legal question troubled courts and contributed to Durham’s eventual abandonment.

Contemporary Standards:

Most American jurisdictions today apply variations of M’Naghten, though many have adopted hybrid approaches. The Model Penal Code test, used in federal courts and many states, combines cognitive and volitional elements, asking whether defendants lacked “substantial capacity” to appreciate wrongfulness or conform conduct to law. Some states have abolished the insanity defence entirely, replacing it with “guilty but mentally ill” verdicts that recognise mental illness while maintaining criminal responsibility.

The practical reality is that insanity defences rarely succeed under any standard. Studies show insanity defences are raised in less than one percent of felony cases and succeed in only a fraction of those. Public scepticism, concerns about faking, and the challenge of proving mental states at a past moment all contribute to low success rates. Even when insanity defences succeed, defendants typically face lengthy involuntary commitment in psychiatric facilities, often exceeding the prison sentences they would have served if convicted.

Conclusion:

The M’Naghten and Durham Rules represent fundamentally different approaches to criminal insanity. M’Naghten’s cognitive test emphasises rationality and moral understanding, rooted in classical concepts of free will and moral responsibility. It maintains strict limits on the insanity defence by requiring proof that mental illness destroyed the defendant’s capacity to know right from wrong. Durham’s product test adopted a more expansive, medically-oriented approach, recognising that mental illness affects human behaviour in ways that extend beyond simple cognitive impairment.

While Durham attempted to modernise criminal law by incorporating psychiatric insights, its practical difficulties, such as vague definitions, inconsistent applications and excessive reliance on expert testimony, led to its abandonment. M’Naghten, despite criticism as outdated and overly narrow, remains the dominant standard, valued for its relative clarity and limitation of the insanity defence to cases of profound cognitive impairment.

Contemporary debates continue to address questions these rules raise: How should criminal law account for mental illness? What role should psychiatric expertise play in determining criminal responsibility? And fundamentally, when does mental illness sufficiently compromise free will to justify excusing someone from criminal liability? These questions ensure that the tension between M’Naghten’s cognitive focus and Durham’s causal approach remains relevant to modern criminal law, even as courts seek standards that balance justice, public safety and compassionate treatment of mentally ill offenders.

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