Insanity and diminished responsibility

by TheEditor

Categories: Law, Reporting

This article is an exploration of two legal defences to unlawful homicide. The materials here are for educational purposes and subject to our disclaimer.

The word ‘insanity’ is obviously related to ‘insane’. In the UK people are not ‘certified as insane’, contrary to popular social media belief – driven by movies. If you wish to learn more this will be a substantial read and study. I will go into legal history, the law and how it connects to cases in current application. [Read our disclaimer – please].

Hereafter: LID = legal insanity defence, DR = diminished responsibility.

Diminished Responsibility [DR]

In July 2012 (over 10 years ago) I wrote on DR when the law was new. I reviewed and rechecked that analysis for accuracy. It was an intricate piece of law. Table 3 below gives the criteria for DR and compares with LID.

History of DR

The concept of diminished responsibility has a rich history in the uk legal system, evolving significantly over time. here’s an overview of its development:

Early concepts and development: The idea of diminished responsibility, while not formally recognised in early English law, has roots in the understanding that mental illness can impact culpability. This concept was evident in various legal systems, including Islamic law, where the principle of linking an individual’s actions to their intentions and mental state was recognised.

Scottish influence: Before its formal introduction in English law, the concept of diminished responsibility was established in Scottish case law. In 1867, a murder charge in Scotland was downgraded due to the accused’s ‘weakness of mind’. The term ‘diminished responsibility’ started being regularly used in Scottish courts in the early 20th century to reduce murder convictions to manslaughter.

Formal introduction in English law: The Homicide Act 1957 formally introduced the concept of diminished responsibility into English law. This act provided a partial defence for individuals charged with murder, which, if proven, could reduce the conviction to manslaughter. This development was partly influenced by the desire to provide an alternative to the death penalty for mentally disordered offenders, as seen in the Infanticide Act of 1922.

Evolution and modern adaptation: The criteria for diminished responsibility were further refined and updated by the Coroners and Justice Act 2009. This amendment reflected the evolving understanding of mental health issues and their impact on criminal responsibility.

Insanity law evolution: Alongside the development of diminished responsibility, the insanity defence also evolved. Historical cases, such as Arnold (1724) with the “Wild Beast” test and Hadfield (1800) with the “Good and Evil” test, laid the groundwork for what would become the M’Naghten Rules.

Legal and medical perspectives: Throughout its history, the definition and application of diminished responsibility have been influenced by both legal and medical perspectives. The legal interpretation often focuses on the defendant’s ability to understand their actions and distinguish right from wrong, while medical perspectives provide insights into the nature and impact of mental disorders.

Continued adaptation: The doctrine of diminished responsibility has continuously adapted to reflect current understandings of mental health, legal principles, and societal values. This evolution is evident in the significant changes and refinements made over the years in response to legal precedents, psychiatric advancements, and public discourse.

The history of diminished responsibility in the UK illustrates the legal system’s response to the complex interplay between mental health and criminal responsibility. It highlights the shift from a more punitive approach to one that considers the nuances of mental disorders in assessing culpability

Diminished Responsibility – Homicide Act 1957 – As Amended by Coroner’s Act 2009

I note importantly that:
1. S2(1) – contains a number of clauses that work conjunctively.
2. S2(1A) underpins S2(1)b
3. S2(1)c in being underpinned by S2(1B), creates an exceedingly high bar by introducing ‘causation’ as a legal test.

Most importantly, note that S2(1A) and S2(1B) feed back into S2(1). So in essence S2(1) is complete. 2(1A) and 2(1B) are there to raise the bar and narrow the ‘crack’.

Even if S2(1)a is satisfied in part for abnormality of mental functioning (AMF) arising from a recognised medical condition, the hurdles become more difficult to surmount, as the defendant’s AMF must be substantially impaired in three important respects. S2(1B) is really difficult to satisfy because ‘causation’ in whole or in part, is introduced. This test is likely to be failed in quite a number of cases if expert opinion evidence is not persuasive in:

  1. Showing path and the strength of causation, with robust references to the evidence.
  2. Showing how the AMF explains the defendant’s conduct (in whole), or (in part) that the AMF is a significantly contributory factor in causing  the conduct. But there is no escape from causation.

At least 5 of 6 primary tests must be satisfied and feed positively into S2(1) to meet the requirements for a successful defence.

Legal insanity defence [LID]


The history of ‘insanity’ goes back well before the 1300s. One could easily get lost in the very ‘addictive’ reading. Some key references are given at the end for those who may be interested.

Table 1 – summary of insanity defence from 1200s
The ‘Wild Beast’ test (1256): originating from Judge Henry de Bracton’s treatise written in 1256. This test posited that a person is insane if they were “totally deprived of his understanding and memory, and doth not know what he is doing, no more than an infant, a brute, or wild beast.”
Evolution in common law: The insanity defence saw significant development under common law, particularly with the case Rex v. Arnold in 1724. Here, the judge instructed the jury that for a defendant to be acquitted by reason of insanity, they must be a person “totally deprived of his understanding and memory, and doth not know what he is doing, no more than an infant…a brute, or a wild beast.”
James Hadfield case (1800): Lord Thomas Erskine, representing James Hadfield charged with the attempted assassination of King George III, advanced the “offspring of a delusion test.” This did not require “total insanity” but suggested acquittal if the defendant’s conduct was the result of their disease.
The Criminal Lunatics Act of 1800: Following Hadfield’s acquittal, this act mandated the continued confinement of defendants acquitted by reason of insanity, reflecting a shift in how such cases were handled post-verdict.

The main story starts in the mid-1800s in England.

Daniel M’Naghten, at the centre of the famous M’Naghten case, shaped the insanity defence in UK law. He was afflicted by a complex set of mental health issues. By the time he was 30, M’Naghten was experiencing intrusive paranoid thoughts, particularly fixated on police and church officials. His paranoia extended to a belief that he was being persecuted by the Tory party, following his voting against their candidate in an election. This led him to travel to London and France, where he felt the persecution continued.

M’Naghten’s mental state deteriorated further, leading him to purchase pistols under the guise of shooting birds. By 1842, he spent several weeks around Westminster and Whitehall, and on January 20, 1843, he shot Edward Drummond, the private secretary of Prime Minister Robert Peel, mistaking him for Peel himself. Drummond initially survived the shooting but succumbed to his injuries five days later.

Following his arrest, M’Naghten’s statements reflected his disturbed state of mind. He believed he was being persecuted by the Tories, who were, in his view, responsible for his deteriorating health and peace of mind. The trial that followed involved significant medical evidence, with experts attesting to M’Naghten’s delusions and the belief that these had deprived him of restraint over his actions. The defence focused on the concept of monomania, an insane fixation on a particular issue, subject, or person, arguing that M’Naghten’s delusions had eradicated his ability to distinguish right from wrong.

M’Naghten’s case was pivotal in shaping the legal understanding of insanity. It led to the formulation of the M’Naghten Rules, which centred around the ability of the accused to understand the nature and quality of their act and whether they knew it was wrong. The case underscored the principle that a person could be rational and coherent in most aspects but could still be considered insane due to a specific fixation over which they had little control.

This historical case illustrates the complexities involved in understanding and applying the insanity defence, highlighting the interplay between legal standards and mental health evaluations in determining criminal responsibility​

Table 2 – Analysis of M’Naghten – Insanity Defence
Aspect of M’Naghten’s ActionsDescription
Identification of TargetM’Naghten mistakenly identified Edward Drummond as Prime Minister Robert Peel, whom he intended to kill.
Preparation and PlanningHe purchased weapons (pistols) and spent time around Westminster and Whitehall, indicating premeditation.
Execution of the CrimeM’Naghten shot Drummond at point-blank range, demonstrating a deliberate act.
Awareness of His ActionsHe was aware of the physical act of shooting Drummond.
Response Post-ActM’Naghten made no attempt to escape after the shooting, suggesting some level of resignation or lack of concern for the consequences.
Expression of MotiveHe claimed that the Tories were persecuting him, showing an understanding of a motive, albeit based on delusional beliefs.
Perception of RealityHis statements and actions were influenced by delusions, indicating an impaired perception of reality.
Understanding of Right and WrongThe trial concluded that his ability to distinguish right from wrong was compromised by his delusions.


Table 3 – Legal criteria compared
No.Insanity Defence CriteriaDiminished Responsibility Criteria
1Disease of the Mind: The defendant must be suffering from a mental disorder, often described as a disease of the mind.Abnormality of Mental Functioning: Arising from a recognised medical condition, this abnormality is key to the defence.
2Defect of Reason: This disorder must cause a defect of reason, such that the defendant either lacks the ability to understand the nature and quality of their act, or does not know that what they are doing is legally or morally wrong.Substantial Impairment: The abnormality must substantially impair the defendant’s ability to understand the nature of their conduct, form rational judgments, or exercise self-control.
3Knowledge of Wrongfulness: The defendant, due to their mental disorder, either did not understand the nature and quality of the act, or did not know it was wrong.Link to the Offence: The abnormality must provide an explanation for the defendant’s conduct in committing the offence.
4Legal Standard: The defence is raised by the defendant and must be proven to the required legal standard.Burden of Proof: The defence bears the burden of proving the claim of diminished responsibility on the balance of probabilities.
6Focus on Cognitive Ability: The emphasis is on the cognitive ability to recognise right from wrong at the time of the act.Focus on Mental Abnormality: The emphasis is on how a specific mental abnormality impacted the defendant’s actions and decision-making processes.

Supplemental reading

  1. The Origins and Development of the “Wild Beast” Concept of Mental Illness and Its Relation to Theories of Criminal Responsibility (1965).

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