The Insanity Defence: M’Naghten Rules and Modern Application in England and Wales

In clinical negligence, personal injury, and criminal proceedings, the question of a defendant’s mental state at the time of an alleged offence can be pivotal. The insanity defence, governed by the M’Naghten Rules, remains a cornerstone of criminal law in England and Wales. For legal practitioners, understanding the clinical frameworks underpinning this defence and the role of expert psychiatric evidence is essential when advising clients or preparing cases involving allegations of diminished responsibility or lack of criminal intent.

Clinical Context: The M’Naghten Rules and Psychiatric Assessment

The M’Naghten Rules, established in 1843 following the case of Daniel M’Naghten, provide the legal test for insanity in criminal proceedings. To succeed in an insanity defence, the defendant must prove, on the balance of probabilities, that at the time of the alleged offence:

• They were suffering from a defect of reason, caused by a disease of the mind; and
• As a result, they either did not know the nature and quality of their act, or did not know that what they were doing was wrong.

Key Clinical Considerations

A disease of the mind is not limited to psychiatric disorders recognised in the DSM-5 or ICD-11; it may include organic conditions such as epilepsy, diabetes-related hypoglycaemia, or neurodegenerative diseases. The critical factor is whether the condition affects the defendant’s cognitive or volitional capacity at the material time.

Psychiatric expert witnesses typically assess:

• The presence of a recognised mental disorder;
• The impact of the disorder on the defendant’s understanding of their actions;
• Whether the disorder meets the threshold for a defect of reason under the M’Naghten Rules;
• The temporal relationship between the disorder and the alleged offence.

In cases with a history of psychiatric treatment, expert witnesses scrutinise medical records, particularly entries close to the date of the alleged offence, as these may provide insight into the defendant’s mental state at the relevant time.

Legal Relevance: Proceedings and Statutory Frameworks

The insanity defence is most commonly encountered in criminal proceedings but also informs other areas of medico-legal practice, including:

• Criminal trials: May result in a special verdict of not guilty by reason of insanity, leading to a hospital order, supervision order, or absolute discharge.
• Inquests: Where the deceased’s mental state is relevant to the cause of death, psychiatric expert evidence can assist in determining capacity.
• Civil proceedings: In clinical negligence claims involving psychiatric injury, the claimant’s mental state may be relevant to issues of consent or causation.
• Fitness to plead: Under the Criminal Procedure (Insanity) Act 1964, the court must determine whether a defendant is fit to stand trial.

Statutory Framework

The key statutes include:

• The M’Naghten Rules (common law);
• The Criminal Procedure (Insanity) Act 1964;
• The Mental Health Act 1983;
• The Mental Capacity Act 2005.

In criminal cases, the prosecution may challenge the insanity defence with its own psychiatric evidence, potentially leading to disputes between experts. Instructing solicitors should consider whether a Single Joint Expert (SJE) or party-appointed experts are appropriate.

Common Pitfalls and Disputes in Expert Evidence

Key Challenges

• Over-reliance on diagnostic labels: The M’Naghten Rules focus on functional impact, not specific diagnoses.
• Failure to consider organic causes: Conditions like epilepsy or brain injury may qualify as a disease of the mind.
• Inadequate scrutiny of records: Missing records may lead to incomplete or misleading opinions.
• Confusion with automatism: Automatism involves external factors, not insanity.
• Temporal element oversight: The defect of reason must be present at the time of the offence.

Expert Selection

Instructing solicitors should ensure the expert has the appropriate subspecialty expertise:

• Psychotic disorders: Consultant forensic psychiatrist.
• Neurological conditions: Consultant neurologist or neuropsychiatrist.
• Metabolic disorders: Endocrinologist or general physician.
• Substance misuse: Consultant addiction psychiatrist.

A multi-disciplinary approach may be necessary for complex cases involving multiple clinical factors.

Role of the Expert Witness: Report Content and Methodology

Essential Report Components

A psychiatric expert report should address:

• Background and instructions: Clear statement of the legal framework and questions.
• Sources of information: All documents reviewed, including limitations.
• Clinical history: Summary of psychiatric and medical history.
• Assessment of the defendant: Mental state examinations and cognitive testing.
• Diagnostic formulation: Opinion on recognised mental disorders and relevance to M’Naghten criteria.
• Impact on capacity: Retrospective analysis of the defendant’s symptoms.
• Alternative explanations: Consideration of automatism, intoxication, or malingering.
• Conclusion: Reasoned opinion on whether the defendant meets the M’Naghten criteria.

Joint Statements

Under CPR Part 35, experts may be required to produce a joint statement identifying areas of agreement and disagreement. This process ensures clarity and professionalism in contested opinions.

Practical Guidance for Solicitors

Best Practices

• Early instruction: Allows timely review of records and assessments.
• Subspecialty matching: Ensures the expert has relevant expertise.
• Provision of records: All relevant medical records should be provided.
• Clear instructions: Legal framework and specific questions must be outlined.
• Consideration of joint instruction: SJEs may reduce costs and disputes.
• Preparation for joint statements: Facilitate discussions between experts.
• Addressing capacity issues: Fitness for trial reports may be necessary.

Multi-Disciplinary Approach

In complex cases, a multi-disciplinary approach ensures all relevant clinical factors are considered. Multi-disciplinary medico-legal chambers can provide matched subspecialist expertise.

Conclusion

The insanity defence, governed by the M’Naghten Rules, is a nuanced area of medico-legal practice. Legal practitioners must understand the clinical frameworks and the role of expert psychiatric evidence when advising clients or preparing cases involving allegations of diminished responsibility. Key considerations include subspecialty-matched expertise, thorough scrutiny of medical records, and a clear grasp of the applicable legal tests.

In cases involving multiple clinical factors, a multi-disciplinary approach may be essential to ensure all relevant evidence is considered. Multi-disciplinary medico-legal chambers can provide the necessary expertise to strengthen the evidence base.