Mental Capacity Defences

Mental capacity defences in criminal law address situations where the defendant’s mental state affects their ability to form the required criminal intent. These defences include insanity, automatism, and intoxication. Understanding how each operates is essential for A-Level Law students, as they play a crucial role in determining criminal liability.

Insanity

• Definition: Insanity is a legal defence where the defendant is deemed not responsible for their actions due to a "defect of reason" arising from a "disease of the mind".
• Starting Point: The presumption is that every defendant is sane unless proven otherwise. The burden of proof for insanity lies with the defence, on the balance of probabilities.

The M’Naghten Rules:

• Established in R v M’Naghten (1843).

Three key criteria:

• There must be a defect of reason.
• This defect must be caused by a disease of the mind.
• As a result, the defendant did not know the nature and quality of the act, or did not know it was wrong.

Consequences of an Insanity Verdict:

Historically, a verdict of "not guilty by reason of insanity" could lead to indefinite detention in a mental hospital.

Under the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, courts can now order hospitalisation, supervision, or an absolute discharge, depending on the case.

Note: The insanity defence is not available for offences of strict liability.

Automatism

• Definition: Automatism is a defence where the defendant’s actions are involuntary due to an external factor, meaning they lack control over their movements.
• Starting Point: The law presumes voluntary behaviour; the defence must prove automatism.

Non-Insane Automatism:

• Occurs when involuntary actions arise from an external cause (e.g., concussion, hypoglycaemia).
• Successful use of this defence results in a complete acquittal.
• Examples: A person acting unconsciously due to a blow to the head or an adverse reaction to medication.

Insane Automatism:

If the cause is internal (e.g., epilepsy, diabetes-induced hyperglycaemia), it is classified as insanity, not automatism.

This follows the M’Naghten rules and leads to the consequences of an insanity verdict.

Intoxication

• Definition: Intoxication refers to a defendant’s mental state being affected by alcohol, drugs, or other substances.

Voluntary Intoxication:

• Where the defendant willingly consumes the substance.
• If the offence requires specific intent (e.g., theft, murder), intoxication may be a defence if it prevents forming intent.
• No defence for basic intent crimes (e.g., assault) – the law assumes recklessness in becoming intoxicated.

Involuntary Intoxication:

Occurs when the defendant is unaware they are consuming an intoxicating substance (e.g., spiked drinks).

If intoxication prevents the formation of mens rea, it may be a defence to both specific and basic intent offences.

Key Distinction:

Voluntary intoxication rarely succeeds as a defence for basic intent crimes.

Involuntary intoxication provides a better chance of acquittal if mens rea is absent.

Summary

• Insanity requires a "disease of the mind" and a defect of reason, proven via the M’Naghten rules.
• The consequences of an insanity verdict can include hospitalisation, supervision, or discharge.
• Automatism must arise from an external factor for a complete acquittal (non-insane automatism); internal causes fall under insanity.
• Intoxication is only a defence for specific intent crimes if involuntary, or if voluntary and intent cannot be formed.
• Always clearly distinguish between legal and medical definitions in your answers.
• Use case law to support your points: R v M’Naghten, R v Quick (non-insane automatism), R v Majewski (voluntary intoxication).

Exam Tips

• Address each defence individually and compare them where relevant.
• Use clear definitions and apply the rules methodically.
• Refer to relevant statutes and case law to enhance your answers.
• Remember, mental capacity defences are highly technical,  keep your answers structured and concise.