General Defences Part 1: Insanity, Automatism, Intoxication

This lesson covers three general defences that relate to the mental state of the defendant at the time of the offence: insanity, automatism, and intoxication. These defences are fundamental to criminal law because they address situations where the defendant may lack the mental capacity or awareness required for criminal liability.

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Key Terms

Term	Definition
Insanity	A defence based on the M'Naghten Rules (1843) — D was suffering from a defect of reason caused by a disease of the mind such that they did not know the nature and quality of their act, or did not know it was wrong
Automatism	A defence where D's actions were involuntary due to an external cause — D had no conscious control over their bodily movements
Intoxication	Where D's mental state is affected by alcohol or drugs — may negate mens rea for specific intent offences
Disease of the mind	A legal (not medical) term — any condition (internal to D) that affects the functioning of the mind
External cause	A cause originating outside D's body — distinguishes automatism from insanity
Specific intent offence	An offence that can only be committed with intention (e.g., murder, s.18 GBH, theft)
Basic intent offence	An offence that can be committed with recklessness (e.g., manslaughter, s.20 GBH, ABH, criminal damage)

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Insanity — The M'Naghten Rules (1843)

Background

The defence of insanity is governed by the M'Naghten Rules, which originate from M'Naghten's Case [1843]. Daniel M'Naghten shot and killed Edward Drummond, the private secretary to the Prime Minister Sir Robert Peel, believing Drummond was Peel. M'Naghten was suffering from paranoid delusions that Peel and the Tory party were persecuting him. He was found not guilty by reason of insanity.

The House of Lords subsequently formulated the M'Naghten Rules, which require the defence to prove on the balance of probabilities that at the time of the offence:

The Three Elements

Element	Requirement
1. Defect of reason	D's powers of reasoning must be impaired — not just a failure to use those powers (R v Clarke [1972])
2. Caused by a disease of the mind	The defect must be caused by an internal condition (disease of the mind is a legal, not medical, concept)
3. D did not know the nature and quality of the act, or did not know it was wrong	Either (a) D did not understand what they were physically doing (nature and quality), or (b) D did not know the act was legally wrong

Disease of the Mind

"Disease of the mind" is a legal concept, not a medical one. The courts have given it a very wide interpretation:

Case	Condition	Disease of the Mind?
R v Kemp [1957]	Arteriosclerosis (hardening of the arteries) causing temporary lack of consciousness	Yes — the condition affected the mind, even though it was a physical illness
R v Sullivan [1984]	Epilepsy causing an involuntary violent act during a seizure	Yes — any condition that manifests in violence and is prone to recur is a disease of the mind
R v Hennessy [1989]	Diabetes (hyperglycaemia) — high blood sugar causing a dissociative state	Yes — an internal condition, even though it could be controlled by medication
R v Burgess [1991]	Sleepwalking	Yes — an internal condition causing automatic behaviour

R v Windle [1952]

Windle killed his wife by administering a fatal dose of aspirin. He told the police, "I suppose I will hang for this." He raised the insanity defence, arguing he did not know what he was doing was wrong. The court held that "wrong" means legally wrong — not morally wrong. Since Windle knew his act was against the law (evidenced by his statement), the defence failed.

Principle: "Wrong" in the M'Naghten Rules means legally wrong, not morally wrong.

Outcome of a Successful Insanity Defence

If the insanity defence is successful, the verdict is "not guilty by reason of insanity" (under the Trial of Lunatics Act 1883, as amended by the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991). The judge has a range of disposals:

• Hospital order (with or without a restriction order)
• Supervision order
• Absolute discharge

For murder, a hospital order is mandatory.

Flowchart: Insanity

flowchart TD
    A[D raises insanity defence] --> B{Was D suffering from a defect of reason?}
    B -->|No — mere absent-mindedness: R v Clarke| C[Defence fails]
    B -->|Yes| D{Was the defect caused by a disease of the mind — internal cause?}
    D -->|No — external cause| E[Consider automatism instead]
    D -->|Yes — R v Kemp, R v Sullivan, R v Hennessy| F{Did D not know the nature and quality of the act?}
    F -->|Yes| G[Insanity defence succeeds — not guilty by reason of insanity]
    F -->|No| H{Did D not know the act was legally wrong? — R v Windle}
    H -->|Yes| G
    H -->|No| C

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Automatism

Definition

Automatism is a complete defence resulting in an acquittal. It applies where D's actions were involuntary — D had no conscious control over their bodily movements — and the cause of the involuntary behaviour was external (not a disease of the mind).

Hill v Baxter [1958]

The court gave classic examples of automatism: being stung by a swarm of bees while driving, having a stone thrown through the windscreen, suffering a sudden and unexpected blow to the head.

Distinction Between Insanity and Automatism

The critical distinction is the source of the involuntary behaviour:

Feature	Insanity	Automatism
Cause	Internal (disease of the mind)	External
Examples	Epilepsy, diabetes (hyperglycaemia), schizophrenia, sleepwalking	Blow to the head, effect of medication, hypoglycaemia (insulin), reflex actions
Verdict	Not guilty by reason of insanity	Complete acquittal
Burden of proof	On the defendant (balance of probabilities)	On the prosecution to disprove (beyond reasonable doubt)

The Diabetes Cases: Quick v Hennessy

The distinction is illustrated by two contrasting diabetes cases:

R v Quick [1973]

Quick, a diabetic, took insulin and then failed to eat. His blood sugar dropped dangerously low (hypoglycaemia), causing him to become violent and assault a patient. The Court of Appeal held that the cause of the automatism was external — the insulin injection — and therefore the defence was automatism (not insanity).

R v Hennessy [1989]

Hennessy, a diabetic, failed to take his insulin. His blood sugar rose dangerously high (hyperglycaemia), causing a dissociative state in which he took a car and drove while disqualified. The Court of Appeal held that the cause was internal — the diabetes itself — and therefore the condition was a disease of the mind (insanity, not automatism).