What you'll learn

• How insanity, automatism and intoxication affect criminal liability.
• The key rules from the M’Naghten rules and major case law.
• The difference between internal causes and external causes.
• How to apply these defences in problem questions using clear legal reasoning.

The starting point: criminal liability needs a voluntary, blameworthy act

Most crimes require both actus reus and mens rea.

Mental capacity defences ask whether the defendant’s mental state or control over their body was so affected that it would be unfair, or legally inaccurate, to convict them in the usual way.

Use this route map to separate the three main areas before you start applying cases.

Insanity

Definition

Insanity is a legal defence where the defendant proves that, because of a recognised legal “disease of the mind”, they did not understand what they were doing or did not know that it was legally wrong.

The defence is based on the M’Naghten rules.

The M’Naghten rules

In M’Naghten (1843), the defendant suffered paranoid delusions and shot the Prime Minister’s secretary, believing he was being persecuted. The case established the classic test for insanity.

To succeed, the defendant must show:

1. There was a defect of reason;
2. Caused by a disease of the mind;
3. So that the defendant either:
   • did not know the nature and quality of the act; or
   • did not know that the act was wrong.

Presumption of sanity and burden of proof

The starting point is that every defendant is presumed sane. If the defendant raises insanity, the defendant must prove it on the balance of probabilities, meaning it is more likely than not.

This is unusual in criminal law because normally the prosecution must prove guilt beyond reasonable doubt.

Key parts of the test

A defect of reason means the defendant’s power of reasoning was impaired. Mere absent-mindedness is not enough.

• R v Clarke (1972): the defendant, who was diabetic and depressed, absent-mindedly took items from a shop. The court held that forgetfulness or confusion was not a defect of reason.

A disease of the mind is a legal phrase, not just a medical diagnosis. It means an internal condition that affects the defendant’s mental faculties.

• R v Kemp (1957): arteriosclerosis caused the defendant to attack his wife. A physical illness affecting the mind could count as a disease of the mind.
• R v Sullivan (1984): an epileptic seizure led to an assault. Epilepsy was treated as an internal condition and therefore insanity.
• R v Hennessy (1989): hyperglycaemia caused by diabetes and failure to take insulin was treated as an internal cause, so the correct defence was insanity.

The defendant must not know the nature and quality of the act, meaning they do not understand the physical character of what they are doing. Alternatively, they must not know the act is legally wrong.

• R v Windle (1952): after killing his wife, the defendant said, “I suppose they will hang me for this.” This showed he knew the act was legally wrong, so insanity failed.

Consequences of an insanity verdict

If insanity succeeds, the verdict is not guilty by reason of insanity. This is not the same as a simple acquittal.

Under s5 Criminal Procedure (Insanity) Act 1964, as amended, the court may impose:

• a hospital order;
• a supervision order;
• an absolute discharge.

For murder, the court must impose a hospital order with restrictions.

Automatism

Definition

Automatism is where the defendant’s body acts without voluntary control by the mind. If successful, non-insane automatism is a complete defence and leads to a full acquittal.

In Bratty v Attorney-General for Northern Ireland (1963), Lord Denning described automatism as an act done by the muscles without control of the mind, such as a spasm, reflex action or convulsion.

Non-insane automatism

For non-insane automatism, the defendant must usually show:

1. A total loss of voluntary control;
2. Caused by an external factor;
3. Not self-induced in a blameworthy way.

A partial loss of control is not enough.

• Attorney-General’s Reference (No 2 of 1992) (1993): a lorry driver claimed he had been driving “without awareness”. The court held that reduced awareness was not enough; automatism requires total loss of voluntary control.

External factors can include a blow to the head, some medical treatment, or another outside event.

• R v Quick (1973): the defendant, a diabetic nurse, became hypoglycaemic after taking insulin and failing to eat. Insulin was an external factor, so automatism, not insanity, could apply.

This contrasts with R v Hennessy (1989), where high blood sugar from diabetes itself was treated as internal and therefore insanity.

Intoxication

Definition

Intoxication means the defendant’s mental state is affected by alcohol, drugs or another substance. It is not usually a defence by itself. Instead, it matters because it may show the defendant did not form the required mens rea.

There are two main types: voluntary intoxication and involuntary intoxication.

Voluntary intoxication

Voluntary intoxication occurs where the defendant knowingly takes alcohol or drugs.

The law then distinguishes between specific intent offences and basic intent offences.

For voluntary intoxication:

• It may be a defence to a specific intent offence if the defendant was so intoxicated that they did not form the required intention.
• It is not a defence to a basic intent offence.

Key cases:

• DPP v Majewski (1977): the defendant took drugs and alcohol and assaulted people. Voluntary intoxication was no defence to basic intent offences because getting intoxicated was itself reckless.
• R v Lipman (1970): the defendant took LSD and killed his girlfriend while hallucinating. He lacked the intent for murder but was guilty of manslaughter.
• A-G for Northern Ireland v Gallagher (1963): the defendant drank whisky to build up courage to kill his wife. This “Dutch courage” did not help him because he had formed the intent before becoming intoxicated.

Involuntary intoxication

Involuntary intoxication occurs where the defendant did not choose to become intoxicated, for example because their drink was spiked or they had an unexpected reaction to medication.

However, it only helps if the defendant did not form the required mens rea.

• R v Kingston (1994): the defendant was drugged without his knowledge but still formed the intent to commit the offence. The House of Lords held that involuntary intoxication was no defence where mens rea was still present.
• R v Hardie (1984): the defendant took Valium, expecting it to calm him, and then started a fire. Because Valium was not known to him as a dangerous intoxicant, the jury could consider whether he lacked mens rea.

AO3 evaluation: fairness and effectiveness

The law tries to balance principle and policy. On principle, a person should not be punished if they lacked voluntary control or could not understand what they were doing. On policy, the law must protect the public and avoid easy false claims.

Insanity can be criticised as outdated. The M’Naghten rules come from 1843 and use language that does not match modern psychiatric understanding. It may also be unfair that conditions such as epilepsy or diabetes can lead to the stigmatising label of “insanity”.

Automatism is stricter and can be effective because it requires a total loss of control. This prevents weak claims based merely on panic, tiredness or confusion. However, the internal/external distinction can be artificial: the contrast between R v Quick (1973) and R v Hennessy (1989) shows how similar diabetic situations can lead to very different legal outcomes.

The intoxication rules are policy-driven. It seems fair that defendants should not escape basic intent liability after choosing to get drunk or take drugs. But the distinction between specific and basic intent can be technical, and it may be difficult for juries to decide whether intoxication truly prevented intention.