Legal basis: Common law, defined in M’Naghten’s Case (1843).

Definition:

A person is legally insane if, at the time of the act, they were suffering from a defect of reason arising from a disease of the mind, causing them not to know the nature and quality of the act or that it was wrong.

Elements (M’Naghten Rules):

• Defect of reason – impaired ability to think rationally (R v Clarke [1972] – absent-mindedness not enough).

• Disease of the mind – can be a physical or mental condition affecting mental function (R v Kemp [1957] – arteriosclerosis counted).

• Not knowing nature/quality or wrongness – must not understand what they were doing or that it was legally wrong (R v Windle [1952] – knew killing was wrong → no defence).

Effect: Special verdict: “Not guilty by reason of insanity.” The court can impose hospitalisation or supervision under the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991.

Definition:

An involuntary act caused by an external factor that prevents the defendant from controlling their actions.

Elements:

• Total loss of voluntary control (AG’s Reference (No 2 of 1992) – partial control is not enough).

• External cause – e.g., concussion, hypoglycaemia, or a blow to the head.

• Not self-induced – must not result from voluntary intoxication or recklessness.

Cases:

• R v Quick [1973] – hypoglycaemia (external insulin effect) = automatism.

• R v Hennessy [1989] – hyperglycaemia (internal condition) = insanity, not automatism.

Effect: Full acquittal.

Definition:

Intoxication can negate mens rea if the defendant was so affected by alcohol or drugs that they could not form the required intent.

Key Distinctions:

• Voluntary intoxication – only a defence to specific intent crimes (e.g., murder, s.18 GBH), not basic intent crimes (e.g., assault, s.20 GBH).

  • DPP v Majewski [1977] – voluntary intoxication is no defence to basic intent offences.

• Involuntary intoxication – can be a defence to both if it removes mens rea entirely (R v Hardie [1985] – unexpected reaction to Valium).

Effect: May lead to acquittal if mens rea genuinely absent, but not if intoxication simply made the defendant reckless.

Definition:

A genuine mistake of fact can prevent conviction if it means the defendant lacked mens rea.

Key Principles:

• Must be a mistake of fact, not law (“ignorance of the law is no excuse”).

• The belief must be honest, even if unreasonable (R v Williams (Gladstone) [1984] – mistaken belief in defending another).

• Does not apply to offences of strict liability or voluntary intoxication.

Effect: If the mistake is genuine and removes mens rea, the defendant is acquitted.