Insanity and automatism

This note examines the two principal defences that address involuntary conduct and impaired mental functioning: insanity and automatism. Both defences challenge the foundational requirement that criminal liability presupposes a voluntary act performed by a responsible agent. Yet their operation, scope, and consequences diverge sharply.

Insanity, governed by the common law M'Naghten Rules (1843), applies where a defendant suffers from a disease of the mind rendering them unable to appreciate the nature and quality of their act or to know it was wrong. A successful plea results in a special verdict of not guilty by reason of insanity, which until recent reforms led to indefinite detention. The defence is notoriously narrow, anachronistic, and has generated extensive academic and judicial criticism.

Automatism applies where the defendant acts in a state of impaired or absent consciousness attributable to an external cause—be it concussion, hypoglycaemia induced by external factors, or involuntary intoxication. Automatism yields an unqualified acquittal. The distinction between insanity (internal cause) and automatism (external cause) turns on the internal/external factor test articulated in Quick [1973] and refined in Hennessy [1989] and Burgess [1991].

The intersection of these defences with concepts covered in Week 1 (voluntariness of actus reus) and Week 12 (intoxication) is substantial. Both defences raise difficult normative and empirical questions: should diabetes mellitus be treated as a disease of the mind? Must a sleepwalker be labelled insane? To what extent should the law defer to psychiatric expertise?

This note proceeds historically (§02), sets out key principles (§03), examines the statutory framework (§04), reviews landmark cases (§05), traces doctrinal development (§06), canvasses academic debates (§07), offers comparative perspectives (§08), and concludes with tutorial worked answers (§09), exam traps (§10), practice questions (§11), and further reading (§12).

The modern law of insanity originates in M'Naghten's Case (1843) 10 Cl & Fin 200, which arose after Daniel M'Naghten, labouring under paranoid delusions, shot and killed Edward Drummond (private secretary to the Prime Minister, Sir Robert Peel) believing he was acting in self-defence against persecutors. His acquittal on grounds of insanity provoked public outcry and a debate in the House of Lords. The judges were summoned to provide answers to five questions; their responses constitute the M'Naghten Rules.

The Rules establish a presumption of sanity and require the defendant (on a balance of probabilities) to prove:

1. At the time of the act, he was labouring under such a defect of reason,
2. From disease of the mind,
3. As not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know what he was doing was wrong.

The rationale was retributivist and epistemic: if the defendant lacked rational capacity or moral understanding, he could not fairly be held responsible. The formulation reflects mid-Victorian psychiatry and moral philosophy. Its cognitive focus—knowing right from wrong—contrasts with modern psychiatric emphasis on volitional and emotional impairment.

Historically, the verdict entailed automatic detention in a mental hospital without limit of time (Criminal Procedure (Insanity) Act 1964, s 5). This draconian consequence deterred defendants from pleading insanity, distorting the law's application. The Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 introduced discretionary disposal options (hospital orders, supervision orders, absolute discharge) except for murder, ameliorating but not eliminating the stigma.

Automatism emerged later as a residual category. Early cases such as R v Charlson [1955] 1 WLR 317 (cerebral tumour causing violence) suggested that some involuntary states fell outside insanity. The modern distinction between insane and non-insane automatism crystallised in Bratty v Attorney-General for Northern Ireland [1963] AC 386, where Lord Denning defined automatism as 'an act which is done by the muscles without any control by the mind such as a spasm, a reflex action or a convulsion; or an act done by a person who is not conscious of what he is doing such as an act done whilst suffering from concussion or whilst sleepwalking' [at 409]. Crucially, Lord Denning held that if the involuntariness arises from disease of the mind, the appropriate defence is insanity, not automatism.

This bifurcation has generated friction. Medically, conditions such as epilepsy, diabetes, and sleepwalking resist neat categorisation as internal or external. Legally, the consequence—acquittal versus special verdict—seems arbitrary and disproportionate.

A. The burden and standard of proof

Insanity: Woolmington's golden thread is reversed. Once the defendant raises evidence of insanity, he bears the legal burden of proving it on the balance of probabilities (M'Naghten; R v Podola [1960] 1 QB 325). This is an exception justified by the presumption of sanity and the difficulty of the prosecution disproving mental disorder.

Automatism: The defendant bears the evidential burden only; once credible evidence is adduced, the prosecution must disprove automatism beyond reasonable doubt (Bratty; R v Stripp (1978) 69 Cr App R 318). The same applies to voluntariness generally (Woolmington v DPP [1935] AC 462).

B. The three elements of insanity (M'Naghten Rules)

(i) Defect of reason

Mere failure to use reason does not suffice; there must be a deprivation or impairment of rational faculties. In R v Clarke [1972] 1 All ER 219, the defendant, suffering from depression, shoplifted absentmindedly. The Court of Appeal held this was not insanity: her powers of reason were intact, though she failed to exercise them. The test is capacity, not exercise.

(ii) Disease of the mind

This is a legal, not medical, concept (Kemp [1957] 1 QB 399 per Devlin J). It encompasses any internal condition—organic or functional—affecting the proper functioning of the mind. The leading authority is Sullivan [1984] AC 156: epilepsy constitutes a disease of the mind even though it is a physical disorder, because it affects consciousness and cognition.

Foundation

1. Explain the three elements of the M'Naghten Rules and illustrate each with case law.

1. What is the difference between insane and non-insane automatism, and why does it matter?

Standard

1. Diane, an epileptic, fails to take her medication and has a seizure during which she strikes Victor, breaking his jaw. Advise Diane on any defences. How would your answer differ if the seizure were caused by a blow to the head sustained in a car accident the previous day?

1. 'The internal/external distinction in Quick and Hennessy is arbitrary and unjust.' Discuss with reference to case law and proposals for reform.

Challenge

1. Eric, suffering from paranoid schizophrenia, believes that government agents are poisoning the water supply and that he must kill the local water company manager to save the nation. He does so, and at trial psychiatric evidence confirms his delusion but also that he knew killing was against the law. Eric claims the defence of insanity. Advise the court on (a) whether the defence is available under current law, and (b) whether the law should be reformed to allow the defence in these circumstances. Consider comparative and ECHR perspectives.

Essential

• Law Commission Report No 364 (2013), Insanity and Automatism (comprehensive reform proposals; essential for essays)
• RD Mackay, 'The Abnormality of Mind Factor in Diminished Responsibility' [1999] Crim LR 117; and Mental Condition Defences in the Criminal Law (Oxford: Clarendon Press, 1995) (leading empirical and doctrinal analysis)
• AP Simester & GR Sullivan (eds), Simester & Sullivan's Criminal Law: Theory and Doctrine, 7th edn (Oxford: Hart, 2019), ch 11 (authoritative treatment)

Recommended articles

• Glanville Williams, 'Automatism and Insanity' [1965] Crim LR 381 (classic critique)
• Peter Alldridge, 'The Coherence of Defences' [1983] Crim LR 665 (conceptual clarity)
• Ronnie Mackay & Warren Brookbanks, 'The New Insanity and Unfitness to Plead Provisions in New Zealand' [2005] Crim LR 517 (comparative reform)
• Nicola Lacey, 'Responsibility and Modernity in Criminal Law' (2001) 9 Journal of Political Philosophy 249 (philosophical context)

Advanced and critical

• Stephen Morse, 'Culpability and Control' (1994) 142 University of Pennsylvania Law Review 1587 (neuroscience and responsibility)
• Jeremy Horder, 'Pleading Involuntary Lack of Capacity' (1993) 52 Cambridge Law Journal 298 (conceptual analysis)
• Law Commission Working Paper No 129 (1994), Insanity and Automatism (background to 2013 Report)
• Victor Tadros, Criminal Responsibility (Oxford: OUP, 2005), ch 12 (philosophical critique of insanity defence)

Case notes

• Note on R v Coley; R v McGhee; R v Harris [2013] EWCA Crim 223, [2013] Crim LR 923 (intoxication and insanity)
• Note on R v Johnson [2007] EWCA Crim 1978, [2008] Crim LR 132 (sleepwalking and Burgess)