Defences — self-defence, duress, necessity

This note addresses three fundamental defences in English criminal law: self-defence (including defence of another and defence of property), duress (both by threats and of circumstances), and necessity. These defences share a common structure—each provides a complete acquittal where established—yet they differ markedly in scope, underlying rationale, and degree of judicial recognition.

Self-defence is a justificatory defence grounded in common law but partially codified in s. 76 of the Criminal Justice and Immigration Act 2008 (as amended by s. 43 of the Crime and Courts Act 2013). It permits the use of reasonable force to prevent crime, effect lawful arrest, or protect oneself, another, or (in limited circumstances) property. The defence requires both an honest belief in the need to use force and an objectively reasonable degree of force in response.

Duress comprises two forms: duress by threats (a recognised common law defence) and duress of circumstances (a judicially developed variant). Duress exculpates when the defendant commits a crime because of an imminent threat of death or serious injury to himself or another. It is an excusatory defence: the conduct remains wrongful, but the law recognises that a person of reasonable firmness might yield to such pressure. Duress is unavailable for murder (as principal or secondary party), attempted murder, and certain forms of treason.

Necessity, by contrast, remains highly controversial. While the Court of Appeal in Re A (Children) (Conjoined Twins: Surgical Separation) [2001] Fam 147 appeared to accept a residual defence of necessity of dire circumstances, the House of Lords in R v Hasan [2005] UKHL 22, [2005] 2 AC 467 and earlier authorities have repeatedly confined or denied its existence. The boundary between duress of circumstances and necessity remains contested.

Together, these defences raise profound questions about individual autonomy, the balance between communitarian values and personal security, and the proper limits of state punishment. They sit at the intersection of doctrinal precision, moral theory, and public policy, and are mainstays of FHS examination papers.

The common law has long recognised that not all technically criminal acts merit punishment. Medieval jurists acknowledged the right to repel force with force in self-preservation, viewing it as an incident of natural law. Blackstone described self-defence as 'the primary law of nature' (Commentaries, IV, 186). Early cases treated self-defence and necessity as overlapping concepts rooted in the instinct for survival.

Self-defence evolved from the medieval requirement of retreating 'to the wall' before employing lethal force. By the nineteenth century, the duty to retreat had softened to a consideration relevant to the reasonableness of force used. R v Palmer [1971] AC 814, a Privy Council decision from Jamaica, restated the modern law: the jury should ask whether the defendant used reasonable force, and a person under attack cannot be expected to 'weigh to a nicety the exact measure of his necessary defensive action'. The Criminal Law Act 1967, s. 3 codified the permissibility of reasonable force to prevent crime, and s. 76 CJIA 2008 later consolidated common law rules for self-defence.

Duress emerged separately. Medieval law allowed duress as an excuse for certain felonies short of treason and murder. The rationale, articulated by Lord Wilberforce in Lynch v DPP for Northern Ireland [1975] AC 653, was that 'the law must…make proper allowance for human frailty' when a person is subjected to threats he cannot reasonably resist. The scope of duress expanded in the twentieth century. R v Hudson and Taylor [1971] 2 QB 202 held that the threat need not be immediately executable, provided it is imminent and effective. Yet in R v Howe [1987] AC 417, the House of Lords ruled that duress could never excuse murder, even for a secondary party, a principle reaffirmed in R v Gotts [1992] 2 AC 412 for attempted murder.

Duress of circumstances emerged in the 1980s as a variant. R v Willer (1986) 83 Cr App R 225 and R v Conway [1989] QB 290 recognised that objective external threats (e.g., a violent mob forcing a driver to mount a pavement) could operate analogously to threats by persons. R v Martin (DP) [1989] 1 All ER 652 consolidated the test: the defendant must reasonably believe circumstances give rise to a threat of death or serious injury, and a sober person of reasonable firmness sharing the defendant's characteristics would have responded as he did.

Necessity has a murkier pedigree. R v Dudley and Stephens (1884) 14 QBD 273, the notorious lifeboat cannibalism case, rejected a defence of necessity to murder. Lord Coleridge CJ observed that it was impermissible to balance lives against lives and condemned the moral hazard of allowing such a defence. The scepticism persisted: in Southwark LBC v Williams [1971] Ch 734, Lord Denning MR warned that necessity would open 'a door which no man could shut'. Despite this, Re A (Children) accepted necessity in separating conjoined twins where one would die to save the other, though the judgments diverged sharply on rationale.

Self-defence

Structure of the defence

Self-defence comprises two elements:

1. Subjective element: The defendant must have honestly believed it was necessary to use force. The belief need not be reasonable, but reasonableness is evidence of honesty (s. 76(3) CJIA 2008; R v Williams (Gladstone) [1987] 3 All ER 411).
2. Objective element: The force used must be objectively reasonable in the circumstances the defendant believed them to be (s. 76(3)).

Mistaken belief, even if induced by voluntary intoxication, suffices for the subjective limb only if the mistake relates to the need to use force, not the degree (R v O'Grady [1987] QB 995; affirmed in R v Hatton [2005] EWCA Crim 2951). However, s. 76(5) provides that if the mistaken belief is attributable to voluntary intoxication, the defendant cannot rely on it.

Reasonableness and the 'householder' provisions

What is 'reasonable' is assessed from the defendant's perspective at the moment of action. Section 76(7) instructs that 'a person acting for a legitimate purpose may not be able to weigh to a nicety the exact measure of any necessary action'. Evidence that the defendant acted only to do what he honestly and instinctively thought necessary is 'strong evidence' that the action was reasonable.

Section 76(5A)–(5F) (inserted by the Crime and Courts Act 2013) creates a special regime for householders defending themselves in dwellings. In such cases, force is unreasonable only if it was grossly disproportionate. 'Disproportionate but not grossly disproportionate' force is lawful. A 'householder case' is one where:

• the force is used by D while in or partly in a building (or part of a building) that is a dwelling;
• D is not a trespasser; and
• D believed V to be a trespasser.

This relaxation reflects political concern for householders facing intruders.

Foundation

1. What are the two elements of self-defence, and how does s. 76 of the Criminal Justice and Immigration Act 2008 codify them?

1. Explain the difference between duress by threats and duress of circumstances. Why does the law treat them similarly?

Standard

1. Alice, threatened by her abusive partner Bob with death unless she helps him steal a car, drives the getaway vehicle. Bob kills the car's owner during the robbery. Discuss Alice's potential liability for murder and any defences available to her.

1. 'The exclusion of duress from murder in R v Howe is unprincipled and unjust.' Discuss with reference to the normative foundations of criminal defences.

Challenge

1. 'The concepts of self-defence, duress, and necessity are doctrinally distinct but normatively overlapping. English law's failure to rationalise them into a coherent framework undermines the legitimacy of the criminal law.' Critically evaluate this claim with reference to statutory and common law authorities, academic commentary, and comparative perspectives.

Essential

• Ashworth, A. & Horder, J., Principles of Criminal Law (9th edn, OUP 2019), ch 6 (Defences).
• Ormerod, D. & Laird, K., Smith, Hogan, and Ormerod's Criminal Law (16th edn, OUP 2021), ch 12 (General Defences).
• Law Commission, Murder, Manslaughter and Infanticide, Law Com No 304 (2006), Part 6 (Duress).

Academic articles

• Dennis, I., 'Duress, Murder and Criminal Responsibility' (2013) 129 LQR 208.
• Rogers, J., 'Necessity, Private Defence and the Killing of Mary' [2001] Crim LR 515 (analysis of Re A).
• Ashworth, A., 'Self-Defence and the Right to Life' (1975) 34 CLJ 282.
• Herring, J., 'Familial Homicide, Failure to Protect and Domestic Violence: Who's the Victim?' [2007] Crim LR 923 (includes discussion of duress and battered defendants).
• Loveless, J., 'Domestic Violence, Coercion and Duress' [2010] Crim LR 93.

Advanced and comparative

• Tadros, V., Criminal Responsibility (OUP 2005), ch 9 (Duress and Necessity).
• Uniacke, S., Permissible Killing: The Self-Defence Justification of Homicide (CUP 1994).
• Horder, J., Excusing Crime (OUP 2004), ch 3.
• Law Commission, Legislating the Criminal Code: Offences Against the Person and General Principles, Law Com No 218 (1993) (proposes codification of duress and necessity).
• Elliott, C., 'Necessity, Duress and Compassionate Killing' [2010] Crim LR 937.

Case notes

• Milgate, D., 'Case Comment: R v Hasan' [2005] Crim LR 678.
• Gardner, S., 'Case Note: Re A (Children)' (2001) 117 LQR 400.