Defences — self-defence, duress, necessity

This week completes the Part IA treatment of substantive criminal responsibility by examining three defences which are often grouped together because each concerns pressure, emergency, or threatened harm: self-defence, duress, and necessity. That grouping is pedagogically useful but doctrinally dangerous. The defences do not share a single structure. Self-defence is principally a justificatory defence: the defendant claims that the use of force was warranted, not merely understandable. Duress is ordinarily excusatory: the defendant admits that the offence was wrong, but says that the law should not condemn him because threats overbore the standards which the criminal law can fairly demand. Necessity is the most unstable category: English law recognises pockets of necessity, particularly in medical and property contexts, but resists a broad defence of choosing the lesser evil.

For Cambridge purposes, this topic is best learned as a set of contrasting doctrinal tests rather than as a loose moral theme. Self-defence requires attention to two questions: did D honestly believe that force was necessary, and was the degree of force used reasonable in the circumstances as D believed them to be? The first question is substantially subjective, though unreasonable belief may be evidentially relevant; the second is normative and external, though it is applied to D’s believed circumstances. The statutory restatement in Criminal Justice and Immigration Act 2008, s 76, is indispensable, but it does not abolish the common law. It codifies and modifies parts of it, especially after the householder amendments.

Duress must be divided into duress by threats and duress of circumstances. The former concerns threats made by a human source, typically to kill or cause serious injury unless D commits an offence. The latter concerns surrounding circumstances creating a comparable threat. The leading modern authority is R v Hasan [2005] UKHL 22, which tightened the availability of duress where D voluntarily associated with criminals and ought reasonably to have foreseen the risk of compulsion. Duress is unavailable to murder, and to attempted murder. That rule is controversial but examination candidates must state it clearly before criticising it.

Necessity is not a general licence to commit crime whenever D thinks the balance of harm favours breach of law. R v Dudley and Stephens (1884) remains the symbolic rejection of necessity as a defence to intentional killing. Yet English law cannot avoid necessity altogether. It appears in the doctrine of duress of circumstances, in statutory lawful excuse for criminal damage, and in cases such as Re A (Conjoined Twins) [2001] Fam 147, where the Court of Appeal treated a life-saving separation operation as lawful despite the inevitable death of one child. The result is a law of emergency action which is doctrinally fragmented, morally rich, and especially attractive to Tripos examiners.

The historical development of these defences reflects a gradual move from rigid public-order concerns towards a more discriminating account of agency under pressure. Early common law was reluctant to allow private judgment to override public authority. That reluctance explains much of the narrowness of necessity. If individuals may decide that breach of the criminal law produces a better social outcome, the rule of law seems to yield to private arithmetic. R v Dudley and Stephens (1884) 14 QBD 273 is the canonical expression of that anxiety. Shipwrecked sailors killed and ate a cabin boy to survive. The court refused to recognise necessity as a defence to murder. Its continuing importance is not that the facts recur, but that it stands for a principled suspicion of consequentialist defences to intentional killing.

Self-defence, by contrast, has always occupied a more secure place. The law has long accepted that a person under attack need not wait for the state to intervene. Older doctrine sometimes spoke the language of retreat and last resort, but modern law treats failure to retreat as evidence rather than as an absolute bar. The Privy Council’s decision in Palmer v R [1971] AC 814 gave classic expression to the realities of emergency judgment: a defendant cannot be expected to measure precisely the exact force required when confronted with imminent danger. Beckford v R [1988] AC 130 then emphasised that defensive force may be anticipatory: a person need not wait to be struck first. The modern statutory formulation in s 76 of the 2008 Act draws directly on this line of authority.

Duress developed more unevenly. The common law accepted that threats may excuse some crimes, but it feared that easy recognition would reward cowardice or collusion. The defence was therefore confined by requirements of serious threat, immediacy, lack of reasonable escape, and reasonable firmness. The modern law is shaped by R v Graham [1982] 1 WLR 294 and R v Hasan [2005] UKHL 22. Graham formulated the two-stage inquiry into D’s belief in the threat and the response expected of a sober person of reasonable firmness. Hasan refined that inquiry and restricted duress where D exposed himself to coercive pressure by voluntary association with criminals.

The prohibition on duress for murder has deeper historical roots. In R v Howe [1987] AC 417 the House of Lords confirmed that duress is no defence to murder for principal offenders or secondary parties. R v Gotts [1992] 2 AC 412 extended the rule to attempted murder. The rule rests on the proposition that the law should not permit one innocent life to be taken to save another. Yet that proposition sits uneasily beside the excusatory rationale of duress: if the question is not whether the killing was justified but whether D can fairly be blamed, why should murder be categorically excluded?

Necessity has re-entered the law through narrower doors. Statutory lawful excuse under the Criminal Damage Act 1971 permits damage to protect property where D holds the required beliefs. Medical law has also required exceptions: emergency treatment of unconscious patients, withdrawal of futile treatment, and separation of conjoined twins cannot be explained by a crude denial that necessity matters. The result is historically characteristic of English criminal law: resistance to large theoretical categories, combined with pragmatic exceptions where hard cases make absolute rules untenable.

The first organising distinction is between justification and excuse. A justified act is one which the law treats as right, permissible, or at least not wrongful in the circumstances. An excused act remains wrongful, but D is not fairly condemnable. Self-defence is usually justificatory: if D uses reasonable force against an aggressor, the law does not merely forgive D; it says that D was entitled to defend himself, another, property, or the prevention of crime. Duress is usually excusatory: D’s act remains an invasion of the victim’s rights, but the threat means that the criminal law should not insist on heroic resistance. Necessity may be either justificatory or excusatory depending on context. Re A is often read as a justification based on lesser evil; duress of circumstances may operate more like excuse.

Self-defence has two components. First, D must honestly believe that the use of force is necessary. This belief may be mistaken. Under Criminal Justice and Immigration Act 2008, s 76(4), if D genuinely held the belief, he may rely on it whether or not the mistake was reasonable. The reasonableness of the belief is relevant evidentially to whether it was genuine. There is an important exception: D cannot rely on a mistaken belief attributable to voluntarily induced intoxication. This follows both the statute and the wider policy that voluntary intoxication should not generate defensive permissions against innocent persons.

Secondly, the degree of force must be reasonable in the circumstances as D believed them to be. This is not wholly subjective. D’s perception supplies the factual setting, but the law assesses the proportionality of the response. The ordinary formulation asks whether the force was reasonable; after the householder amendments, force in a householder case is not unreasonable merely because it is disproportionate, but it is unreasonable if grossly disproportionate. The householder rule does not authorise revenge, punishment, or gratuitous violence. It adjusts the threshold in recognition of the fear and urgency of confronting intruders in the home.

The self-defence cases establish a humane but bounded approach to emergency action. Palmer v R is the classic statement that defensive force cannot be judged with armchair precision. It rejects mathematical proportionality, but it does not abandon reasonableness. The jury must make allowance for the urgency and stress of attack. Beckford v R adds that defensive force may be anticipatory. If D honestly believes that an attack is imminent, he need not wait passively. R v Owino then sharpens the modern test: the jury asks whether the force was reasonable in the circumstances as D believed them to be. R v Martin illustrates the limits of the defence. A householder shot fleeing burglars; his psychiatric evidence could not convert excessive force into self-defence, though it mattered to diminished responsibility on the murder count.

For duress, R v Graham and R v Hasan are central. Graham gives the two-limb test: was D impelled to act because he reasonably believed in threats of death or serious injury, and would a sober person of reasonable firmness sharing relevant characteristics have acted similarly? Hasan is now the leading House of Lords authority. It tightened the defence by emphasising immediacy, reasonable evasive action, and self-induced exposure to threats through voluntary criminal association. It also confirmed that the threat must be one of death or serious injury, not mere economic pressure or lesser harm.

R v Howe is indispensable because it excludes duress as a defence to murder. The House of Lords treated the sanctity of innocent life as overriding the excusatory claim. R v Gotts applies the same exclusion to attempted murder. These cases are often criticised, but in a problem question they are not optional. If the offence charged is murder or attempted murder, duress cannot provide an acquittal.

Necessity is anchored by two contrasting authorities. Dudley and Stephens rejects necessity as a defence to murder even in conditions of extreme survival pressure. Re A (Conjoined Twins), by contrast, permitted surgical separation where one twin would inevitably die but the other could live. The case is difficult because the judges used overlapping concepts: necessity, self-defence, lesser evil, and intention. Its significance is that English law sometimes recognises necessity where the circumstances are exceptional, the harm avoided is grave, and the court can control the category tightly.

R v Shayler is useful as a limiting authority. The defendant sought to rely on necessity or duress of circumstances after disclosing official secrets. The House of Lords held the defence narrowly confined. Political or public-interest motives do not become necessity merely because D believes illegality will avert harm. The case is an important corrective to over-expansive exam arguments.

See practice questions section below.

See further reading section below.