Defences — self-defence, duress, necessity

Defences complete the architecture of criminal responsibility. Weeks 1–10 have examined the positive conditions of liability: conduct, causation, mental states, homicide, non-fatal offences, sexual offences and principal property offences. Week 11 asks the complementary question: when, despite the apparent satisfaction of an offence definition, should the defendant not be convicted?

Self-defence, duress and necessity are often grouped as “defences of pressure”. That grouping is useful but imprecise. Self-defence is ordinarily a justification: the defendant’s use of force is treated as lawful because it responds to an unjust threat, or prevents crime, or assists lawful arrest. Duress is ordinarily an excuse: the defendant’s conduct remains wrongful, but punishment is withheld because the law accepts that the pressure on the defendant’s will was extreme. Necessity is the most unstable category. English criminal law recognises some specific necessity-like defences, accepts duress of circumstances in limited form, but remains deeply suspicious of a general defence of choosing the lesser evil.

The topic is doctrinally examinable because it draws together two recurrent themes in criminal law. First, the law alternates between subjective and objective standards. Self-defence asks whether the defendant honestly believed force was necessary, but whether the degree of force was reasonable in the circumstances as believed. Duress asks whether the defendant was impelled by threats of death or serious injury, but also whether a person of reasonable firmness sharing certain characteristics would have acted as the defendant did. Secondly, the law polices moral boundaries. It refuses duress for murder, restricts reliance on threats connected with voluntary association with criminals, and resists allowing necessity to become a general charter of private moral choice.

For Durham assessment, the danger is not usually ignorance of the leading authorities. It is misclassification. A problem question may contain a householder, a frightened mistaken defender, a getaway driver coerced by threats, a person carrying a weapon to avert an imminent attack, and a defendant damaging property to prevent greater harm. Each requires a distinct route. Do not write a single generic paragraph on “pressure”. Identify the defence, its elements, its burden, its limits, and its consequence. In essay questions, distinguish conceptual justification from excuse, and use the homicide exclusion in duress and the narrowness of necessity as evidence of a criminal law committed to both moral principle and institutional caution.

The modern law of self-defence has old common law roots, but its current form is a mixture of common law principle, statutory clarification and politically charged legislative amendment. At common law, force used in self-preservation or in defence of another could be lawful if necessary and proportionate. Section 3(1) of the Criminal Law Act 1967 added a statutory ground for reasonable force in the prevention of crime and in lawful arrest. Later, section 76 of the Criminal Justice and Immigration Act 2008 sought to state the governing principles, especially after public controversy about householders confronting intruders. It did not abolish the common law; it attempted to clarify it. The result is a hybrid: the defence still speaks the language of honest belief, necessity and reasonable force, while statute now supplies much of the examination framework.

Duress has had a less linear history. Older cases were reluctant to allow threats to excuse serious offending, and the law has never accepted duress to murder. In the later twentieth century, the Court of Appeal gave the defence a more structured test in Graham, requiring both subjective pressure and an objective standard of reasonable firmness. The House of Lords in Howe reaffirmed the moral boundary: a defendant may not rely on threats as a defence to murder. Gotts extended that exclusion to attempted murder. Hasan then tightened the law of self-induced duress, reflecting a judicial concern that criminal association may foreseeably expose defendants to coercion and should not create an escape route from liability.

Necessity has the most troubled history. Dudley and Stephens is the symbolic starting point: shipwrecked sailors killed and ate a cabin boy to survive, but the court rejected necessity as a defence to murder. The case is not a general denial that emergencies matter. Rather, it marks a powerful anxiety: if defendants may decide that one innocent life may be sacrificed to save others, criminal law loses its categorical protection of life. Later law has recognised necessity in narrower settings: medical necessity in cases such as Re A, lawful excuse in criminal damage, duress of circumstances, and statutory or common law provisions for emergency action. But English law still refuses a broad defence that the defendant committed the lesser wrong.

This history matters because the three defences express different institutional instincts. Self-defence trusts citizens to repel unlawful threats, within objective limits. Duress acknowledges human frailty, but insists on firm limits where life is taken or criminal associations are chosen. Necessity is constrained because it asks courts to ratify the defendant’s private balancing of harms. In Durham criminal law, where Year 2 builds on the conceptual method learned in Year 1, the best answers show how doctrine reflects these underlying tensions rather than merely reciting tests.

The first organising distinction is between justification and excuse. A justified act is, in the relevant circumstances, not wrongful: the defendant was entitled to act as he did. An excused act remains wrongful, but the defendant is not fairly blameworthy. Self-defence is usually justificatory. Duress is usually excusatory. Necessity may be either, depending on the form invoked: medical necessity may justify; duress of circumstances more closely resembles excuse; statutory lawful excuse in criminal damage is offence-specific.

Self-defence has two main questions. First, did the defendant honestly believe that force was necessary? This is subjective. The defendant may rely on an honest mistake, even if unreasonable, provided it is not attributable to voluntarily induced intoxication. The reasonableness of the belief is evidentially relevant: an implausible belief may not be accepted as genuine. But if genuinely held, the belief supplies the factual basis on which the jury assesses the defence. Secondly, was the degree of force used reasonable in the circumstances as the defendant believed them to be? This is objective, but anchored to the defendant’s believed facts. The defendant is not required to weigh the precise measure of defensive force in a crisis; instinctive action may be strong evidence of reasonableness. But disproportionate force defeats the ordinary defence; in householder cases the statutory language draws a special distinction between disproportionate and grossly disproportionate force.

Self-defence may arise before an attack has fully materialised. A person need not wait to be struck. Pre-emptive force can be lawful if the defendant honestly believes it necessary and uses reasonable force. Equally, there is no general duty to retreat, though an opportunity to withdraw may be relevant to necessity and reasonableness. The defence is available not only for personal protection, but also for defence of another, defence of property, prevention of crime and lawful arrest. Care is required in property offences. Criminal damage may be better analysed under lawful excuse in section 5 of the Criminal Damage Act 1971 rather than generic self-defence.

The leading self-defence cases move from broad common law principle to statutory consolidation. Palmer remains the classic authority for the practical allowance made to a defendant acting in crisis: the law does not demand fine calibration where a person is under attack. Williams confirmed that an honest mistake can found self-defence even if unreasonable, although unreasonable beliefs may be disbelieved as a matter of evidence. Owino supplies the compact modern formula: reasonable force is assessed by reference to the circumstances as the defendant believed them to be. Martin illustrates both the reach and the limit of the defence: a householder’s fear and personal characteristics may be relevant to diminished responsibility, but they do not convert excessive force into reasonable defensive force.

Duress is built around Graham, Howe, Gotts and Hasan. Graham established the two-limb framework of subjective compulsion and objective reasonable firmness. Howe is the moral centre of the modern law, refusing duress as a defence to murder even where the defendant acts under grave threats. Gotts extended the exclusion to attempted murder. Hasan then tightened the doctrine by narrowing the circumstances in which self-induced duress may be pleaded, especially where the defendant voluntarily associates with criminals and foresees, or ought reasonably to foresee, the risk of coercion by violence.

For duress of circumstances, Abdul-Hussain and Pommell are essential. Abdul-Hussain rejected an over-rigid understanding of immediacy: a threat may be imminent although not instantaneous, provided it is operating on the defendant’s mind with sufficient urgency. Pommell allowed duress of circumstances to be raised where a defendant possessed a firearm for a short period to prevent greater danger, subject to strict limits of immediacy and proportionality.

Necessity is anchored by Dudley and Stephens and Re A. Dudley and Stephens denies a general necessity defence to murder, particularly where one innocent life is intentionally sacrificed for others. Re A is exceptional: the Court of Appeal permitted surgical separation of conjoined twins where one would inevitably die and the other could live, using reasoning that drew on necessity but was confined by the extraordinary medical facts. The contrast between these cases is exam gold. It shows that necessity is not unknown to English law, but it is carefully confined where the criminal law fears private choice over the value of life.

See practice questions section below.

See further reading section below.