Mastering Criminal Law Defences: Capacity and Necessity
Welcome to your complete study guide for Defences (Capacity and Necessity) for AQA A-Level Law (Paper 1: Criminal Law). In criminal law, proving that a defendant (D) committed the actus reus (guilty act) with the required mens rea (guilty mind) is often only half the battle. A defendant may still avoid criminal liability if they can successfully raise a legal defence.
Don't worry if this topic feels a bit overwhelming at first! We will break these defences down into simple, step-by-step tests with clear case examples, memory tricks, and common traps to avoid in your exam.
Overview: How Defences Are Divided
The AQA specification splits these defences into two main families:
1. Capacity Defences: These focus on whether D had the mental capacity or voluntary control to be held legally responsible for their actions (Insanity, Automatism, Intoxication).
2. Necessity Defences: These focus on situations where D was forced or compelled to act due to threats, extreme circumstances, or the need to protect themselves or others (Self-Defence / Prevention of Crime, Duress by Threats, Duress of Circumstances).
PART A: Capacity Defences
1. Insanity (Insane Automatism)
In law, "insanity" is a legal term, not a medical diagnosis. The legal foundation comes from the historic M'Naghten Rules (1843).
Burden and Standard of Proof
Every person is presumed sane. If D raises the defence of insanity, the defence bears the burden of proving it on the balance of probabilities.
The Three-Part M'Naghten Test
To succeed, D must prove that at the time of committing the act, they suffered from:
1. A Defect of Reason: D’s ability to reason must be genuinely impaired. Mere confusion, absent-mindedness, or temporary poor memory is not enough (R v Clarke [1972]).
2. Caused by a Disease of the Mind: This means an internal condition that affects mental faculties (such as memory, reasoning, or understanding). It can be mental or physical in origin:
• Arteriosclerosis / hardening of the arteries (R v Kemp [1957])
• Epilepsy (Bratty v A-G for Northern Ireland [1963]; R v Sullivan [1984])
• Diabetes causing hyperglycemia (high blood sugar due to failing to take insulin — R v Hennessy [1989])
• Sleepwalking (R v Burgess [1991])
3. Resulting in D not knowing:
• The nature and quality of their act: D is in a delusional or unconscious state and does not understand what they are physically doing (e.g. believing they are slicing a loaf of bread when they are actually cutting someone's arm); OR
• That what they were doing was wrong: "Wrong" strictly means legally wrong (contrary to English law), not morally wrong. If D knows the act is against the law, the defence fails, even if D believes God commanded it (R v Windle [1952] – "I suppose they'll hang me for this"; reaffirmed in R v Johnson [2007]).
The Special Verdict
If successful, D is found Not guilty by reason of insanity (under s.2 Trial of Lunatics Act 1883). Under the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 (amended by the 2004 Act), the judge has flexible disposal options: a hospital order (with or without restriction), a supervision order, or an absolute discharge. However, for murder, a hospital order with restriction remains mandatory.
2. Automatism (Non-Insane Automatism)
Automatism is defined in Bratty [1963] as: "an act done by the muscles without any control by the mind".
The Two Key Elements
1. Total Loss of Voluntary Control: There must be a complete destruction of voluntary control. Partial loss, impaired control, or reduced awareness is not automatism (Broome v Perkins [1987]; A-G's Reference (No 2 of 1992) [1993]).
2. Caused by an External Factor: The state must be triggered from outside the body:
• Swarm of bees attacking a driver (Kay v Butterworth [1945])
• Diabetes causing hypoglycemia (low blood sugar caused by taking insulin but failing to eat enough food — R v Quick [1973])
• A physical blow to the head causing severe concussion
• Post-traumatic stress disorder triggered by an exceptional external traumatic event (R v T [1990])
• Involuntary intoxication or an unexpected reflex reaction.
Self-Induced Automatism
If D brings about their own automatism through their own recklessness (e.g. failing to eat after taking insulin), the rules from R v Bailey [1983] apply:
• Specific intent crimes (e.g. Murder, s.18 OAPA): Self-induced automatism can be used if D lacked the required intent.
• Basic intent crimes (e.g. s.20 OAPA, s.47 OAPA, Assault, Battery): Self-induced automatism fails because D's prior recklessness supplies the mens rea.
Effect of Automatism
Automatism leads to a complete acquittal (D walks free without any disposal orders).
Quick Review: The Diabetes Trap!
• R v Hennessy: D failed to take insulin \(\implies\) high blood sugar (hyperglycemia) \(\implies\) Internal cause \(\implies\) Insanity.
• R v Quick: D took insulin but failed to eat \(\implies\) low blood sugar (hypoglycemia) caused by the medicine \(\implies\) External cause \(\implies\) Automatism.
3. Intoxication
Intoxication covers drugs, alcohol, or other substances. The legal outcome depends on whether the intoxication was voluntary or involuntary, and whether the offence is one of specific intent or basic intent.
Specific vs Basic Intent Offences
• Specific Intent: Crimes where the mens rea requires intention only (e.g. Murder, s.18 OAPA 1861, Theft, Robbery).
• Basic Intent: Crimes where the mens rea can be satisfied by recklessness (e.g. Manslaughter, s.20 OAPA 1861, s.47 OAPA 1861, Assault, Battery).
Voluntary Intoxication
• For Specific Intent Crimes: Voluntary intoxication can negate the required specific intent (DPP v Beard [1920]). If D genuinely did not form the intent, D is acquitted of that specific intent crime. However, D will usually be convicted of a fallback basic intent crime (e.g. Murder falls back to Involuntary Manslaughter; s.18 drops to s.20) (DPP v Majewski [1977]). Remember: a "drunken intent is still an intent" (R v Sheehan and Moore [1975]).
• For Basic Intent Crimes: Getting voluntarily intoxicated is legally considered a reckless course of conduct. That recklessness transfers to satisfy the mens rea of the crime. Therefore, voluntary intoxication is no defence to basic intent crimes (DPP v Majewski [1977]).
• Dutch Courage: If D deliberately drinks or takes drugs to gather the courage to commit a crime, the defence is completely barred (A-G for Northern Ireland v Gallagher [1963]).
Involuntary Intoxication
Applies when D’s drink was spiked, or D suffered unexpected side effects from prescribed non-soporific drugs, or took soporific drugs properly without reckless disregard (R v Hardie [1985]).
• Involuntary intoxication is a defence to both specific and basic intent crimes if it prevented D from forming the required mens rea.
• However, if D still formed the mens rea despite being drugged/spiked, they remain guilty (R v Kingston [1994]).
Intoxication and Mistake
• Drunken mistake about self-defence: D cannot rely on a mistaken belief caused by voluntary intoxication (s.76(5) Criminal Justice and Immigration Act 2008; R v O'Grady [1987]; R v Hatton [2005]).
• Drunken mistake in criminal damage: Under s.5(2)(a) Criminal Damage Act 1971, an honest belief in consent provides a defence, even if induced by voluntary intoxication (Jaggard v Dickinson [1980]).
Key Takeaway for Capacity Defences: Always identify whether the root problem is internal (Insanity), external (Automatism), or substance-induced (Intoxication), and check whether the crime is specific or basic intent.
PART B: Necessity Defences
1. Self-Defence / Prevention of Crime / Defence of Property
Self-defence operates under both common law (defending oneself, another person, or property) and statute (s.3(1) Criminal Law Act 1967 – prevention of crime and lawful arrest). The rules are clarified under s.76 of the Criminal Justice and Immigration Act (CJIA) 2008.
The Two-Limb Test
Limb 1: Necessity of Force (Subjective Limb)
• Was force necessary based on the facts as D genuinely and honestly believed them to be? (s.76(3) CJIA 2008; R v Williams (Gladstone) [1987]).
• Pre-emptive strikes: D does not have to wait to be attacked before defending themselves (Beckford v R [1988]).
• Duty to retreat: There is no legal duty to retreat, although retreating can be evidence of reasonable behaviour (s.76(6A) CJIA 2008; R v Bird [1985]).
• Drunken mistake: D cannot rely on a mistaken belief if it was caused by voluntary intoxication (s.76(5) CJIA 2008).
Limb 2: Reasonableness / Proportionality of Force (Objective Limb)
• Was the level of force used objectively reasonable in the circumstances as D believed them to be? (s.76(6) CJIA 2008).
• The courts recognise that a person acting in the heat of the moment cannot "weigh to a nicety the exact measure of their action" (Palmer v R [1971]).
• Householder Cases (s.76(5A) CJIA 2008): When an occupant uses force against an intruder in a dwelling, the force is reasonable unless it was grossly disproportionate (R (Collins) v Secretary of State for Justice [2016]; R v Ray (Steven) [2017]).
Effect of Self-Defence
Self-defence is an all-or-nothing complete defence. If successful, D is fully acquitted. If D uses excessive force, the defence fails entirely (R v Clegg [1995]).
2. Duress by Threats
Duress by threats occurs when D commits a crime because someone has threatened them with death or serious injury.
The Two-Stage Graham Test (R v Graham [1982]; R v Hasan [2005])
1. Subjective Limb: Was D impelled to act because they reasonably believed they had good cause to fear death or serious injury?
2. Objective Limb: Would a sober person of reasonable firmness, sharing the characteristics of D, have acted in the same way?
Recognised Characteristics (R v Bowen [1996])
The jury may consider: age, sex, pregnancy, serious physical disability, or a recognised psychiatric condition. Low IQ cannot be taken into account.
Key Limits and Restrictions on Duress
• Excluded Offences: Duress is NEVER a defence to murder (R v Howe [1987]) or attempted murder (R v Gotts [1992]).
• Nature of Threat: The threat must be of death or serious physical injury (R v Valderrama-Vega [1985]). Threats to property or blackmail are not enough on their own.
• Target of Threat: Must be directed at D, immediate family, or someone for whom D reasonably feels responsible (R v Wright [2000]; R v Hasan [2005]).
• Nexus: There must be a direct link between the threat and the specific crime nominated by the threat-maker (R v Cole [1994]).
• Immediacy & Escape: The defence fails if D had a safe avenue of escape or an opportunity to seek police protection (R v Gill [1963]; R v Hudson and Taylor [1971]; tightened in R v Hasan [2005]).
• Self-Induced Duress: Duress is barred if D voluntarily joined a criminal gang or associated with criminals and foresaw (or ought to have foreseen) the risk of being subjected to threats (R v Sharp [1987]; R v Hasan [2005]).
3. Duress of Circumstances and Necessity
Sometimes the threat does not come from a specific person making a direct demand, but from an external threatening situation or circumstance.
Duress of Circumstances
• Developed originally in road traffic cases (R v Willer [1986]; R v Conway [1989]; R v Martin [1989]) and extended to criminal law generally (R v Pommell [1995]).
• Uses the same two-stage Graham test:
1. Did D act from a reasonable belief that the circumstances posed an imminent danger of death or serious physical injury?
2. Would a sober person of reasonable firmness have responded in the same way?
General Defence of Necessity
• English law has historically been very reluctant to recognise a general defence of necessity.
• Necessity is not a defence to murder (R v Dudley and Stephens [1884] – shipwrecked sailors who killed and ate a cabin boy to survive were guilty of murder).
• However, necessity has been recognised in exceptional medical situations (Re A (Conjoined Twins) [2000] – surgery to separate conjoined twins where one would inevitably die to save the other).
Key Takeaway for Necessity Defences: Remember that neither duress by threats nor duress of circumstances is ever available for murder or attempted murder!
Common Student Pitfalls to Avoid in Paper 1
• Mistaking Householder Rules: The 2013 amendment does not allow householders to use "grossly disproportionate" force. It permits force that is disproportionate, as long as it is not grossly disproportionate (R v Ray).
• Confusing Self-Defence with Loss of Control: Self-defence is a complete necessity defence leading to a full acquittal. Loss of control (under s.54 Coroners and Justice Act 2009) is only a partial defence to murder that reduces the conviction to voluntary manslaughter.
• Thinking Windle was about Morals: Knowing an act is morally wrong is irrelevant under M'Naghten. The test is strictly whether D knew it was legally wrong.
• Applying Duress to Murder: Never argue duress for murder or attempted murder on a problem question. Under Howe and Gotts, the defence is strictly barred.
Quick Summary Checklist
• Insanity: Defect of reason + Disease of the mind (Internal) + Not knowing nature/quality or legal wrongfulness (\(\implies\) Special verdict).
• Automatism: Total loss of voluntary control + External cause + Not self-induced recklessness (\(\implies\) Complete acquittal).
• Intoxication: Specific vs Basic intent; Majewski rule for recklessness; Kingston rule for involuntary intoxication.
• Self-Defence: Subjective necessity (Gladstone Williams) + Objective reasonable force (Palmer) + Householder threshold (Ray).
• Duress: Graham test (Subjective belief + Objective reasonable firmness) + Bowen characteristics + No murder (Howe/Gotts) + No self-induced gang membership (Hasan/Sharp).