The structure of actus reus
The actus reus is best understood as the definitional elements of an offence other than those relating to fault. It has three possible components:
- Conduct: a positive act (or, in omissions liability, a failure to act). Conduct must be willed and voluntary. Involuntary movements caused by reflex, spasm, automatism, or external physical force do not constitute an act in the legal sense.
- Consequences: the prohibited result. For result crimes (murder, manslaughter, causing grievous bodily harm, criminal damage), the actus reus requires proof that the defendant's conduct caused the relevant harm. For conduct crimes (perjury, possession offences), no further consequence need be proved.
- Circumstances: the external facts that must exist at the time of the conduct. For theft, the property must belong to another; for rape, the absence of consent is a circumstance element.
The requirement of voluntariness
The act must be the product of the defendant's will. Acts performed during an epileptic seizure, under hypnosis, or while sleepwalking are not voluntary and do not satisfy the actus reus. The requirement is fundamental but under-theorised. In Hill v Baxter [1958], the Divisional Court acknowledged that a driver who lost control due to a sudden illness (a swarm of bees, a stroke) would not be performing a voluntary act. The defendant bears an evidential burden to raise the issue; once raised, the prosecution must disprove involuntariness beyond reasonable doubt.
Voluntariness is analytically distinct from automatism (a defence), though the concepts overlap in practice. Automatism goes to whether the defendant was in control of his faculties; voluntariness asks whether the particular movement was willed. The distinction matters because even a conscious defendant may perform involuntary acts (reflex movements, actions under physical compulsion). Conversely, some automatic states may still permit voluntary acts if the defendant retains partial control.
Causation in result crimes
Where the actus reus includes a prohibited result, the prosecution must prove two forms of causation:
Factual causation: the 'but for' test
Would the result have occurred but for the defendant's act? If the answer is no, factual causation is established. If yes, the defendant did not cause the result. This test is necessary but not sufficient. In R v White [1910], the defendant put cyanide in his mother's drink intending to kill her, but she died of a heart attack before the poison could take effect. The 'but for' test was not satisfied: she would have died anyway. White was convicted of attempted murder, not murder.
Legal causation: operative and substantial cause
Even where factual causation is made out, the prosecution must prove that the defendant's act was an 'operative and substantial cause' of the result. This is a normative, evaluative judgment. The defendant's act need not be the sole or even the main cause; it is sufficient that it made a significant contribution. As Lord Parker CJ put it in R v Smith [1959], the defendant's act must be 'a cause' and 'more than minimal'.
Three recurrent problems arise:
*1. Intervening acts (novus actus interveniens)*
An intervening act by a third party or the victim may break the chain of causation if it is 'free, deliberate and informed' (R v Kennedy (No 2) [2007]) or so independent and potent that it renders the defendant's contribution merely part of the background. Medical negligence rarely breaks the chain: in R v Cheshire [1991], the Court of Appeal held that even 'palpably wrong' medical treatment would not relieve the defendant of liability unless it was 'so independent of his acts, and in itself so potent in causing death, that the jury regard the contribution made by his acts as insignificant'. The original wound need not still be an operating cause at the time of death.
In R v Smith [1959], a soldier stabbed in a fight was dropped twice by those carrying him to the medical station and then received treatment that the court described as 'thoroughly bad'. He died. The Courts-Martial Appeal Court upheld the conviction: the stab wound remained an operating and substantial cause.
2. Victim's own acts
If the victim's response to the defendant's unlawful act is within the range of reasonable responses that might be expected, it will not break the chain of causation. In R v Roberts (1971), the victim jumped from a moving car to escape the defendant's sexual advances. The Court of Appeal held that the defendant caused her injuries: the victim's act was 'within the ambit of reasonableness' and therefore foreseeable. The test is objective: would a reasonable person in the victim's position have reacted in that way, or was the reaction 'so daft' or 'so unexpected' as to break the chain?
Refusal of medical treatment, even on religious grounds, does not break the chain: R v Blaue [1975]. The defendant stabbed a young Jehovah's Witness who refused a blood transfusion and died. The Court of Appeal held that the defendant must take the victim as he finds him, including his religious beliefs.
3. Pre-existing frailty: the 'thin skull' rule
The defendant must take the victim as he finds him, whether physically or psychologically fragile. If the defendant assaults a person with a thin skull or a weak heart, and death results, the defendant is liable for that death even though a healthy victim would have survived. This is a rule of legal causation, not a statement about foreseeability. The rule extends to psychological frailty and to refusal of treatment on religious grounds (Blaue).
Omissions
The general rule is that there is no criminal liability for a failure to act. R v Miller [1983] at 961 (Lord Diplock): 'The conduct… must be either an act or an omission to do an act which there was a legal duty to do.' A legal duty may arise in the following situations:
1. Statute
Numerous statutes impose express duties to act: for example, duties to report road traffic accidents, duties on company directors to file accounts, duties under health and safety legislation. Breach is typically a summary offence, though some regulatory offences are indictable.
2. Contract
A contractual duty owed to a third party may give rise to criminal liability if the breach causes harm. In R v Pittwood (1902), a railway crossing keeper failed to close the gate; a cart was struck by a train and a man was killed. The keeper was convicted of manslaughter. His duty arose from his contract of employment, even though the victim was not party to that contract.
3. Special relationship
Parents owe duties to their children; spouses may owe duties to one another (though the scope is controversial). In R v Gibbins and Proctor (1918), a father and his partner deliberately starved the father's seven-year-old daughter to death. Both were convicted of murder. The father's duty arose from the family relationship; the partner's duty arose from her having assumed care of the child.
4. Voluntary assumption of responsibility
A person who voluntarily undertakes the care of another, particularly a vulnerable person, may come under a duty to act. In R v Stone and Dobinson [1977], the defendants, who were of low intelligence, took in Stone's elderly sister, who was anorexic and became bedridden. They made ineffectual attempts to summon help but did not do so in time; she died. Both were convicted of manslaughter. The Court of Appeal held that by taking her in, they had assumed a duty of care.
The decision has been criticised. The defendants were poorly educated and may not have understood the gravity of the situation. The case raises the question: at what point does passive tolerance of another's presence in one's home amount to a voluntary assumption of responsibility?
5. Creation of a dangerous situation
If the defendant inadvertently creates a danger and then becomes aware of it, a duty arises to take reasonable steps to avert the harm. R v Miller [1983] is the leading authority. Miller, a squatter, fell asleep while smoking; he woke to find the mattress on fire. Rather than extinguish it or summon help, he moved to another room. The house caught fire. The House of Lords upheld his conviction for arson. Lord Diplock held that when Miller became aware of the fire he had started, he was under a duty to take reasonable steps to counteract the danger he had created.
The Miller principle has since been extended and refined. In DPP v Santana-Bermudez [2003], the defendant, when asked by a police officer searching him whether he had any sharps, said no. The officer was injured by a needle in his pocket. The Divisional Court held that by creating a danger (the false assurance), the defendant came under a duty to speak truthfully.