Actus reus — conduct, causation, omissions

Criminal law begins with a deceptively simple proposition: liability ordinarily requires both an external element and a mental element. The external element is conventionally called the actus reus. It is not confined to an act. It may consist of conduct, circumstances, consequences, or a combination of all three. In homicide, for example, the prohibited consequence is death; in theft, the conduct is appropriation and the relevant circumstances include property belonging to another; in possession offences, the external element may consist primarily in a state of affairs.

This first week of Durham Criminal Law therefore supplies the architecture for the whole module. Before one can analyse murder, non-fatal offences, sexual offences, dishonesty, complicity or inchoate liability, one must ask what the defendant is said to have done, what circumstances made that conduct criminal, and whether any prohibited result can properly be attributed to that conduct. Actus reus performs a limiting function. It prevents punishment for bad character, dangerous thoughts, abstract wickedness, or mere moral failure unless the law has identified an external basis for liability.

Three topics dominate this note. First, conduct must ordinarily be voluntary. The criminal law does not usually punish bodily movements which are not the defendant’s movements in an agency sense: spasms, reflexes, or movements caused by overwhelming external force. The requirement is not always expressed with care in the cases, but it reflects a central premise of responsibility: a person is liable for what he does, not merely for what happens through him.

Secondly, where a result offence is charged, causation must be proved. It is not enough that D behaved badly and V suffered harm. The prosecution must show factual causation, usually by the ‘but for’ inquiry, and legal causation, requiring that D’s contribution be sufficiently operative and not displaced by a legally significant intervening act. Causation is not metaphysics; it is a normative doctrine for attributing criminal responsibility.

Thirdly, omissions raise the most difficult boundary question. English law is reluctant to impose liability for a mere failure to act, even where intervention would have been easy and morally expected. Yet that reluctance is qualified by recognised duties: duties arising from statute, contract, special relationships, voluntary assumption of responsibility, creation of danger, and certain public or office-based roles. A central task in this topic is to distinguish moral blameworthiness from legal duty.

For Durham purposes, this is also an exercise in legal method. The compulsory first-year subjects have already trained you to separate rule, rationale and remedy. Criminal law requires the same discipline. In tort, causation often allocates losses; in contract, obligations are voluntarily or legally recognised; in constitutional law, legality controls state power. In criminal law, the stakes are conviction and punishment. The actus reus must therefore be identified with particular precision before the mens rea or any defence is considered.

The language of actus reus and mens rea is inherited from a long common-law tradition, but it should not be mistaken for a neat Roman or civilian code. English criminal law developed piecemeal: particular wrongs were defined through indictment practice, judicial elaboration and later statutory intervention. The general part — the principles about conduct, fault, causation, participation and defences — emerged unevenly from litigation about particular offences.

Historically, criminal liability was often described through the maxim actus non facit reum nisi mens sit rea: an act does not make a person guilty unless the mind is guilty. The maxim is useful as a reminder that criminal liability normally has both external and internal elements. But it is not a rule from which cases can be deduced. It does not tell us which external elements matter, whether omissions count, when consequences are attributable, or how far strict liability may be imposed. Modern criminal law uses the actus reus/mens rea distinction as an organising device rather than as a complete theory.

The common law was early concerned with the difference between voluntary human action and mere occurrence. A person who is physically carried into a prohibited place, or whose body moves involuntarily, does not act in the legally relevant sense. This concern is visible in older cases such as Larsonneur and later discussions such as Hill v Baxter. The principle is partly moral and partly evidential: punishment presupposes conduct attributable to the defendant as an agent.

Causation developed most prominently through homicide. Murder and manslaughter forced courts to ask whether D’s act caused V’s death where medical treatment, victim reactions, refusal of treatment, negligent third parties, or self-administered drugs complicated the causal story. The resulting law reflects two pressures. On the one hand, criminal law must not allow a defendant to escape liability merely because the exact route to harm was unusual. On the other hand, it must not attribute every later misfortune to an initial wrong. Thus English law came to distinguish factual causation from legal causation, and to insist that D’s conduct remain a significant and operating cause.

Omissions developed even more cautiously. The common law did not embrace a general duty of easy rescue. A passer-by who sees a stranger drowning and fails to help is morally censurable but, absent more, not guilty of homicide. That position reflects liberal anxieties about coercing citizens into general benevolence, evidential difficulties about what could have been done, and the criminal law’s concern with fair labelling. Yet the common law also recognised that some failures are not mere non-intervention. Where D has undertaken care, created a danger, assumed responsibility, or occupies a legally defined role, failure to act may be equivalent to culpable wrongdoing.

The modern law therefore represents a compromise. It rejects liability for bare moral indifference, but accepts liability for omissions where a legal duty can be identified. It accepts that causation is necessary in result crimes, but treats it as a normative judgment rather than a scientific formula. It requires conduct, but recognises that conduct may include continuing acts, possession, states of affairs and failures to discharge legally recognised duties. This layered history explains why Week 1 is not a preliminary technicality. It is the foundation for every later topic in Durham Criminal Law.

1. Actus reus means the external elements of the offence. It is better translated as ‘the prohibited conduct, circumstances and consequences’ than as ‘the guilty act’. Some offences require conduct only: for instance, driving while disqualified. Some require conduct in specified circumstances: handling stolen goods, or appropriation of property belonging to another. Some require a prohibited consequence: murder requires death; section 18 of the Offences Against the Person Act 1861 requires wounding or causing grievous bodily harm. A good answer begins by disaggregating the offence: what conduct is alleged, what surrounding circumstances must exist, and what result must be proved?

1. Conduct must normally be voluntary. A criminal offence ordinarily presupposes human agency. If D’s arm is forced by another person, or D suffers an unforeseeable reflex movement, there may be no actus reus. This is not a defence in the strict sense; it is a denial that D performed the conduct element. The distinction matters. Automatism, involuntariness and insanity raise more specialised questions later in the module, but at this stage the point is basic: the criminal law punishes responsible action, not merely bodily movement.

1. A state of affairs may sometimes constitute the actus reus. English law has occasionally imposed liability where D did not positively act at the relevant moment. Larsonneur is the classic illustration: D was found in the United Kingdom after being deported from Ireland against her will. The decision is controversial because it appears to treat presence as sufficient, even though the presence was involuntary in an ordinary sense. The better view is to confine such cases to their statutory context and to treat them as warnings about strict or regulatory offences, not as a general denial of voluntariness.

1. The coincidence principle remains important. Normally, the actus reus and mens rea must coincide in time. Fagan shows how this can be satisfied where the conduct is a continuing act: D accidentally drove onto a police officer’s foot but then knowingly refused to move. The later mens rea coincided with the continuing actus reus. This doctrine prevents liability where fault arises only after the offence is complete, while also preventing artificial acquittals where a single course of conduct continues over time.

The early actus reus cases should be read as illustrations of principle rather than as isolated facts. They teach a method: isolate the prohibited external element, ask whether it was voluntary or continuing, and then ask whether the defendant caused any prohibited result.

Larsonneur is often introduced as a harsh state-of-affairs case. The defendant was physically returned to the United Kingdom and then convicted of being found here in breach of immigration conditions. Its continuing significance lies less in its moral persuasiveness than in its warning that some offences criminalise circumstances rather than chosen acts. It should not be allowed to swallow the principle of voluntariness.

Hill v Baxter is commonly used for the opposite point. The Divisional Court accepted, in principle, that a driver who lost control because of an unforeseeable event such as a swarm of bees or sudden illness might lack voluntary conduct. Although the defendant failed on the facts, the case remains a convenient starting point for involuntariness. It also shows the evidential burden that defendants face when asserting that apparent conduct was not voluntary.

Fagan is the leading continuing-act case. The defendant drove onto a police officer’s foot accidentally but then refused to move. The court avoided the coincidence problem by treating the actus reus as continuing while the car remained on the foot. Fagan is therefore indispensable in any answer involving a sequence in which fault develops after an initial innocent movement.

Miller is the foundation of liability for failing to avert a danger one has created. The defendant accidentally started a fire and, on becoming aware of it, moved away rather than taking steps to extinguish it or call for help. The House of Lords treated his failure to act after awareness of the danger as capable of founding liability. The case does not impose a general rescue duty; it imposes a duty where D’s own conduct has created or contributed to the danger.

Stone and Dobinson shows voluntary assumption of responsibility. The defendants allowed an anorexic relative, who was living with them and dependent upon them, to deteriorate and die. Their liability was grounded in the care they had undertaken and the dependency they had accepted. The case is uncomfortable because the defendants themselves had limited capacity, but doctrinally it remains central for omission liability.

Blaue is the classic thin-skull case in criminal causation. The defendant stabbed the victim, who refused a blood transfusion because of her religious beliefs and died. The refusal did not break the chain of causation. D takes V as found, including religious conviction. Blaue is especially useful because it demonstrates the court’s unwillingness to treat victim vulnerability as an intervening cause.

Cheshire sets a demanding threshold for medical treatment to break the chain of causation. The defendant shot the victim, who later died after negligent medical treatment. The Court of Appeal held that treatment will break the chain only in exceptional circumstances where it renders the defendant’s acts insignificant. This is a policy-laden causation rule designed to prevent defendants from escaping liability because doctors struggled with the consequences of the initial injury.

Kennedy (No 2) marks the modern limit of causal attribution where an informed adult voluntarily acts. The defendant prepared a syringe of heroin; the deceased self-injected and died. The House of Lords held that the deceased’s voluntary and informed self-administration broke the causal chain for unlawful act manslaughter. Kennedy is crucial because it treats autonomous human intervention as different from mere physical vulnerability, medical negligence, or predictable panic.

See practice questions section below.

See further reading section below.