Actus reus — conduct, causation, omissions

Criminal liability requires proof of actus reus (external element) and mens rea (mental element). The actus reus comprises three potential components: conduct (a voluntary act or, exceptionally, an omission where a legal duty exists), consequences (the prohibited result in result crimes), and circumstances (surrounding facts that must exist). Two threshold requirements apply: the conduct must be voluntary (reflex actions, unconscious movements, or acts under physical compulsion do not suffice), and where result crimes are charged, causation must be proved—both factual ('but for' test) and legal (defendant's act must be an 'operative and substantial cause'). The criminal law generally punishes acts, not omissions; liability for failing to act arises only where specific legal duties exist through statute, contract, special relationships, voluntary assumption of responsibility, or creation of danger.

R v White [1910] 2 KB 124: Established the 'but for' test for factual causation—defendant put poison in mother's drink but she died of heart attack before it took effect. No factual causation meant no murder conviction. Essential for distinguishing factual from legal causation.

R v Cheshire [1991] 1 WLR 844: Medical negligence only breaks causation if 'so independent' and 'so potent' as to render defendant's acts insignificant. Original wound need not still be operating at death. Sets high bar for breaking causation chain.

R v Miller [1983] 2 AC 161: Created duty to act when defendant inadvertently causes danger and becomes aware of it. Miller fell asleep smoking, woke to find mattress burning, moved rooms without acting. Establishes principle that awareness of self-created danger generates positive duty.

R v Kennedy (No 2) [2007] UKHL 38: Victim's free, deliberate and informed act breaks causation. Dealer prepared heroin syringe; victim self-injected and died. House of Lords held self-injection broke chain. Crucial for autonomous third-party acts.

R v Stone and Dobinson [1977] QB 354: Voluntary assumption of care creates legal duty. Defendants of low intelligence took in anorexic sister who died; convicted of manslaughter. Shows how easily courts find assumed duties.

Courts approach actus reus systematically. First, identify whether defendant performed voluntary act—if involuntary (reflex, unconsciousness, physical compulsion), no actus reus. Second, for result crimes, apply causation tests sequentially: (1) factual causation—but for defendant's act, would result have occurred? If no, proceed to (2) legal causation—was defendant's act operative and substantial cause? Consider intervening acts: medical negligence rarely breaks chain (apply Cheshire test); victim's reasonable response maintains causation (Roberts); victim's free deliberate act breaks it (Kennedy). For omissions, identify source of legal duty—statute, contract, relationship, assumption of responsibility, or danger creation (Miller). Without established duty category, no liability for failing to act. Apply 'thin skull' rule absolutely—defendant takes victim as found, including pre-existing conditions and religious beliefs affecting treatment decisions (Blaue).

Students confuse factual and legal causation—both must be proved separately; satisfying 'but for' test alone insufficient. Medical negligence assumed to break causation when Cheshire establishes opposite presumption. Kennedy over-applied—only victim's free deliberate act breaks chain, not any third-party intervention. Forgetting absolute nature of thin skull rule—defendant liable even if normal victim would survive. Assuming moral duties create legal duties for omissions—must identify specific legal duty category. Missing voluntariness threshold—involuntary acts mean no actus reus, making mens rea irrelevant. Misreading Miller—duty arises at awareness, not danger creation.