Causation in UK Criminal Law — factual and legal causation

For a result-crime conviction, the prosecution must prove the defendant caused the prohibited result. Causation has two limbs: factual and legal.

1. Factual causation: the ‘but-for’ test. Would the result have occurred but for the defendant’s conduct? If yes, the defendant is not the factual cause. R v White [1910] 2 KB 124— defendant put cyanide in mother’s drink intending to kill her; she died of an unrelated heart attack before drinking. No factual causation; conviction reduced to attempt.

2. Legal causation. Beyond factual causation, the defendant’s act must be a substantial and operatingcause — not merely de minimis. The leading principles:

• Substantial cause: R v Pagett (1983) 76 Cr App R 279— defendant used girlfriend as human shield; police shot through her; defendant’s act was a substantial cause of her death.
• Thin-skull rule: you take your victim as you find them — R v Blaue [1975] 1 WLR 1411: Jehovah’s Witness refused life-saving blood transfusion; defendant who stabbed her remained the cause of death.
• Medical treatment: ordinary medical treatment, even if negligent, doesn’t usually break the chain — R v Smith [1959] 2 QB 35: original wound was the operating cause despite poor medical care. But R v Jordan (1956) 40 Cr App R 152— palpably wrong medical treatment did break the chain.
• Victim’s own act: if the victim’s response is reasonable and foreseeable, the chain holds — R v Roberts (1971) 56 Cr App R 95 (victim jumped from moving car to escape sexual assault). If the response is ‘daft’ or unforeseeable, it can break the chain — R v Williams [1992] 1 WLR 380.
• Drug supply / self-injection: R v Kennedy (No 2) [2007] UKHL 38— supplying drugs to a competent adult who self-injects is not unlawful act manslaughter; the victim’s free, voluntary act broke the chain.

3. Novus actus interveniens. A new intervening act breaks the chain if it is sufficiently independent of the defendant’s conduct. Three classic categories: (a) free, deliberate, informed acts of a third party (Pagett exception); (b) extraordinary natural events; (c) gross medical negligence (Jordan).

Omissions and causation. Where the actus reus is an omission, causation focuses on what the defendant failed to do that would have prevented the harm (R v Stone and Dobinson [1977] QB 354; Airedale NHS Trust v Bland [1993] AC 789).

Exam approach. Always run the but-for test first. Then apply the legal-causation principles, going through each potential break in the chain. Consider the thin-skull rule for any pre-existing vulnerability. Cite the leading case for each step. Don’t conflate medical- treatment cases with victim-conduct cases — they’re analysed separately.

See also our mens rea guide and the glossary entry on novus actus interveniens.