The ‘But For’ Test
Some crimes require the defendant to cause a particular result. If this is the case, the prosecution must prove factual and legal causation. Factual causation exists if but for the defendant’s act or omission, the result would not have come about: R v White  2 KB 124. The defendant’s acts do not to be the sole cause, or even the main cause, of the proscribed result: R v Hennigan  3 All ER 133.
‘Breaks in Causation’
Certain subsequent events may ‘break the chain of causation’ between the defendant’s action and the proscribed result. In these circumstances, the defendant is not taken to have caused the result in law.
Act of a Third Party
Reasonable or foreseeable third-party actions will not break causation: R v Pagett (1983) 76 Cr App R 279. Completely unforeseeable acts, or fully informed, unreasonable and voluntary acts of third parties may break causation, however: R v Latif  2 Cr App r 92. Generally, the criminal law is far less willing than tort to find that the act of third parties (even negligent or deliberate acts) are unforeseeable or unreasonable: R v Benge (1865) 4 F&F 504; R v Warburton & Hubbersty  EWCA Crim 627.
Act of the Claimant
If the victim’s actions are a natural or foreseeable consequence of the defendant’s actions, they will not break causation: R v Roberts  EWCA Crim 4; R v Pitts (1842) Car & M 284. Only completely ‘daft’ or ‘unexpected’ actions in the circumstances will break causation: R v Williams & Davis  Crim LR 198. Free, informed and voluntary actions of a capable victim (particularly in drug-taking cases) will break the chain of causation, however: R v Kennedy (No 2)  UKHL 38. However, not even outright suicide will be ‘voluntary’ if the defendant’s actions provide a strong reason or compulsion for it: R v Wallace  EWCA Crim 690.
Act of Nature
A completely unforeseeable non-human act may break the chain of causation if it is not a natural result of what the defendant did. For example, a defendant is a legal cause of death if he knocks the victim unconscious and leaves him in a puddle to drown, but not if the building the unconscious victim is left in subsequently collapses on his head for unrelated reasons: R v Hallet  SASR 141.
The Eggshell Skull Rule
The defendant must take the victim as they find them. This has two implications:
The result does not need to be foreseeable. This means that if the claimant has a special condition that makes them more likely to suffer harm (or more serious harm) than a normal person, the defendant is still taken to cause the harm: R v Hayward (1908) 21 Cox 692.
The Defendant’s Contribution Must be an ‘Operative’ Cause
If the defendant’s contribution is merely background setting, they are not a legal cause.
For example, the defendant invites the victim to his house. The victim is killed in a car accident on the way. The defendant is not a legal cause of death even though without their invitation the victim would be alive.
Making the distinction between operative and non-operative causes can be difficult and causes particular problems in negligent medical treatment cases:
For example, in R v Jordan (1956) 40 Cr App E 152, the victim died of pneumonia more than a week after being stabbed by the defendant. At the time of death, the stab wounds had started to heal. The cause of the pneumonia was the negligent administration of fluids and antibiotics which the victim was allergic to. The defendant’s act was deemed not to be an operative cause of death.
By contrast, in R v Smith  2 QB 35, the defendant stabbed the victim. The victim was dropped several times on the way to receive medical treatment. The doctor then failed to properly diagnose the victim’s injuries, meaning he was not treated for a punctured lung. The victim probably would have survived if he had been given proper medical treatment. Nevertheless, the defendant was found to have caused his death since the victim died of the punctured lung.
In R v Cheshire  1 WLR 844, the defendant shot the victim in the leg and stomach. The victim developed respiratory problems in hospital and was given a tracheotomy as a result. He later died from complications from the tracheotomy, at a time when his shooting wounds were healing. The defendant’s shooting was deemed to be an operative cause of death. This was because the subsequent medical treatment was not ‘so potent’ that it made the defendant’s contribution unimportant.
The Cause Must Be ‘Culpable’
The act which the prosecution relies on as being the ’cause’ must be culpable: R v Dalloway (1847) 2 Cox CC 273. This does not mean that the defendant must have acted deliberately or negligently, merely that there is something about what the defendant did which was ‘open to proper criticism’: R v Hughes  UKSC 56.