Law of Torts
Factual Causation and Legal Causation: What Produced the Harm and What the Law Will Charge Him With
Causation in the law is two enquiries wearing one name. Factual causation is a question of history: did the breach in fact bring the harm about, tested by asking whether the harm would have happened but for it. Legal causation is a question of attribution: of the consequences the breach in fact produced, which will the law charge to the defendant, tested by remoteness, by the intervention of other causes, and by the purpose of the duty broken. The first is necessary and never sufficient. A defendant who fails it wins outright; a defendant who passes it may still win on the second.
1. The Comparison
Factual causation | Legal causation | |
The question | Did the breach in fact produce the harm | Which of the consequences will the law attribute to him |
The test | But for the breach, would the harm have occurred | Remoteness; novus actus interveniens; the scope of the duty |
Its nature | A question of fact | A question of legal policy, expressed in the language of causation |
Decided by | Evidence, including expert evidence | The court, on grounds of legal responsibility |
Also called | Causation in fact, the cause in fact, sine qua non | Causa causans, proximate cause, remoteness of damage |
If the defendant succeeds | He wins outright. Nothing he did made any difference | He wins as to the consequences excluded, and may remain liable for others |
Where it sits | After breach and before remoteness | After factual causation and before assessment |
2. Factual Causation: the But For Test
- Ask whether the harm would have occurred if the defendant had not been in breach. If it would, the breach is not a cause in fact, and the claim fails however gross the carelessness.
- It is a test of necessity and not of sufficiency. The breach need not be the only cause, or even the principal one; it need only be a cause without which the harm would not have happened.
- Cork v. Kirby MacLean Ltd., [1952] 2 All ER 402 states it: if the damage would not have happened but for a particular fault, that fault is the cause of the damage; if it would have happened just the same, fault or no fault, the fault is not the cause.
- It disposes of the case where the defendant's breach changed nothing, which is the commonest way a negligence claim fails on causation.
📖 Barnett v. Chelsea and Kensington Hospital Management Committee, [1969] 1 QB 428 Facts Three night watchmen went to a hospital casualty department in the early hours complaining of vomiting after drinking tea. The nurse telephoned the casualty officer, who did not examine them and told them to go home and consult their own doctors. One of the men died some hours later of arsenical poisoning. The evidence established that even with prompt admission and treatment the antidote could not have been administered in time to save him. Held The hospital owed a duty and was in breach of it, but the claim failed. Ratio Breach of duty does not by itself found liability. The plaintiff must show that the breach caused the death, and here the deceased would have died in any event. Where the harm would have occurred regardless of the negligence, the but for test is not satisfied and no action lies. |
3. Where the But For Test Fails
- It breaks down where several causes operate and none is individually necessary, or where the state of scientific knowledge cannot say which of them did the damage. The law has developed three relaxations.
- Material contribution to the damage. Bonnington Castings Ltd. v. Wardlaw, [1956] AC 613: where the injury was caused by cumulative exposure to dust from a guilty and an innocent source, it is enough that the guilty source made a material contribution to the disease.
- Material contribution to the risk. McGhee v. National Coal Board, [1973] 1 WLR 1: where the defendant's breach materially increased the risk of the very injury that occurred, and the state of knowledge cannot say whether it caused it, the increase in risk may be treated as sufficient.
- The Fairchild exception. Fairchild v. Glenhaven Funeral Services Ltd., [2003] 1 AC 32: where a claimant contracted mesothelioma after exposure by several employers, and medical science could not identify which exposure caused it, each employer who materially increased the risk is liable, the alternative being that a plainly injured claimant recovers from nobody.
- Wilsher v. Essex Area Health Authority, [1988] AC 1074 marks the limit. Where there were five possible causes of the infant's blindness and the defendant's negligence accounted for only one of them, proving that the negligence added one candidate to a list is not proving that it caused the harm. The relaxation applies where there is one causal agent, not several competing ones.
4. Legal Causation
- Once the breach is shown to be a cause in fact, the question becomes how far down the chain of consequences the defendant should answer. This is a question of attribution, and the courts decide it under three headings.
- Remoteness. The defendant answers only for damage of a kind that was reasonably foreseeable: The Wagon Mound. Damage of an unforeseeable kind is too remote, though its unforeseeable extent is not: TORT 123.
- Novus actus interveniens. An intervening act may break the chain. It does so where it is free, deliberate and informed, so that responsibility passes to the intervener; and it does not do so where the intervening conduct was the very thing the defendant should have guarded against.
- The scope of the duty. A defendant answers only for the consequences falling within the risk against which the duty was imposed. Where a duty exists to guard against one kind of harm, the defendant does not answer for a different kind merely because his breach was a but for cause of it.
5. Novus Actus Interveniens
The intervening cause | Does it break the chain |
An act of nature | Yes, where it is extraordinary and unconnected with the risk created. No, where it is the ordinary operation of the elements |
A third party's deliberate wrongful act | Ordinarily yes, being free, deliberate and informed. But no, where the defendant's duty was precisely to guard against such an act |
A third party's negligent act | Usually not, unless it was so unreasonable as to be outside anything the defendant should have contemplated |
An instinctive act, or an act done in the agony of the moment | No. A person put in peril by the defendant is not held to a nice judgment. Scott v. Shepherd, where a lighted squib thrown into a market was twice thrown on in self preservation |
A rescuer's intervention | No. Rescue is a foreseeable response to danger, and the rescuer has his own claim. Haynes v. Harwood |
The plaintiff's own unreasonable conduct | It may break the chain where it is wholly unreasonable, and otherwise reduces the award as contributory negligence |
Medical treatment of the injury | Ordinary or even negligent treatment does not break the chain; grossly negligent treatment may |
6. Successive and Supervening Causes
- A second injury after the first raises the question whether the first defendant continues to answer for the original loss.
- Baker v. Willoughby, [1970] AC 467. The plaintiff's leg was injured by the defendant's negligence and was later amputated after a shooting in a robbery. The first defendant remained liable for the continuing disability, since otherwise the plaintiff would fall between two wrongdoers and recover fully from neither.
- Jobling v. Associated Dairies Ltd., [1982] AC 794. A back injury caused by the defendant was later overtaken by an unrelated and naturally occurring spinal disease which would itself have disabled the plaintiff. The damages were limited to the period before the disease supervened, the vicissitudes of life being taken into account in assessing loss of earnings in any event.
- The two are reconciled on the footing that a supervening tort is treated differently from a supervening natural illness, since the first leaves a wrongdoer to answer and the second does not, though the distinction is not free from difficulty.
⚠ Why legal causation is policy, and why it is better to say so Judges speak of the chain of causation being broken, of a cause being proximate rather than remote, and of an event being the real or effective cause, and this language suggests that the question is one of physical or logical fact which careful analysis will settle. It is not. As a matter of physical fact every necessary condition of an event is equally a cause of it, and the chain runs backwards without limit: the driver who was careless, the employer who rostered him, the manufacturer who built the vehicle, the parents of the driver. What the law is doing when it selects one of these and calls it the cause is making a judgment about responsibility, using causal language to express it. That is why the tests are not consistent with one another and why the same intervening act is a novus actus in one case and not in another: the answer depends on what duty the defendant owed and what risk it was meant to guard against. The practical value of recognising this is that an argument about legal causation is really an argument about the scope of the duty, and it is more persuasively made in those terms, by asking what the defendant was supposed to be protecting the plaintiff from, than by asserting that a chain was or was not broken. |
7. The Position Stated Shortly
1. Factual causation asks whether the breach in fact produced the harm; legal causation asks which consequences the law will attribute to the defendant.
2. The test of factual causation is the but for test: would the harm have occurred if the defendant had not been in breach.
3. Barnett v. Chelsea and Kensington Hospital is the leading illustration: a man who would have died anyway recovers nothing though the doctor was in breach.
4. The but for test is one of necessity and not of sufficiency, and the breach need not be the only or the principal cause.
5. Where several causes operate, Bonnington allows material contribution to the damage and McGhee and Fairchild allow material contribution to the risk.
6. Wilsher marks the limit: adding one candidate to a list of five possible causes is not proof of causation.
7. Legal causation operates through remoteness, novus actus interveniens and the scope of the duty.
8. An intervening act breaks the chain where it is free, deliberate and informed, and does not where it is the very thing the defendant should have guarded against.
9. Baker v. Willoughby and Jobling v. Associated Dairies govern supervening causes, distinguishing a later tort from a later natural illness.
10. Legal causation is a judgment about responsibility expressed in causal language, and an argument about it is in substance an argument about the scope of the duty.