Law of Torts
Novus Actus Interveniens: When a New Intervening Act Breaks the Chain of Causation
A novus actus interveniens is a new intervening act so independent of the defendant's breach, and so unreasonable, that the law treats it rather than the breach as the cause of what followed. The defendant's breach remains a cause in fact: the but for test is still satisfied. What the doctrine does is cut the defendant's liability off at the point where the second event takes over. Three kinds of intervening event arise: the act of a third party, the act of the claimant himself, and a natural event. In each case the question is the same, whether the intervention was reasonably foreseeable and whether the defendant's breach remained an operative cause when the harm occurred.
The three kinds of intervening event, the test stated three ways, and what does not break the chain
1. What the Doctrine Does
- It is a doctrine of legal causation, not of factual causation. The breach is still a cause in fact; the question is whether the law will continue to treat it as the cause.
- Its effect is to end the defendant's liability for everything after the intervening act, which makes it an all or nothing doctrine as against that later harm.
- It is the reason the but for test needs a filter. Without it, a defendant would be liable for an indefinite chain of consequences however bizarre.
- It is a question of law and degree, and there is no formula that decides it. The courts have expressed the test in several ways, all pointing in the same direction.
- The burden of establishing it is on the defendant, since it is he who seeks to cut off a chain the plaintiff has otherwise established.
2. The Test, Stated Three Ways
1. Was the intervening act reasonably foreseeable as a consequence of the breach? If it was, it does not break the chain, because the defendant ought to have had it in contemplation.
2. Was it a free, deliberate and informed act, or was it instinctive, constrained or made under pressure? Only a genuinely free and independent act is likely to break the chain.
3. Did the defendant's breach remain an operative cause of the ultimate harm, or had it spent itself by the time the second event occurred? If the breach was still working, the chain is unbroken.
4. The three tests converge, and a court will often use more than one of them in the same judgment.
5. The more culpable and the more deliberate the intervention, the more likely it is to break the chain, and conversely a merely careless intervention usually does not.
3. Intervening Act of a Third Party
📖 Scott v. Shepherd (1773) 2 Wm Bl 892 Facts The defendant threw a lighted squib into a crowded covered market. It landed on the stall of one Yates. Willis, a bystander, picked it up and threw it across the market to protect himself and the goods. It landed on another stall, and Ryal threw it on again. It struck the plaintiff in the face and exploded, putting out his eye. Held The original thrower was liable in trespass. Ratio The intermediate acts of Willis and Ryal did not break the chain. They acted instinctively and under a compulsive necessity for their own safety, and were not free and deliberate actors. Their conduct was a natural and probable consequence of the defendant's own act, and the defendant remained answerable for the ultimate injury. The case is the classic illustration of an intervention that does not amount to a novus actus. |
- The deliberate wrongdoing of a third party ordinarily breaks the chain, because the law treats a free and informed human choice as the effective cause.
- But it does not where the defendant was under a duty to guard against it. In Home Office v. Dorset Yacht Co. [1970] AC 1004, borstal officers left trainees unsupervised on an island; the trainees took a yacht and damaged the plaintiff's vessel. The Home Office was liable, having control over the boys and knowledge of the very risk that materialised.
- Nor where the intervention was the very thing the defendant ought to have prevented, as where an occupier leaves premises insecure and they are entered by vandals whose conduct was foreseeable.
- Ordinary medical treatment of the injury does not break the chain, even if given somewhat carelessly. Only grossly negligent treatment, so egregious as to be regarded as a fresh cause, will do so.
4. Intervening Act of the Claimant
- The claimant's own act breaks the chain only if it is unreasonable. Conduct that is a natural and foreseeable response to the situation the defendant created does not.
- A reasonable attempt to extricate himself does not break the chain, even if it turns out badly.
- An unreasonable act is usually treated as contributory negligence rather than as a break, because that apportions the loss instead of defeating the claim, and the courts prefer apportionment.
- The question is one of degree. A plaintiff with an injured leg who attempts a steep flight of stairs without assistance may be contributorily negligent; one who does something reckless and wholly unnecessary may break the chain.
- A suicide following a negligently inflicted injury has been treated as within the chain where the injury caused the depression that led to it, which shows how reluctant the courts are to find a break.
5. Intervening Natural Event
- A natural event breaks the chain only if it is extraordinary. The ordinary operation of nature on a situation the defendant created does not.
- So rain falling into a hole the defendant negligently dug is not a novus actus, rain being an ordinary incident of the climate.
- An unprecedented flood or storm may be, and where the tort alleged is under Rylands v. Fletcher the same facts will usually be considered under the heading of act of God.
- The test overlaps with act of God but is not identical. Act of God is a defence with two settled requirements; a natural novus actus is a question of causation.
- The natural deterioration of a thing left in a dangerous condition is ordinarily foreseeable, and does not break the chain.
6. What Does Not Break the Chain
The intervening event | Why the chain holds |
A rescue attempt | Danger invites rescue, and the defendant's own act invited it: Haynes v. Harwood |
Ordinary medical treatment | Treatment of the injury is foreseeable, and even somewhat careless treatment is within the risk the defendant created |
An instinctive act of self preservation | It is not a free and deliberate choice: Scott v. Shepherd |
The claimant's reasonable attempt to extricate himself | A natural and foreseeable response to the position the defendant put him in |
An act the defendant was under a duty to guard against | The defendant cannot rely on the very event he ought to have prevented: Home Office v. Dorset Yacht |
The ordinary operation of nature | Foreseeable, and therefore within the risk |
7. Novus Actus or Contributory Negligence
⚠ The choice between them decides how much the plaintiff recovers Where the intervening act is the claimant's own, the court has a choice between two analyses, and the difference is not academic. If the act is treated as a novus actus, the defendant's liability for everything after it ends, and the plaintiff recovers nothing for that later harm. If it is treated as contributory negligence, the damages are reduced in proportion to the plaintiff's share of the fault, and he recovers the balance. Modern practice strongly prefers the second, because apportionment produces a result matched to the degree of the plaintiff's fault rather than an all or nothing outcome. A break in the chain is therefore reserved for conduct that is not merely careless but so unreasonable as to be properly regarded as the effective cause of what followed. |
8. The Position Stated Shortly
1. A novus actus interveniens is a new intervening act that breaks the chain of causation.
2. It is a doctrine of legal causation: the breach remains a cause in fact, and the doctrine cuts off liability for what follows.
3. The test asks whether the intervention was reasonably foreseeable, whether it was a free and deliberate act, and whether the breach remained an operative cause.
4. Scott v. Shepherd holds that instinctive acts of self preservation do not break the chain.
5. The deliberate wrongdoing of a third party ordinarily does, unless the defendant was under a duty to guard against it: Home Office v. Dorset Yacht.
6. Ordinary medical treatment does not break the chain; only grossly negligent treatment will.
7. The claimant's own act breaks the chain only if unreasonable, and a reasonable attempt to extricate himself does not.
8. A natural event breaks the chain only if extraordinary; the ordinary operation of nature does not.
9. A rescue does not break the chain, because danger invites rescue.
10. Where the intervening act is the claimant's own, the courts prefer contributory negligence to a break, because it apportions rather than defeats.