Causation is the link between the defendant's breach of duty and the claimant's damage. Even if you can prove duty and breach, you still need to show that the defendant's negligence actually caused the harm. This topic covers two main questions: (1) did the defendant factually cause the damage? and (2) should the defendant be legally responsible for it? You will also deal with situations where something else happened in between (novus actus interveniens), where multiple causes exist, and the eggshell skull rule.
Causation questions often appear alongside duty and breach. You need to work through both factual and legal causation every time. The examiner loves testing the but-for test with tricky fact patterns, and novus actus interveniens comes up frequently with medical negligence or claimant conduct scenarios.
Factual causation asks a straightforward question: but for the defendant's negligence, would the claimant have suffered the harm? If the answer is yes, the defendant is not factually responsible. If the answer is no, the defendant did cause the damage on the balance of probabilities.
Three night watchmen drank tea at the defendant's hospital. The tea was contaminated with arsenic. They all went to the casualty department, but the receptionist sent them home without seeing a doctor. One of them, Mr Barnett, later died from the arsenic poisoning. The court held that the hospital was not liable. But for the receptionist's failure to call a doctor, Mr Barnett would still have died — the arsenic was going to kill him regardless. The negligence did not cause the death on the balance of probabilities.
When you apply the but-for test, think about it literally. Remove the defendant's negligence from the scenario and ask: would the result be the same? If yes, the defendant is not a factual cause. The burden of proof is on the claimant — they must show causation on the balance of probabilities (more likely than not).
What happens when two separate negligent acts each would have been sufficient to cause the same harm on their own? In this situation, the but-for test fails for both defendants — you cannot say that but for defendant A's negligence the harm would not have occurred, because defendant B's act alone would have caused it. The courts have developed solutions to this problem.
A brand-new car was damaged in a road accident caused by the defendant. It needed repairs and was restored to its original condition. Later, the car was damaged again in a second accident (caused by someone else) in the same area. The claimant argued that the first accident had reduced the car's resale value. The House of Lords held that where the first defendant's negligence alone would have caused the full damage, the first defendant is liable for all of it. The second accident made no difference to the claimant's loss.
When two independent acts are each sufficient to cause the harm, the but-for test fails for both. This is because but for either one, the result would have been the same. The courts get around this using alternative approaches, which you will see in the material contribution test and the Fairchild exception below.
Where the but-for test fails because the claimant cannot prove on the balance of probabilities that the defendant's negligence caused the harm, courts may still find causation if the defendant materially contributed to the risk of harm. This is a more generous approach that helps claimants who would otherwise be left without a remedy.
Mr Wardlaw worked at the defendants' foundry for many years and developed pneumoconiosis (a lung disease). The foundry contained both siliceous dust (harmful) and non-siliceous dust (harmless). The claimant could not prove that the siliceous dust, which the defendants failed to control, had actually caused his illness on the balance of probabilities — the harmless dust might have been the cause. However, the House of Lords held that the defendants' breach had materially contributed to the risk of harm, and that was sufficient to establish causation. The defendants were liable.
Remember the distinction: the but-for test asks "did this cause the harm?" while the material contribution test asks "did this contribute to the risk of the harm?" Material contribution is a fallback when scientific uncertainty makes the but-for test impossible to satisfy. It is not a separate test of causation — it is an alternative route to the same conclusion.
Once you have established factual causation, you still need to check legal causation. Legal causation asks whether the defendant should be held responsible for the type of damage that actually occurred, or whether the damage was too remote. The test is reasonable foreseeability: was this type of damage a reasonably foreseeable consequence of the defendant's negligence?
The defendants negligently discharged furnace oil into Sydney Harbour. The oil drifted and fouled the claimant's wharf. The claimant's employees were welding on the wharf, and molten metal set fire to cotton waste floating in the oil. The fire spread and destroyed the wharf. The Privy Council held that the defendants were only liable for the contamination damage, not the fire damage. The fire was not a reasonably foreseeable consequence of spilling oil — oil on water was known to be difficult to ignite. The type of damage must be foreseeable.
It is important to understand that the test looks at the type of damage, not the exact way it happened. The damage does not need to be foreseeable in its precise details or in the exact way it occurred. It just needs to be of a general type that a reasonable person would foresee. For example, physical injury is a foreseeable type of damage from a car crash — the fact that the injury was to a specific bone in a specific way does not make it too remote.
This was a second action arising from the same incident, but brought by the owners of the ships destroyed by the fire. This time, evidence showed that the defendants' chief engineer had known that oil on water could be ignited by molten metal. The Privy Council held that the defendants were liable for the fire damage. Once the type of damage is shown to be foreseeable, the defendant is liable for the full extent of that damage — even if the actual damage is greater than was foreseeable. If the damage is foreseeable in principle, the defendant takes the consequences as they actually unfold.
| Case | Facts | Principle |
|---|---|---|
| The Wagon Mound (No 1) [1961] | Oil spilled in harbour; fire from welding destroyed wharf | Damage must be of a foreseeable type. Fire was not foreseeable, so not recoverable. |
| The Wagon Mound (No 2) [1967] | Same incident; evidence that oil could ignite was available | If damage is foreseeable in type, defendant liable for full extent even if unusually severe. |
| Hughes v Lord Advocate [1963] | Post office workers left manhole open with tent; child climbed in and dropped a paraffin lamp causing explosion | Damage foreseeable in a general sense is sufficient. Burns from an explosion were foreseeable even though the exact circumstances were unusual. |
In an exam answer, always distinguish between the type of damage and its extent. First, ask: was this type of damage foreseeable? If no, the claim fails on remoteness. If yes, then ask: even though the actual damage was more severe than expected, the defendant is still liable for the full amount. The Wagon Mound cases are your go-to authorities.
Novus actus interveniens means "a new act intervening." Sometimes, after the defendant has been negligent, something else happens that contributes to or causes the claimant's damage. The question is: does this new act break the chain of causation between the defendant's negligence and the harm? If it does, the defendant is no longer liable for the damage caused by the intervening act.
The general rule is that an intervening act will NOT break the chain of causation unless it is so extraordinary or unreasonable that it makes the original negligence no longer the effective cause of the harm. The courts are reluctant to let defendants escape liability just because something else happened along the way.
If the original injury requires medical treatment and the treatment is negligent, this does not usually break the chain of causation. The original tortfeasor remains liable. The reasoning is that it is foreseeable that an injured person will need medical attention, and the original wrongdoer must take the consequences of that foreseeable need for treatment.
Mr Smith was burned on the lip by molten metal at work (the defendant's negligence). The burn was minor and would have healed on its own. However, the burn triggered a pre-existing cancerous condition, and Mr Smith died from cancer three years later. The court held the defendant liable for the death. The original tortfeasor must take the victim as they find them — the eggshell skull rule applied. The chain of causation was not broken by the pre-existing condition.
The chain can be broken if the medical negligence is so gross or independent that it supersedes the original injury as the cause of the harm. For example, if a surgeon amputates the wrong limb, that is so far removed from the original accident that it would likely break the chain. The question is whether the medical treatment was so extraordinary that it constituted a completely new and independent cause of the harm.
Do not confuse these two situations: (1) Medical treatment that makes an existing injury worse — chain NOT broken (original tortfeasor still liable). (2) Completely new and independent medical negligence that creates an entirely new injury — chain MAY be broken. The distinction turns on whether the medical negligence is a foreseeable consequence of the original injury.
If the claimant takes reasonable steps to mitigate their injury — for example, following medical advice or attending physiotherapy — this does not break the chain of causation. The defendant remains liable. It would be unfair to punish the claimant for trying to get better.
If the claimant acts unreasonably after the original negligence, and this unreasonable conduct causes further harm, the chain of causation may be broken. The key question is whether the claimant's actions were so unreasonable that they constituted a new and independent cause of the damage.
The claimant was injured in a fall caused by the defendant's negligence. His left leg was left weaker. Later, while walking down stairs, his leg gave way. Instead of holding the handrail or sitting down, he tried to jump to the bottom and injured himself more seriously. The court held that his unreasonable actions broke the chain of causation. The defendant was not liable for the further injury because the claimant's conduct was not a reasonable response to his original injury.
Be careful to distinguish between breaking the chain of causation (a complete defence) and contributory negligence (a partial defence under the Law Reform (Contributory Negligence) Act 1945). If the claimant's actions are unreasonable but not extraordinary, the chain is not broken — but damages may be reduced for contributory negligence. Only truly unreasonable or reckless conduct breaks the chain entirely.
As a general rule, the acts of third parties do not break the chain of causation. The original wrongdoer remains liable even if someone else's actions contributed to the harm. This is because it is often foreseeable that third parties might act in certain ways, and the defendant should not escape liability simply because someone else was also involved.
Borstal boys (young offenders) escaped from a detention centre on an island. The Home Office had been negligent in supervising them. The boys boarded a yacht and drifted it out to sea, causing damage. The House of Lords held that the boys' actions did not break the chain of causation. It was entirely foreseeable that young offenders left unsupervised might cause mischief. The Home Office was liable for the damage caused by the boys.
A third-party act will break the chain if it is free, deliberate, and informed — meaning the third party acted voluntarily, with full knowledge of the consequences, and their actions were not a foreseeable response to the defendant's negligence. In these rare cases, the third party's conduct becomes the new effective cause of the harm.
A motorcyclist was injured in a road accident. A police officer negligently instructed another officer to ride a police motorcycle down a tunnel (which was closed) to warn traffic. The second officer crashed. The court held that the first officer's negligence did not cause the second crash because the second officer had made a free, deliberate, and informed decision to ride into the tunnel. The second officer's actions broke the chain of causation. The first officer was not liable for the second crash.
| Case | Third-Party Act | Chain Broken? |
|---|---|---|
| Home Office v Dorset Yacht [1970] | Young offenders caused damage after escaping from negligent supervision | No — foreseeable that unsupervised offenders might cause mischief |
| Knightley v Johns [1982] | Police officer rode into tunnel following negligent instructions | Yes — the officer made a free, deliberate, informed choice |
| Lamb v Camden LBC [1981] | Squatters entered a vacant house after the council left water pipes exposed | No — the squatters' actions were foreseeable |
The test for a third-party act breaking the chain is deliberately high. Courts are reluctant to allow defendants to shift blame. For the chain to break, you need to show that the third party's actions were truly independent, voluntary, and not a foreseeable consequence of the original negligence. This is a difficult argument to run successfully.
Natural events can potentially break the chain of causation, but only if they are truly extraordinary and unforeseeable. Ordinary weather events or natural processes that are reasonably foreseeable will not break the chain. The defendant must take the victim and their circumstances as they find them.
A ship was damaged by the defendant's negligence and needed repairs. While waiting for a dry dock, a storm (a natural event) caused further damage. The court held that the storm did not break the chain of causation. The storm was not sufficiently extraordinary or unforeseeable — ships face the risk of storms as a normal part of their existence. The defendant was liable for all the damage.
In practice, natural events very rarely break the chain. You would need something truly exceptional — like an earthquake in a region where earthquakes never happen. Ordinary weather, tides, and natural deterioration are all foreseeable and will not break the chain of causation.
Sometimes the claimant has been exposed to negligence by multiple defendants, and it is impossible to tell which one actually caused the harm. This situation arises most commonly in industrial disease cases, where workers were employed by several companies over many years and were exposed to harmful substances (like asbestos) by more than one employer. The but-for test fails because you cannot prove which employer's negligence was the actual cause.
Several employees developed mesothelioma (a fatal lung cancer) after working for multiple employers, each of whom had negligently exposed them to asbestos dust. It was impossible to identify which employer's asbestos had actually caused the disease. The House of Lords held that each employer who had materially increased the risk of the disease was liable. The claimant did not need to prove which specific exposure caused the harm — it was enough that each defendant had wrongfully contributed to the risk.
The House of Lords confirmed the Fairchild principle and held that liability should be apportioned between the defendants according to their share of the risk they created. Each defendant was liable only for a proportionate share of the damage, reflecting the degree to which they had contributed to the risk. This was controversial because it meant claimants could not recover full compensation from any single defendant.
Parliament intervened to address the injustice caused by the Barker decision. Section 3 of the Compensation Act 2006 provides that, in mesothelioma cases where the claimant has been exposed to asbestos by multiple employers, the court may find each employer jointly and severally liable for the full damage. This means the claimant can recover the full amount from any one of the defendants, regardless of their individual share of responsibility. The defendant who pays can then seek contribution from the others.
| Authority | Principle | Effect on Liability |
|---|---|---|
| Fairchild [2002] | Each defendant who materially increased the risk is liable | Overcomes the but-for test failure in mesothelioma cases |
| Barker [2006] | Liability apportioned by share of risk created | Each defendant pays only their proportionate share |
| Compensation Act 2006 s.3 | Court may impose joint and several liability in mesothelioma cases | Claimant can recover full damages from any one defendant |
The Fairchild exception is limited to cases where it is impossible to identify which defendant caused the harm on the balance of probabilities. It is not a general licence to ignore causation requirements. The Compensation Act 2006 s.3 specifically applies to mesothelioma cases. For other types of multiple-cause scenarios, the standard causation rules still apply.
The eggshell skull rule (also called the thin skull rule) means that the defendant must take the claimant as they find them. If the claimant has a pre-existing condition that makes them more vulnerable to injury, the defendant is liable for the full extent of the damage — even if a normal person would have suffered far less. You do not get a discount just because your victim was unusually fragile.
This is the leading case. Mr Smith was burned on the lip by molten metal. The burn itself was minor. However, Mr Smith had a pre-existing cancerous condition, and the burn activated the cancer, which killed him. The defendants argued they should only be liable for the burn, not the death. The court held that the defendants were liable for the full consequences, including the death. The original burn was a foreseeable type of injury, and the defendants had to take Mr Smith as they found him — cancer and all.
Think of it this way: you tap someone lightly on the head, and because they have an unusually thin skull, they suffer a serious brain injury. You are liable for the full brain injury, not just the light tap you intended. The phrase "you must take your victim as you find them" is the one to remember for the exam.
The eggshell skull rule does not let the claimant recover for unforeseeable types of damage. It only applies where the type of damage is foreseeable but its severity is not. For example, if you negligently cause a small cut and the claimant happens to be a haemophiliac who bleeds to death, you are liable for the death — because bleeding is a foreseeable consequence of a cut. But if your negligence causes only minor property damage and the claimant develops an unrelated psychiatric condition, that is not recoverable.
| Area | Key Test | Leading Case |
|---|---|---|
| Factual causation | But-for test: would the harm have occurred without the negligence? | Barnett v Chelsea and Kensington Hospital [1969] |
| Multiple sufficient causes | Each sufficient cause is liable for the full damage | Performance Cars v Abraham [1962] |
| Material contribution | Defendant materially contributed to the risk of harm | Bonnington Castings v Wardlaw [1956] |
| Legal causation (remoteness) | Type of damage must be reasonably foreseeable | The Wagon Mound (No 1) [1961] |
| Novus actus — medical negligence | Ordinary treatment: chain NOT broken. Gross negligence: chain MAY break | Smith v Leech Brain [1962] |
| Novus actus — claimant's actions | Reasonable mitigation: chain NOT broken. Unreasonable: chain MAY break | McKew v Holland [1969] |
| Novus actus — third-party acts | Generally do NOT break chain unless free, deliberate, informed | Home Office v Dorset Yacht [1970] |
| Novus actus — natural events | Rarely break chain unless truly extraordinary | Carslogie Steamship [1952] |
| Multiple causes (indivisible) | Each defendant who increased the risk is liable | Fairchild v Glenhaven [2002] |
| Compensation Act 2006 s.3 | Joint and several liability for mesothelioma | Statutory provision |
| Eggshell skull rule | Defendant must take claimant as they find them | Smith v Leech Brain [1962] |