Supreme Court Clarifies Law of Causation – Clements v Clements

by John McKiggan

Last week, the Supreme Court of Canada released its decision in Clements v Clements. The decision offers some useful clarification regarding the law of causation.

The Facts

In the Clements case the Defendant, Mr. Clements, was driving his motorcycle with his wife, the Plaintiff, riding behind him as a passenger. The bike was overloaded by almost a hundred pounds, and there was a nail that punctured the rear tire of the motorcycle.

travelingelling in a 100 km zone, Mr. Clements accelerated to 120 km/hr to pass a vehicle. The nail fell out of the rear tire causing it to deflate. Mr. Clements lost control of the bike, and Mrs. Clements suffered a severe traumatic brain injury in the resulting crash. Mrs. Clements sued her husband claiming her injuries were caused by his negligence in exceeding the speed limit, and overloading the motorcycle.

At trial, the judge found that the Mr. Clements’ negligence contributed to Mrs. Clements’ injury. However, the accident reconstruction experts were not able to prove on the “But For” test that Mr. Clements’ negligence had caused the crash. The trial judge ruled in Mrs. Clements’ favour on the basis that Mr. Clements’ actions had materially contributed to her injury.

The British Columbia Court of Appeal set aside the trial judgment and dismissed the Plaintiff’s claim on the basis that the material contribution test did not apply.

The Supreme Court of Canada ruled, seven to two, that a new trial should be ordered.

Must Use “But For” Test

In order to be successful in a claim for negligence, the plaintiff must prove “on the balance of probabilities” that the defendant was the cause of his or her injuries.

I have explained this in past articles and in this educational video that you can find on our website and our You Tube channel. 


In most cases the Court applies “But For” test to determine causation. In other words, But For the defendant’s negligence with the injury have occurred?

Except When You Can’t

However, in certain circumstances it may be difficult to determine exactly who or what caused a plaintiff’s injury. This is frequently the case in medical malpractice claims where the patient had an underlying illness or where a number of doctors or nurses were involved in the care of the patient and it may not be clear how the care of each person contributed to the patients bad outcome.

Material Contribution

So in limited circumstances the Court may apply a “material contribution” test. Simply put, did the defendant’s conduct materially contribute to the risk of injury?

I have posted before about the difficulty the Courts have had in applying these two standards.

Proving Causation in Medical Malpractice Cases – Sienkiewicz v. Greif

Medical Malpractice Compensation Limited to Harm Caused by Doctors Negligence: Ontario Court of Appeal

Proving causation can be particularly challenging in cases with complicated scientific medical evidence or in cases with multiple wrongdoers where in number of different wrongful acts contributed to the injury but no one wrongful act caused the entire injury.

Chief Justice Summarizes State of the Law

Writing for the majority, Chief Justice McLachlin offered a useful summary of the law of causation, and when “But For” test applies and when the “material contribution” test applies.

Increasing the Risk not Increasing the Injury

Justice McLachlin stated that although some cases have said that a defendant will be liable where they materially contribute to the plaintiff’s injury the correct application of the material contribution test requires the plaintiff to show that the defendant’s conduct materially contributed to the plaintiff’s risk of injury.

In other words, it is not the injury itself that requires examination but the circumstances surrounding the increase in risk of injury that is relevant.

No More Pointing Fingers

The court clarified that the “material contribution” test will generally apply in cases of multiple defendants where the plaintiff has proved that “but for” the negligence of one or more of the defendants, the injury would not have occurred – but the plaintiff cannot determine which defendant is at fault because they are all pointing their fingers at each other.

[43] It is important to reaffirm that in the usual case of multiple agents or actors, the traditional “but for” test still applies. The question, as discussed earlier, is whether the plaintiff has shown that one or more of the defendants’ negligence was a necessary cause of the injury. Degrees of fault are reflected in calculations made under contributory negligence legislation. By contrast, the material contribution to risk approach applies where “but for” causation cannot be proven against any of multiple defendants, all negligent in a manner that might have in fact caused the plaintiff’s injury, because each can use a “point the finger” strategy to preclude a finding of causation on a balance of probabilities.


J. McLachlin concluded in her summary of the state of the law in Canada as follows:

The foregoing discussion leads me to the following conclusions as to the present state of the law in Canada:

(1) As a general rule, a plaintiff cannot succeed unless she shows as a matter of fact that she would not have suffered the loss “but for” the negligent act or acts of the defendant. A trial judge is to take a robust and pragmatic approach to determining if a plaintiff has established that the defendant’s negligence caused her loss. Scientific proof of causation is not required.

(2) Exceptionally, a plaintiff may succeed by showing that the defendant’s conduct materially contributed to risk of the plaintiff’s injury, where (a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and (b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone.

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