Proving 'but for' Causation in a Claim for Damages

A woman sustained a severe brain injury in a motorcycle accident and as a result, a personal injury claim was filed on her behalf.  There were several contributing factors to the accident; however, defense counsel argued that the primary cause was a nail that was driven over and punctured the rear tire of the bike, unbeknownst to the motorcycle driver.  Leading up to the accident, the driver (the woman’s husband) accelerated to about 120 km/hr to pass a car, when the nail fell out and the bike's tire deflated. These circumstances caused the couple's motorcycle to wobble, allegedly causing the driver to lose control and crash the bike. The accident occurred in wet weather and also, the bike was approximately 100 pounds overloaded. 

The injured woman sued her husband, the driver, alleging that her injury resulted because he was driving an overloaded bike at an excessive speed.  In the original trial, Clements v. Clements, the judge decided that Mr. Clements’ negligence contributed to his wife’s injury. The judge noted that the plaintiff was not able to prove ‘but for’ causation which, he reasoned, was due to the lack of scientific reconstruction evidence.  The trial judge then applied a material contribution test to the circumstances of the accident and found the defendant, Mr. Clements, liable on that basis.

The original judgement was appealed and the B.C. Court of Appeal ruled that ‘but for’ causation was not proven in the trial and further, the material contribution test did not apply in this case. As a result, the claim against the defendant was dismissed.  In his conclusions in the Appeal, Justice Frankel noted that once the trial judge found that the plaintiff failed to show that the motorcycle would not have crashed if not for Mr. Clements’ negligence, the trial judge should have acknowledged that causation had not been proven. Justice Frankel added that this is not a case involving circular causation where the material contribution test might be appropriate. Instead, it is a fairly common situation where it is not possible to prove that the defendant’s negligent actions caused injury. 

The B. C. Court of Appeal decision was subsequently appealed in the Supreme Court of Canada.  In Clements v. Clements, Chief Justice McLachlin defined the requirements in making a determination that a defendant is liable for a plaintiff’s loss.  "On its own, proof by an injured plaintiff that a defendant was negligent does not make that defendant liable for the loss.  The plaintiff must also establish that the defendant’s negligence (breach of the standard of care) caused the injury.  That link is causation."

Only in special circumstances, may there be a finding of negligence due to material contribution to a risk of injury, without showing ‘but for’ causation.  However, the Court noted that it is potentially dangerous to eliminate proof of causation in a determination of negligence because this conflicts with the fundamental principal in negligence actions, which is that the defendant is ‘at fault’ only with respect to the damage he caused to the plaintiff.  Cases that are successful on the less burdensome ‘material contribution to risk’ approach are uncommon and generally involve multiple defendants, such that, ‘but for’ the actions of one or more of the negligent defendants, the plaintiff would not have suffered injury. For example, a plaintiff may claim compensation for injuries that flowed from several different negligent acts committed by different people, where it is difficult to determine which of the individual acts caused the injury, but which together constitute a ‘but for’ cause of the injury.

Justice McLachlin concluded that the trial judge in Clements committed two errors.  The first is that he required scientific reconstruction evidence as a condition for determining ‘but for’ causation. The trial judge decided that the plaintiff was unable to prove, through no fault of her own, that the defendant’s actions caused her injury because, after the fact, it “is not possible through accident reconstruction modeling to determine at what combination of lower speed and lesser weight recovery from the weave instability would have been practicable”.   However, Justice McLachlin noted that scientific precision of this nature is not needed and ‘but for’ causation requires proof only on a balance of probabilities.

The second error made in the original trial was in applying a material contribution to risk test.  The special conditions requiring this approach do not exist in this single-defendant case, where the only issue is whether the injury would have occurred ‘but for’ the defendant’s negligent actions.

The trial judge accepted evidence from the plaintiff showing that overloading the motorcycle would increase instability when in combination with the weaving caused by deflation of the tire.  He also accepted evidence that higher speeds increase the effect of the instability caused by tire deflation, but believed that it is not possible to determine, without tests, whether this would occur at lower speeds. On the other hand, the trial judge rejected the evidence of an expert witness (for the defendant) who testified that the accident would have occurred even if the motorcycle was not driven too fast or overloaded.  Also, the trial judge relied on his own findings of fact on the issues of weight and speed, which were different than those of the expert speaking on these factors, and the judge specifically noted that the expert conceded that his opinion that speed and weight were not contributing factors could not be supported scientifically.  Given the lack of scientific proof, the judge concluded that ‘ordinary common sense’ supported the existence of a causal relationship between the injury and excessive weight and speed, and accordingly, found that the plaintiff’s injuries resulted from her husband speeding with too heavy a load when the bike’s tire deflated.

In an interpretation of the trial judge’s decision in this matter, Justice McLachlin concluded that the judge had determined that it was impossible to prove how or if each of the factors contributed to the accident, and as a result, he dispensed with ‘but for’ causation and instead applied the ‘material contribution’ test which he believed to have been satisfied. Justice McLachlin asserted that “the trial judge’s finding that the material contribution test was satisfied cannot be reinterpreted as a finding that “but for” causation was established without seriously undermining the important distinction between the two tests and the clarity of the analysis pertaining to causation.”

Justice McLachlin concluded that she is unable to find any basis from the trial judge’s conclusions for inferring that the motorcycle’s excessive speed and weight could have caused the accident.  Therefore, the appeal was dismissed and a new trial was deemed inappropriate.

When an accident victim seeks to recover damages for their injury, they must show, on a balance of probabilities, that ‘but for’ the negligent actions of the defendant, their injury would not have happened. Unfortunately, even when someone has sustained severe injuries, as in the case of Mrs. Clements, they will not be successful in a suit for damages if their injuries cannot be proven to have resulted from a defendant’s actions.  In this case, the overloaded motorcycle and excessive speed were not disputed, however, there was expert opinion that the accident could equally have happened at lower speeds and without excess baggage due to the damage from the nail and other factors; in which case, the defendant's actions were not responsible for causing the plaintiff's injuries.