Scientific precision not required to prove “but for” causation: Supreme Court

Scientific precision is not a necessary condition to find “but for” causation on a balance of probabilities, the Supreme Court of Canada has found.

The court ordered a retrial in a motorcycle accident case, in which the trial judge found that the victim could not prove “but for” causation to explain her injuries because of the limits of scientific reconstruction evidence.

“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,” the Supreme Court ruled in a 7-2 decision. “In this case, the trial judge committed two errors.

“First, he insisted on scientific reconstruction evidence as a necessary condition of finding ‘but for’ causation. Scientific precision is not necessary to a conclusion that ‘but for’ causation is established on a balance of probabilities.”

Second, the court determined, the trial judge incorrectly substituted a “material contribution” test for the “but for” causation test. However, the material contribution test is intended in situations in which several negligent defendants launched the event that led to the plaintiff’s injury, not just one defendant, as in the case of Clements v. Clements.

Joseph Clements was driving a motorcycle with his wife, Joan Clements, riding behind on the passenger seat in 2004. The road conditions were wet and the bike was about 100 pounds overloaded.

Unbeknownst to Joseph Clements, a nail had punctured the rear tire. In a 100 km/h zone, he sped up to 120 km/h to pass a car. The nail fell out and the tire deflated, causing the tire to wobble and the bike to crash. Joan Clements suffered a traumatic brain injury.

The trial judge found that Joseph Clements’ negligence, in fact, contributed to the injury. But he found that Clements’s wife, through no fault of her own, was unable to prove “but for” causation because of the limits of scientific reconstruction evidence. He then used the material contribution test to find that Joseph Clements was liable on this basis.

The Supreme Court ordered a retrial. It found the “but for” test should have been used, and that precise scientific accuracy was not required to establish “but for” causation on the balance of probabilities.