The concept of causation is difficult to grasp at the best of times, as much ink has been spilled over the years since the decision of Athey v Leonati in which the court applied the “material contribution” test.
Eleven years later, the Supreme Court of Canada clarified the law in Hanke v. Resurfice to provide that the “but for” test is to be applied unless there are special circumstances where an exception permits the use of the material contribution test.
In a historical sexual abuse claim, the plaintiff must prove that but for the abuse, he or she would not have suffered psychological harm.
If the plaintiff was abused by several assailants, each of whom had an impact on his or her life trajectory, it may be difficult to prove causation on the “but for” test. In such a case, the plaintiff need only demonstrate that the abusive conduct in issue made a material contribution to the psychological harm.
In 2012, the Supreme Court of Canada in Clements v. Clements added further clarity:
The test for showing causation is the “but for” test. The plaintiff must show on a balance of probabilities that “but for” the defendant’s negligent act, the injury would not have occurred. Inherent in the phrase “but for” is the requirement that the defendant’s negligence was necessary to bring about the injury — in other words that the injury would not have occurred without the defendant’s negligence. This is a factual inquiry. If the plaintiff does not establish this on a balance of probabilities, having regard to all the evidence, her action against the defendant fails
More recently, the Court of Appeal for Ontario in Sacks v. Ross interpreted Clements to mean that the plaintiff must prove that there was a “real and substantial connection” between the defendant’s breach of the standard of care and the plaintiff’s injury.
No doubt the discussion of causation will continue as different cases present varying factual situations.
  3 S.C.R. 458.
  1 S.C.R. 333.
  2 S.C.R. 181.
 2017 ONCA 773.