CanadaHow do I prove that the defendant's actions actually caused my injury?
You must prove causation using the 'but for' test: but for the defendant’s wrongful act, your injury would not have occurred. Courts also apply the 'thin skull rule', meaning the defendant takes you as they find you—even with pre-existing vulnerabilities.
What the Law Says
Canadian tort law does not rely on a single statute for causation in negligence—it is governed by common law principles developed by courts. There is no federal or provincial statute that defines the 'but for' test or the thin skull rule; these are judge-made doctrines applied consistently across provinces.
To succeed in a personal injury claim, you must prove both factual and legal causation. Factual causation asks whether the defendant’s conduct was a necessary cause of your injury — answered by the 'but for' test.
Legal causation (or remoteness) asks whether it is fair and just to hold the defendant responsible for the full extent of the harm — especially when other factors (like pre-existing conditions or later events) are involved.
The law does not require you to prove the defendant’s action was the sole cause — only that it was a 'material contribution' to an injury that would not otherwise have occurred.
What Courts Have Said
The Supreme Court of Canada’s decision in Athey v. Leonati is the leading authority on causation in Canadian negligence law. It clarifies how courts handle cases where injuries result from multiple causes or pre-existing vulnerabilities.
The Court confirmed the 'but for' test as the primary test for factual causation. It reaffirmed the 'thin skull rule' — defendants are liable for the full injury even if the plaintiff had a pre-existing vulnerability that made the harm worse. The Court distinguished this from the 'crumbling skull' scenario, where the plaintiff’s condition would have deteriorated regardless — in which case the defendant is only liable for the incremental harm caused.
What to Do
Gather medical evidence linking your injury directly to the defendant’s action — e.g., expert reports stating 'but for the accident, this injury would not have occurred.'
Document any pre-existing conditions thoroughly — courts need this to apply the thin skull or crumbling skull analysis correctly.
If multiple events contributed to your injury (e.g., two accidents), obtain legal advice to determine whether causation can still be established under Athey.
Do not assume partial responsibility disqualifies your claim — under the thin skull rule, full liability may still attach even if you were more vulnerable than average.
Sources
Not legal advice. This article is general information based on publicly available sources, written for educational purposes. Laws change and individual situations vary. Consult a licensed attorney in your jurisdiction before acting on anything you read here. Last reviewed: 2026-06-08.