Causation in Ontario Personal Injury Claims

By Kevin Souch

Causation is a foundational element of liability in Ontario personal injury litigation. Even where a defendant has breached a duty of care, a plaintiff must still prove that the breach caused the injuries claimed. Ontario courts consistently stress that causation is a question of fact, to be resolved using common sense and a careful assessment of the evidence as a whole.

The “But For” Test

The governing test for causation in Ontario is the “but for” test: the plaintiff must show, on a balance of probabilities, that but for the defendant’s negligence, the injury would not have occurred. This principle was reaffirmed by the Supreme Court of Canada in Clements v. Clements and is now the dominant analytical framework in personal injury cases.

The “but for” test does not require scientific certainty. Courts recognize that causation often cannot be proven with precision, particularly where injuries involve complex medical issues. The trial judge may draw reasonable inferences from the evidence and is entitled to rely on a robust, common‑sense approach rather than demanding proof of a single, exclusive cause.

Material Contribution and Resurfice

In Resurfice Corp. v. Hanke, the Supreme Court clarified that the material contribution to risk test is not an alternative that plaintiffs may freely choose. Rather, it is a narrow exception, available only where it is impossible to prove “but for” causation due to factors outside the plaintiff’s control and where the defendant’s breach materially increased the risk of the type of injury that occurred.

Following Resurfice and later Clements, Ontario courts apply the material contribution test sparingly. In most personal injury claims—particularly motor vehicle accident cases—courts find that the evidentiary record permits a traditional “but for” analysis.

Pre‑Existing Conditions and Athey v. Leonati

Causation frequently arises in cases involving pre‑existing conditions. The leading authority remains Athey v. Leonati, where the Supreme Court confirmed that a defendant need not be the sole cause of an injury. It is sufficient that the defendant’s negligence materially contributed to the damage.

Athey also firmly establishes the thin skull rule: defendants must take plaintiffs as they find them, including any vulnerabilities or latent conditions. If the accident aggravates a pre‑existing condition, the defendant is liable for the full extent of that aggravation. At the same time, Athey recognizes the crumbling skull principle, which limits damages to the extent that the plaintiff’s condition would have deteriorated in any event, even absent the defendant’s negligence.

Multiple Causes and Evidentiary Proof

Ontario law accepts that injuries often have multiple contributing causes. A defendant may be liable even if other factors—such as prior injuries, degenerative conditions, or subsequent events—also played a role. The key inquiry remains whether the defendant’s negligence was a necessary contributing cause of the injury.

Causation is typically established through a combination of lay and expert evidence, including medical opinions, clinical records, and testimony about the plaintiff’s condition before and after the incident. Courts place little weight on speculative or conclusory expert evidence and look for logical coherence between the medical evidence and the factual narrative.

Conclusion

Causation in Ontario personal injury law is a practical, fact‑driven inquiry. Guided by cases such as Athey, Resurfice, and Clements, courts apply the “but for” test flexibly but consistently, reserving exceptional doctrines for truly exceptional circumstances. The ultimate goal remains a fair determination of whether the defendant’s negligence meaningfully caused or contributed to the plaintiff’s injuries.

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