Causation in Negligence — But-For and Material Contribution

The default rule

Causation in Canadian negligence is established under the but-for test: but for the defendant's negligence, would the plaintiff have suffered the injury? The plaintiff bears the burden on a balance of probabilities.

The plaintiff need not eliminate every other possible cause; common-sense inference, not scientific certainty, is the standard (Snell v Farrell 1990).

When but-for fails

Resurfice Corp v Hanke (2007) confirmed that the but-for test governs almost all causation analysis. It also clarified the narrow circumstances in which material contribution to risk may substitute.

Material contribution is available only where:

• It is impossible to prove but-for due to factors outside the plaintiff's control; AND
• It is clear the defendant's breach exposed the plaintiff to an unreasonable risk and the plaintiff's injury falls within the ambit of that risk.

Clements clarification

Clements v Clements (2012) further clarified that material contribution applies primarily in multi-tortfeasor cases where the plaintiff's injury can be attributed to the negligence of multiple defendants but the plaintiff cannot say which one's breach was a but-for cause (the classic "two hunters, one shooter" or "fibrosis worker exposed to asbestos at multiple workplaces" pattern).

Modern application

In single-tortfeasor cases, but-for governs. Mustapha v Culligan (2008) is the standard application: the question is whether but for the dead fly Mustapha would have suffered psychiatric injury — a foreseeability inquiry into ordinary fortitude, not a material-contribution inquiry.

In multi-tortfeasor cases where causation through but-for is impossible, material contribution remains available — but courts apply it sparingly to preserve the rigour of the fault-harm link.

Drafting tip

Plead and argue but-for first. Reach for material contribution only where the multi-defendant evidentiary impasse is squarely on the facts. A general assertion that the case is "complex" or "uncertain" will not unlock the exception.