In 2010 the BC Court of Appeal released reasons for judgement seeking to clarify the law of causation in negligence lawsuits. The Supreme Court of Canada weighed in on this topic in reasons for judgement released today.
In today’s case (Clements (Litigation Guardian of) v. Clements) the Defendant was “driving his motorcycle in wet weather, with his wife riding behind on the passenger seat. The bike was about 100 pounds overloaded. Unbeknownst to (the driver), a nail had punctured the rear tire. Though in a 100 km/h zone, (the driver) accelerated to at least 120 km/h in order to pass a car; the nail fell out, the rear tire deflated, and the bike began to wobble. (the driver) was unable to bring the bike under control and it crashed“. The crash caused a severe brain injury to the passenger.
The BC Court of Appeal dismissed the claim finding that the Plaintiff could not prove the Driver’s speed and overloading of the motorcycle caused the crash. The Supreme Court of Canada, in a 7:2 split, found that errors were made at both the trial and appellate level and ordered a new trial. In doing so the majority provided the following reasons on the “but for” test of causation in negligence claims:
 On its own, proof by an injured plaintiff that a defendant was negligent does not make that defendant liable for the loss. The plaintiff must also establish that the defendant’s negligence (breach of the standard of care) caused the injury. That link is causation.
 Recovery in negligence presupposes a relationship between the plaintiff and defendant based on the existence of a duty of care — a defendant who is at fault and a plaintiff who has been injured by that fault. If the defendant breaches this duty and thereby causes injury to the plaintiff, the law “corrects” the deficiency in the relationship by requiring the defendant to compensate the plaintiff for the injury suffered. This basis for recovery, sometimes referred to as “corrective justice”, assigns liability when the plaintiff and defendant are linked in a correlative relationship of doer and sufferer of the same harm: E. J. Weinrib, The Idea of Private Law (1995), at p. 156.
 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.
 The “but for” causation test must be applied in a robust common sense fashion. There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury. SeeWilsher v. Essex Area Health Authority,  A.C. 1074, at p. 1090, per Lord Bridge; Snell v. Farrell,  2 S.C.R. 311.
 A common sense inference of “but for” causation from proof of negligence usually flows without difficulty. Evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss. See Snell and Athey v. Leonati,  3 S.C.R. 458. See also the discussion on this issue by the Australian courts: Betts v. Whittingslowe,  HCA 31, 71 C.L.R. 637, at p. 649; Bennett v. Minister of Community Welfare,  HCA 27, 176 C.L.R. 408, at pp. 415-16; Flounders v. Millar,  NSWCA 238, 49 M.V.R. 53; Roads and Traffic Authority v. Royal,  HCA 19, 245 A.L.R. 653, at paras. 137-44.
The Court also went on to address the “exceptional” cases where the “material contribution to risk ” doctrine can be used finding its use is appropriate only where:
(a) the plaintiff has established that her loss would not have occurred “but for” the negligence of two or more tortfeasors, each possibly in fact responsible for the loss; and
(b) the plaintiff, through no fault of her own, is unable to show that any one of the possible tortfeasors in fact was the necessary or “but for” cause of her injury, because each can point to one another as the possible “but for” cause of the injury, defeating a finding of causation on a balance of probabilities against anyone.