a) Reversal of Onus or Adverse Inference
The burden of proving causation typically rests on the plaintiff, along the lines already discussed, as part of the case that needs to be met. However, sometimes, courts will quickly transfer on the defendant the burden of explaining how the negligent action is not the cause of the injury. In Snell, supra p. 328, the Supreme Court of Canada explained that “the allocation of the burden of proof is not immutable. Both the burden and the standard of proof are flexible concepts”. The shifting of the burden will happen when a defendant has particular knowledge of a matter that may have caused the injury. In such instance, “very little affirmative evidence on the part of the plaintiff will justify the drawing of an inference of causation in the absence of evidence to the contrary” (Snell, supra pp. 328-329).
Adverse inferences will usually be invoked in cases of complex origin, where science may be lacking or the information is under the control of the defendant. One such example is where a railroad company has control of the information about the breaking capacity of a particular train: Stamper v. Finnigan et al. (1986), 74 NBR (2d) 271 paras 221-237 (QB), confirmed at (1988), 85 NBR (2d) 404 para 60 (CA), although the Court of Appeal saw enough evidence without the need to rely on adverse inferences. Medical malpractice is another field where adverse inferences are used, given the specific knowledge held by the defendants.
In Snell, supra, the ophthalmologist had pursued the cataract operation despite retroorbital bleeding. The removal of the cataract itself had caused some bleeding in the eye and it was impossible to determine from which action the optic nerve atrophy came from. Had the doctor postponed the operation when the bleeding behind the eye was noticed, its effect on the eyesight could have been better determined. The pursuit of the surgery when it should have been stopped made it impossible to identify the precise cause of the blindness. The matter was complicated further by pre-existing conditions of the plaintiff. Nevertheless, all three levels of courts applied the adverse inference rule and the defendant failed to provide a satisfactory explanation to the plaintiff’s condition and was therefore liable.
Technically, the ‘but for’ test remains the same, but the burden resting on the plaintiff will be lightened somewhat. Failure on the part of the defendant to provide an acceptable explanation will lead to an adverse inference of causation.
b) Material Contribution to the Risk of Injury
In specific types of cases, the courts will stray from the usual ‘but for’ test to establish causation. This scenario is distinct from the one of adverse inferences which is available anytime a defendant has control over the relevant information. Where several negligent tortfeasors contribute to an injury and their actions confound which one has caused the damage, the presiding judge will rely on the test of ‘material contribution to the risk of injury’: Cook v. Lewis,  SCR 830; Snell, supra pp. 321-322 & 327. In Clements v. Clements,  2 SCR 181 paras 13-14, the majority of the Supreme Court explained:
 To recap, the basic rule of recovery for negligence is that the plaintiff must establish on a balance of probabilities that the defendant caused the plaintiff’s injury on the “but for” test. This is a factual determination. Exceptionally, however, courts have accepted that a plaintiff may be able to recover on the basis of “material contribution to risk of injury”, without showing factual “but for” causation. As will be discussed in more detail below, this can occur in cases where it is impossible to determine which of a number of negligent acts by multiple actors in fact caused the injury, but it is established that one or more of them did in fact cause it. In these cases, the goals of tort law and the underlying theory of corrective justice require that the defendant not be permitted to escape liability by pointing the finger at another wrongdoer. Courts have therefore held the defendant liable on the basis that he materially contributed to the risk of the injury.
 “But for” causation and liability on the basis of material contribution to risk are two different beasts. “But for” causation is a factual inquiry into what likely happened. The material contribution to risk test removes the requirement of “but for” causation and substitutes proof of material contribution to risk.
The typical scenario invoked to illustrate the test of material contribution to the risk of injury is the one found in Cook v. Lewis, supra. A plaintiff was injured by a gunshot during a hunting trip. Both defendants had fired at the same time and in the same direction, despite being aware of the potential presence of the plaintiff. One of the shots injured the victim but the jury could not determine which one. The plaintiff had established that both defendants were negligent and that he was injured by their tortious act, but could not establish which one had fired the shot. The Supreme Court confirmed the order for a new trial.
Another example of the doctrine can be found in the case of Wood v. Wawanesa, 2019 NBQB 125. Three motorists encountered at least 2 large bags of insulation on a 4-lane highway that had torn open and allowed their content to escape. It created blizzard like conditions which affected the drivers’ visibility. Nobody had seen where the bags came from, but the trial judge concluded that “it is more likely than not that the bags of insulation fell off an unidentified vehicle of some kind, most likely a truck or pick-up, where the operator had not ensured that the load was safely secured” (para 23). The condition created contributed to causing the collision and the unknown driver was found negligent. A chain reaction thus followed where the plaintiff’s SUV rear-ended a car before being also rear-ended. She sustained a fractured wrist in the collision but the evidence could not determine its cause: there had been two highspeed collisions, airbags going off, beating down the airbag to escape a death trap, struggling to open a jammed door, etc. The trial judge was not able to attribute the injury to a specific event (paras 123 & 125) and thus shared liability between all at fault parties (para 127). Although not stating so expressly, he thus applied the ‘material contribution to the risk of injury’ test.
As all other factual determinations, the application of the tests for causation requires a thorough understanding of all the facts of the case. It is wise to have proper evidence of the probable cause of the accident and of the injury, or at least proper evidence excluding all possible causes other than the one relied upon.