What does it mean, in the law of torts, for one to have caused another's injury? The standard legal account of causality has been that of the counterfactual put forward by David Hume as follows: "where, if the first object had not been, the second never had existed." (1748, Section VII). In the law it's the "but for" test and for our purposes then the courts tend to attach legal liability (assuming the existence of a duty and its breach) only upon the following: had Defendant done otherwise, Plaintiff would today be uninjured.
Generally speaking the "but for" test is easy to use and produces a nice clean binary answer of the sort favored by courts trying to resolve disputes. (e.g. "But for" Defendant having run the red light Plaintiff would have passed through the intersection without incident and would not have suffered the broken leg of which she complains.) Of course there are innumerable "but for" causes of the accident, including the Defendant's mother having given birth to him and the rudeness of the fellow with a cart full of groceries in the "10 items or less" line who slowed Plaintiff down so that she wound up in the intersection at the same moment as Defendant. Nevertheless, and almost invariably without much analysis, the parties settle on just one among the countless posibilities as the sine qua non act (or omission) to be subjected to the "but for" test.
However, when deployed in a case in which plaintiff's injury was caused by only one of several identical and indistinguishable acts (see e.g. Summers v. Tice) or by the cumulative effect of some subset of several identical and indistinguishable acts (see e.g. Landers v. East Texas Salt Water Disposal Co.) the "but for" test would, without some other rule, produce the jarring judgment that none of the defendants, considered individually, were more likely than not the sine qua non cause of the plaintiff's injury. In those cases, finding that the acts (divorced from the inquiry of whether they were actually causative) were tortious, the courts held that the defendants should bear the burden of proof that theirs was not the act that caused the plaintiff's injury. The instances in which such a rule would apply were surely few and even a century ago the idea of treating the defendants collectively and shifting the burden on to them was not unheard of. See 2 J. Wigmore, Select Cases on the Law of Torts Section 153, p. 865 (1912) ("When two or more persons by their acts are possibly the sole cause of a harm, or when two or more acts of the same person are possibly the sole cause, and the plaintiff has introduced evidence that one of the two persons, or one of the same person's two acts, is culpable, then the defendant has the burden of proving that the other person, or his other act, was the sole cause of the harm.")
An effort to distill all judicial reasoning about causation down to its essence and to make a rule of it has been underway for some time. Probably the most cited rule, a form of which is incorporated into the Restatement (Third) of Torts, is the NESS test. NESS stands for "necessary element of a sufficient set". The NESS test is itself a restatement of an older test in which a singular putative cause is said to indeed be causative if it, though insufficient, is nevertheless a necessary element of a precipitating event which event, though perhaps itself unnecessary, would nevertheless have been sufficient to have produced the outcome under investigation (see "Causes and Conditions" by J.L. Mackie). What in the world does any of that mean? In Mackie's example investigators of a fire are satisfied that a short-circuit was the cause of a house fire when conditions were such that the short circuit existed, flammable rags were nearby and those rags once ignited were sufficient to cause a conflagration that would burn down the house. The short-circuit alone was insufficient to burn down the house but it was necessary (other causes having been ruled out) to catch nearby flammable rags on fire. Lots of things can ignite and so burn down a house (so burning rags are not necessary to have a burning house) but burning rags are sufficient to torch the place. Whew.
"But for", causal analysis seems then to be about ruling things out and settling on what's left (the short-circuit). On the other hand, the investigation of the product of the remaining thing(s) (sufficient set) seems aimed towards the propensity of one condition (burning rags) to lead to another (burning house). It is out of this second analysis, of the propensity for one thing to lead to another, that queries about reasonableness and foreseeability and risk arise. More on that later though when I get to risk.
Anyway, the reductionist effort itself precipitated a debate about whether the NESS test accounts for all judicial causal reasoning that continues to this day. Take for instance the hypothetical case of the desert traveler. The desert traveler set off to hike across the desert with just enough water to make it to the other side. Unbeknownst to the desert traveler, defendant "A" had poisoned his water such once that he'd consumed all of it he would surely die. Along his way across the desert however, defendant "B" stole the desert traveler's water bottle. The desert traveler was found dead in the desert. If defendant "A's" conduct was not the cause of his death, why not?
Such conundrums would normally make for nothing more than an obscure niche in the law in which a few academics could regularly generate tedious and unread papers. Then forty one years ago Clarence Borel, dying of mesothelioma, filed a products liability claim in Beaumont, TX culminating in an opinion that would usher in the age of mass torts. Concluding that as it was impossible "to determine with absolute certainty which particular exposure to asbestos dust resulted in injury" and because "each exposure may result in an additional and separate injury" the 5th Circuit held in Borel v. Fibreboard that a jury could reasonably conclude "that each defendant was the cause in fact" of Borel's injury.
Twenty seven years after Johns Manville sank under the wave of litigation unleashed by Borel the practical effect of the ruling continues to drive companies into bankruptcy. The Restatement (Third) of Torts was awkwardly silent on the issue of "the mother of all mass torts" but recent musings by its Reporters and others suggest that they thought rather a lot about it and that its impact on the restatement effort was in fact profound. The shame of it is that they almost got it exactly right. All that was needed was to distinguish between risk and causation and to embrace Palsgraf's true meaning: that the risk imparted is the measure of the reasonableness of the man.
Next time: Risk.
To read more from David Oliver, visit his blog MASSTORTS: State of the Art.