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The Common law's tort of negligence, like its equivalents elsewhere, grew up with a basic rule of causation: in order to be liable, the defendant must be proved to have caused harm to the victim. In a series of cases, brought to a head in Sienkiewicz v. Greif,1 the courts have developed an exception to this rule to be applied in cases of ambiguous causation, i.e. when it cannot be shown which one of a number of competing factors was the true cause of the victim's harm. This exceptional rule, now apparently with several sub-forms, has operated in particular where the victim has suffered mesothelioma as a result of exposure to asbestos, a situation in which it is scientifically impossible to identify which fibre or fibres brought about the illness, though it cannot be limited to such cases.
The facts of Sienkiewicz were relatively straightforward. Mrs. Enid Costello died of mesothelioma. She had been tortiously exposed to asbestos while working for the defendants; in addition there was a background exposure to asbestos in the atmosphere. The statistical evidence suggested that the background asbestos would lead to some 24 cases of mesothelioma per million persons, and that the exposure by the defendant had increased this by approximately 18%. In other words, accepting for the moment the statistical evidence, there was an 85% chance that Mrs Costello had died from a non-tortious cause. None the less, it was held that the consequence of the earlier line of cases was that her estate should recover in full from the defendants as if they were the sole cause. The result perhaps appears strange, but it is hard to see how the court could have reached a different decision on the state of the authorities; as Lady Hale suggests, it was a hard case rather than a difficult one.2
The purpose of the present article is to examine the state of the law after the decision in Sienkiewicz, to trace how it reached this state, and to assess how satisfactory it is both in theoretical terms and in terms of the results to which it leads.
The Genesis of the Exceptional Rule of Causation
The normal rule of causation, which...