Overdetermined Causation in Tort Law

Богдан Петрович Карнаух


In vast majority of cases the but-for test works perfectly fine. However, in overdetermined-causation cases it produces falsely negative result. Overdetermined causation is present whenever there are two or more factors each of which alone would have been sufficient for the result. In such cases the but-for test paradoxically indicates that neither of factors is a cause of the result. That is so because the but-for test postulates causation through the concept of necessity and disregards the role of sufficiency.

The drawback can be overcome by using more complicated NESS test (necessary element of a sufficient set). The latter is more precise, it takes into account not just the necessity but the sufficiency as well. Thus, in a set of the real-world factors there may be few subsets each of which is sufficient for the result’s occurrence. Some factor is a cause of the result if it constitutes a necessary element of such a subset and in the absence of the factor the subset is not sufficient.

Overdetermined causation can be symmetrical or asymmetrical. In the case of symmetrical overdetermined causation there are two processes that run parallel with each other and entail the result. In the case of asymmetrical overdetermined causation only one of the processes ends up with the result, meanwhile the second processes would have entailed the result had not it been preempted by the first one. Therefore, asymmetrical overdetermined causation is also called preempted causation (where there is preemptive and preempted causes). In case of symmetrical overdetermined causation both factors should be considered as causes, on the contrary in asymmetrical overdetermined causation only the preemptive cause is a cause. That is why it is crucial to distinguish between the two, as R. Wright underlines. However sometimes it may be difficult to tell one from another, as is the case with McLaughlin’s hypothetical.

The other source of difficulties is the situation where both factors are failures to do something. Though some scholars consider it to be the case of asymmetrical overdetermined causation, the author fortifies the opposite account contending that both failures should be regarded as causes of the result.


causation; causal nexus; overdetermined causation; but-for test


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