The central question in the law of proximate cause is how to divide risks into parts. The leading test of proximate cause, the foreseeability test, requires the jury to decide whether the ‘general type’ of outcome that occurred was too improbable to be foreseeable. Before the jury can address this question, though, it has to aggregate the possible outcomes of the defendant’s conduct into ‘general types’. In effect, then, the foreseeability test requires the jury to divide the risk into parts. So does a promising alternative to the foreseeability test, Judge Posner’s increased-risk test. Nobody has developed a workable, determinate method of dividing risks into parts, however. Instead, adherents of both tests have settled for telling juries vaguely to aggregate possible outcomes according to the ‘sort of mishap’ that occurred. As a consequence, both tests are fundamentally indeterminate.
This Article argues that this aggregation difficulty is solvable, though only within the framework of Judge Posner’s increased-risk test. The solution lies in dividing up risks as Darwin divided up life forms – according to ‘community of descent’. Specifically, outcomes may be situated in relation to one another (1) on the basis of their ‘descent’ from a particular mediating event; and (2) on the basis of their non-descent from a particular extrinsic condition. This method of dividing up the risks isn’t just determinate. When it’s used to frame the increased-risk question – that is, when it’s used to define the aggregate of possible outcomes that must be characterized by increased risk – this method of dividing the risk produces intuitively satisfying answers to a wide range of proximate cause questions.
Johnson, Eric Alan, Dividing Risks (May 15, 2020). University of Illinois Law Review (forthcoming 2021).