Dividing Risks

Toward a Determinate Test of Proximate Cause

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 risk 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 risk produces intuitively satisfying answers to a wide range of proximate cause questions.

a. Professor of Law, University of Illinois College of Law. For their helpful comments on earlier drafts, I am grateful to Margareth Etienne, Ernest Johnson, Michael Moore, and the participants in the 2016 University of Illinois College of Law Faculty Retreat. For their fabulous editorial assistance, I am grateful to the editors and staff of the University of Illinois Law Review.

The full text of this Article is available to download as a PDF.