The Cognitive Psychology of Circumstantial Evidence

Empirical research indicates that jurors routinely undervalue circumstantial evidence (DNA, fingerprints, and the like) and overvalue direct evidence (eyewitness identifications and confessions) when making verdict choices, even though false-conviction statistics indicate that the former is normally more probative and more reliable than the latter. The traditional explanation of this paradox, based on the probability-threshold model of jury decision-making, is that jurors simply do not understand circumstantial evidence and thus routinely underestimate its effect on the objective probability of the defendant’s guilt. That may be true in some situations, but it fails to account for what is known in cognitive psychology as the Wells Effect: the puzzling fact that jurors are likely to acquit in a circumstantial case even when they know the objective probability of the defendant’s guilt is sufficient to convict. This Article attempts to explain why jurors find circumstantial evidence so psychologically troubling. It begins by using a variety of psychological research into judgment and decision-making—Kahneman & Tversky’s simulation heuristic in particular—to argue that jurors decide whether to acquit in a criminal case not through mechanical probability calculations, but on the basis of their ability to imagine a scenario in which the defendant is factually innocent. The Article then examines the basic epistemological differences between direct and circumstantial evidence and shows how those differences normally make it easier for jurors to imagine a factually exculpatory scenario in a circumstantial case. Finally, the Article concludes by discussing how an ease-of-simulation model of jury decision-making improves our understanding of why false verdicts occur.