The dynamics of reasonable doubt

The doctrine of reasonable doubt is deeply entrenched within American culture, but the concept continues to mystify legal scholars, courts and jurors, and a coherent definition remains elusive. Reasonable doubt (RD) can be reified with the tool of decision theory as a tradeoff between acquitting the guilty and convicting the innocent. For instance, Blackstone's maxim that ten erroneous acquittals are equal in cost to one erroneous conviction implies, roughly, that jurors ought to convict only if their confidence in the defendant's guilt exceeds 0.91. This dissertation proposes several descriptive discrepancies from this normative account. First, it is argued that the proffered evidence serves as the focal point for RD, such that jurors listen to the evidence and then determine whether the remaining doubt is reasonable. In short, jurors know RD when they see it. Verdicts do not depend on whether the evidence satisfies an exogenous tradeoff. Second, three studies were conducted to test the hypothesis that mock jurors systematically shift their operational definition or threshold for RD in order to promote cognitive consistency, a state in which attitudes, beliefs and cognitions are congruous. Shifting the decision threshold can theoretically attenuate "close calls"---that is, when the evidence is close to the RD threshold---by inflating the perceived disparity between the two. The studies revealed that mock jurors' implicit threshold regularly shifted in the opposite direction of the proffered evidence. When a piece of evidence increased the likelihood of guilt, it concomitantly decreased the threshold for conviction, and when a piece of evidence decreased the likelihood of guilt, it increased the threshold for conviction. The degree to which the threshold shifted was positively related to decisional confidence, one manifestation of cognitive consistency. Threshold shifting violates an axiom of decision theory, which is that the consequences of an outcome are independent from the chances of its occurrence. This finding has implications for both psychological and legal theory. With respect to the latter, the findings indicate that RD is a relative construct and suggest that the analysis of legal doctrine is more complicated than has been previously supposed.