The Paradox of Conviction Probability: Mock Defendants Want Better Deals as Risk of Conviction Increases

Probability of conviction impacts a defendant’s willingness to accept a plea offer. Existing studies demonstrate that willingness to plead guilty increases as conviction probability increases, but these studies do not reveal whether their participants would have accepted worse offers than they did. The researchers recruited US-based users of Amazon Mechanical Turk and Cloud Research MTurk Toolkit for three second-person vignettes. Each participant assumed the role of a defendant charged as an accomplice in a campus-based drug offense. Participants answered questions about their guilt status, sentence at trial, and conviction probability. Consistent with both the Shadow of Trial Theory and Prospect Theory, participants were more willing to accept plea sentences longer than the expected value of trial when conviction probability was very low and plea sentences shorter than the expected value of trial when conviction probability was nearly certain. Overall, willingness to plead guilty increased as conviction probability increased. However, contravening the Shadow of Trial Theory, the average maximum sentence that participants were willing to accept varied with the probability of conviction. Participants became increasingly risk-seeking as probability of conviction increased, requiring increasingly-better plea offers relative to the expected value of trial before pleading guilty. The researchers concluded that, because of participants’ cognitive biases under conditions of uncertainty, conviction probability affects participants’ appraisals of plea offers on a spectrum, not along a linear model as proposed by the Shadow of Trial Theory.

“The results of Study 3 replicate and extend those of Studies 1 and 2, confirming that, at least under the conditions of our experiments, the minimum plea discount participants were willing to accept was positively (not negatively) correlated with changes in probability of conviction. In the lowest probability conditions, the average maximum offer participants were willing to accept exceeded the expected value of trial, and although willingness to plead guilty to some offer increased with increasing probability of conviction, it took increasingly better plea discounts in order for participants to do so.” p. 49