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

Objective: We examined how probability of conviction affects the maximum plea sentence mock defendants will accept. Hypothesis: Relying on Prospect Theory (Kahneman & Tversky, 1979), we hypothesized that, relative to the expected value of trial, participants would need increasingly better sentences as conviction probability increased and would settle for sentences worse than the expected value of trial when probability was very low. Method: We manipulated conviction probability and potential trial sentence in a series of three between-subjects experiments, with Amazon Mechanical Turk participants assigned to the role of guilty defendants. Participants were majority White (75-82%) and non-Hispanic (92-94%); approximately half (45-51%) identified as female. Study 1 (N = 681) explored the effects of conviction probability (.05, .15, .50, .85, .90) and potential trial sentence (5, 20 years) on the maximum sentence accepted in exchange for a plea. Study 2 (N = 343; (X) over bar (age) = 37.5) clarified results of Study 1 for the upper range of probabilities for two potential trial sentences (5, 10 years). Study 3 (N = 1,035; (X) over bar (age) = 37.6) replicated the effects of probability (.05,.10,.15,.50,.85,.90) and potential trial sentence (5, 10 years). Results: Across all three studies, participants wanted increasingly better deals (relative to the expected value of trial) as conviction probability increased. For example, in Study 3, when probability of conviction was 0.90, plea sentences were, on average, 58% better than the expected value of trial; in contrast, when the probability was 0.05, sentences that were nearly 4 times the expected value of trial were acceptable. Conclusions: The most commonly used model of plea decision-making, Shadow of the Trial (SOT) (Mnookin & Kornhauser, 1979), assumes a direct and constant linear relationship between conviction probability and plea sentence. In contrast, our data suggest that the way conviction probability affects mock defendants' appraisals of plea offers may change across the probability spectrum. These results can facilitate development of a more comprehensive model of plea decision-making.