Probability estimations and delusion-proneness

The present study investigated the reasoning of high and low-delusion-prone individuals on probability estimation tasks using emotionally neutral and delusional narratives. Undergraduate students (N = 33) who were classified as high-delusion-prone or low-delusion-prone based on their scores on a widely used measure of delusion-proneness in reasoning research (the Peters et al. Delusions Inventory) were asked to rate the probability that five neutral and eight unusual situations, presented in narratives that ranged in terms of degree of likelihood, could be true. Results indicated that compared to low-delusion-prone participants, high-delusion-prone participants assigned equivalent probability estimations to neutral narratives, but considered delusional narratives to be more likely (p < .001). Differences emerged between groups at all probability levels for delusional narratives except the least likely. Findings support the presence of a reasoning bias among high-delusion-prone individuals for delusional material similar to that previously found among actively delusional individuals. Results are discussed as they relate to reasoning, psychosis and delusion-proneness.