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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.

Examination of delusion-proneness, or high levels of unusual beliefs in non-clinical populations (Peters, Joseph, & Garety, 1999), has become a popular way to examine the continuum of psychosis and to understand cognition in individuals believed to share features with individuals with psychosis (e.g., Linney, Peters, & Ayton, 1998). A number of researchers promote studying delusion-proneness to determine what sorts of cognitive biases may be present before, as opposed to just after, the onset of delusions ( Colbert & Peters, 2002). If delusion-prone individuals have cognitive biases similar to delusional individuals, it may be possible to target such biases before the onset of psychosis and delay or avoid transition to psychosis. Indeed, there is evidence delusion-prone and delusional individuals have similar reasoning biases, though the tasks used to examine delusion-prone individuals have been limited to neutral or minimally emotionally salient stimuli (e.g., Colbert and Peters, 2002 and Warman and Martin, 2006). While such investigations are useful in understanding general cognitive biases for delusion-prone individuals, they do not demonstrate delusion-prone individuals are actually biased about unusual experiences or believe unusual situations to be more likely than do low-delusion-prone individuals, a documented bias for individuals with delusions ( McGuire, Junginger, Adams, Burright, & Donovick, 2001). In an effort to answer this important question, the aim of the present study was to determine if high-delusion-prone individuals overestimate the likelihood of unusual events relative to low-delusion-prone individuals. Studies investigating individuals with schizophrenia spectrum disorders with active delusions have demonstrated robust differences between how individuals with delusions reason when compared to individuals with other psychiatric disorders and to normal controls (e.g., Huq et al., 1988 and Peters and Garety, 2006). For example, individuals with delusions request significantly less information before making decisions than psychiatric and normal controls, often asking for only one or two stimuli before making decisions when much more information is available (e.g., Garety et al., 2005 and Moritz and Woodward, 2005). Using similar or identical probabilistic reasoning tasks to those used to investigate reasoning biases in delusional populations, studies have demonstrated support for such data gathering biases in delusion-prone individuals when the stimuli presented are emotionally neutral, such as presentation of beads in a jar (Colbert & Peters, 2002), and when stimuli are emotionally salient, such as a “survey task” in which participants are told to decide which of two surveys a word is coming from when the survey is described as conducted about a person very much like the participant (Warman & Martin, 2006). Results, however, are not entirely consistent, suggesting much remains unclear about reasoning and delusion-proneness (e.g., Warman, Lysaker, Martin, Davis, & Haudenschield, 2007). Although asking individuals to make decisions about surveys hypothetically conducted about a person like the participant is likely more emotionally salient than making judgments about beads, the stimuli in the survey task are not related to understanding how delusion-prone individuals reason about a primary variable of interest – delusional material. McGuire et al. (2001) conducted an important study targeting this issue for delusional individuals by examining how individuals with delusions reasoned about the likelihood certain delusional narratives could be true. They found individuals with delusions assigned significantly higher probabilities to delusional narratives (i.e., thought the delusional narratives were more likely to be true or possible) than psychiatric and normal control groups. Of note, when they examined how individuals with delusions performed when narratives included only non-delusional, emotionally neutral material (colored balls), they performed similarly to all other groups studied, indicating their findings cannot be attributed to delusional individuals assigning higher probabilities to narratives in general. In sum, McGuire and colleagues demonstrated that individuals with delusions reason effectively when neutral stimuli are presented when no direct data gathering is required, but their reasoning is significantly affected when the stimuli presented are of delusional content. While previous research has demonstrated many similarities in how delusional and delusion-prone individuals reason (e.g., Colbert & Peters, 2002), we know of no published studies that have investigated how delusion-prone individuals reason about delusional material, such as assessing how they rate the probabilities that delusional narratives are true.1 Given delusional beliefs are more related to interpretations of situations, not to determining from which of two jars beads are being drawn (Warman & Martin, 2006), an investigation of how delusion-prone individuals perceive unusual events appears essential. The present study examined how delusion-prone individuals reason about delusional material. All participants were undergraduate students with no history of psychotic disorder. Participants completed the Peters et al. Delusions Inventory (PDI; Peters et al., 1999), a popular instrument to assess delusion-proneness in reasoning research (Linney et al., 1998 and Warman et al., 2007), and assigned probability ratings (i.e., endorsed how much they believed various delusional scenarios were likely) to each of the narratives used in the McGuire et al. (2001) study. It was expected individuals classified as high-delusion-prone would assign higher probability ratings to delusional material (i.e., be biased in that they would have elevated beliefs that unusual scenarios are likely) than low-delusion-prone individuals. Further, it was predicted high and low-delusion-prone participants would reason equivalently for neutral narratives, mirroring the results McGuire and colleagues found for the delusional group when compared to healthy controls. Because McGuire et al. found familiarity with delusional narratives was linked with higher probability ratings (though a significant relationship between delusional status and probability ratings was present even when familiarity with the narratives was controlled for) we, similarly, assessed participants’ familiarity with the narratives.

Following previous research (e.g., Linney et al., 1998 and Warman et al., 2007) high and low-delusion-prone groups were formed such that individuals whose PDI scores fell above the median for the present sample (median = 90) were classified as high-delusion-prone (n = 15) and individuals whose PDI scores fell below the median (n = 16) were classified as low-delusion-prone. Following Laroi and van der Linden (2005), the two participants whose PDI scores fell at the median were excluded from analyses. The median used to classify participants in the present study is consistent with other studies using similar populations (e.g., Warman, 2008 and Warman et al., 2007).