Reasoning biases in delusion-prone individuals

The objective was to test whether individuals high in delusional ideation exhibit a reasoning bias on tasks involving hypothesis testing and probability judgments. On the basis of previous findings (e.g. Garety, Hemsley & Wessely, 1991), it was predicted that individuals high in delusional ideation would exhibit a 'jump-to-conclusions' style of reasoning and would be less sensitive to the effects of random variation, in comparison to individuals low in delusional ideation.

A non-randomized matched groups design was employed enabling the performance of the delusion prone individuals to be compared to that of a control group.

Forty individuals, selected from the normal population, were divided into groups high and low in delusional ideation, according to their scores on the Peters et al. Delusions Inventory (Peters, Day & Garety, 1996), and were compared on two tasks involving probability judgment and two tasks involving hypothesis testing.

Although no significant differences were found on tasks involving hypothesis testing and the aggregation of probabilistic information, it was found that individuals high in delusional ideation had a 'jump-to-conclusions' style of data gathering and were less sensitive to the effects of random variation, in comparison to individuals low in delusional ideation.

In conclusion, although individuals high in delusional ideation were not found to have a general reasoning bias, some evidence of a more specific bias was found. It is thought that these aberrations may play some role in delusion formation in schizophrenia and paranoia.