Delusions are considered to be one of the primary symptoms of psychosis but until recently have received little empirical investigation. One approach has been to examine the extent to which deluded individuals demonstrate cognitive biases which are different from those of normal controls in inductive reasoning tasks. In this study two hypothesis testing tasks were used to investigate cognitive biases in a group of people with persecutory delusions compared to a group whose delusions had remitted and a normal control group. Participants completed two tasks consisting of a series of visual discrimination problems in which they had to choose between pairs of stimuli presented on cards. Condition 1 examined previously reported biases of deluded participants requiring less information before making judgements and being overconfident in their judgements. Positive or negative feedback was given after every card and participants were unconstrained in giving solutions. Condition 2 partially replicated Young and Bentall's (1995) hypothesis testing study and examined participants' ability to process information sequentially and progressively focus down the set of possible correct solutions. Feedback was restricted and participant responding was constrained. No differences were found between groups in condition 1. In condition 2 deluded participants produced fewer hypothesis and sampled from a smaller range of hypotheses than remitted and control participants. Deluded participants also produced fewer correct hypotheses than the other groups. A trend was found for deluded participants to use fewest sensible responses to feedback, followed by remitted and control groups. The reverse trend was found for use of nonsensical responses to feedback. Limitations of the study, implications for clinical practice and suggestions for future research are considered.

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