Tabraham, Paul; (2001) Major research project: the influence of experience and reasoning on delusion formation. Doctoral thesis (D.Clin.Psy), UCL (University College London).

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Abstract

This study tested two theoretical models of delusion formation. The first suggests that most delusions are caused by the application of normal reasoning to abnormal experiences (Maher, 1974; 1988; 1992). The second suggests that many delusions are caused by an information-processing bias that results in both abnormal reasoning and abnormal experiences (Garety & Hemsley, 1994). The first model predicts that people with delusions, who demonstrate a "jump-to-conclusions" bias, will experience less perceptual anomalies (e.g. hallucinations) than those who do not. The second model predicts that people with delusions, who demonstrate a "jump-to-conclusions" bias, will experience more perceptual anomalies than those who do not. The second model also predicts that the "jump-to-conclusions" bias will be associated with a failure to make use of past regularities when processing new stimuli and a deficiency in the meta-cognitive skill of reality discrimination. The presence of a "jump-to-conclusions" bias in fifty adults with delusions was assessed by their performance on a probabilistic inference task. The performance of the 22 participants who demonstrated the bias, was compared with the performance of the 28 participants, in whom the bias was absent, on three computer-based tasks and two structured interviews, which sought to measure perceptual anomalies, reality discrimination and the use of past regularities in processing new stimuli. Results indicated that people with delusions, who demonstrate a "jump-to-conclusions" bias, experienced less hallucinations than those who do not, thus supporting Maher's model. Experiences of voices commenting, olfactory hallucinations, visual hallucinations and general auditory hallucinations were more common in participants without a "jump- to-conclusions" bias. The "jump-to-conclusions" bias was not associated with a failure to make use of past regularities when processing new stimuli nor with a deficiency in the meta-cognitive skill of reality discrimination, thus contradicting Garety & Hemsley's model. A deficiency in auditory reality discrimination was associated with the experience of voices commenting. A deficiency in visual reality discrimination was associated with a failure to make use of past regularities when processing new stimuli although this information-processing bias was not observed in the sample as a whole. It was concluded that there is considerable variation between individuals in the factors that result in delusions. Despite contradicting some specific predictions of Garety & Hemsley's (1994) model, the findings are consistent with a multi-factorial model of delusion formation. The relationship between these factors would be demonstrated more clearly by the identification of these factors in case studies of individuals with delusions, than by the identification of differences between groups of people with and without delusions. The mechanisms that connect these factors to psychotic phenomena would be elucidated by case studies that specify patients' experiences and cognitive biases in more detail than studies of heterogeneous groups.

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