Thornton, Patricia; (2000) An exploration of reasoning biases in delusions. Doctoral thesis (D.Clin.Psy), UCL (University College London).

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Abstract

Delusions are one of the most prominent and important symptoms of psychosis. Recent attempts to understand delusions from a psychological framework have gathered momentum, and there is evidence that people with delusions show a 'jump to conclusions' reasoning bias, making decisions on the basis of less evidence. Further work has proposed that this is due to a bias in data-gathering. The current study aimed to clarify whether the reasoning biases are related to a tendency towards early cognitive closure or an inability to integrate information. A between-subjects design study compared the reasoning styles of four groups: two groups of psychotic inpatients (one group with delusions, the other group with no cun'ent delusions), and two groups from the normal population (one high on a measure of delusion-proneness, the other low on this measure). Finding evidence for reasoning biases in those in the normal population who are prone to delusional ideation would provide support that these biases are involved in delusion formation, as well as delusion maintenance. A non-deluded psychiatric control group was also used to clarify whether the reasoning biases were related to delusions or the experience of psychosis itself. Five tasks were used to explore the differences in reasoning styles between the groups, and also to further explore the exact nature of the reasoning biases exhibited by delusional individuals.Results showed some evidence that those with delusions demonstrated a 'jump to conclusions' tendency. The evidence gave some support for the hypothesis that this was due to an inability to integrate information. There was no evidence of a general cognitive deficit in those with delusions. Evidence that the biases were due to the presence of delusions rather than psychosis was inconclusive. Also, the subtlety of differences between those high and low in delusion-proneness did not enable conclusions to be drawn on the role of reasoning biases on delusion formation. Implications of these findings for the formation and maintenance of delusional beliefs are discussed. Delusions are seen as being a person's interpretations of their experiences. It is suggested that having an inability to integrate information would lead to interpretations of events that relies only on immediate information, rather than an interpretation that is able to use past information to give context. In this way, the interpretation is more likely to be a false one. Clinical implications for the practice of cognitive behaviour therapy are also discussed.

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