| Summary: | Background: Delusions are characterised by conviction, distress, preoccupation, and disruption. ’Jumping-to-conclusions’ (JTC) and a lack of belief flexibility (BF) have been shown to be associated with delusions. -- The thesis: The overall aim was to investigate psychological processes of change in delusions over time and to examine response to treatment of aspects of delusional experience. Specific questions were: do psychological processes associated with delusions change? And do reasoning biases predict change in delusions? -- Method: Three longitudinal studies were conducted using three separate samples of patients with delusions of at least moderate severity. Studies 1 (N = 40) and 3 (N = 16) involved patients in an acute phase of psychosis, whereas Study 2 participants (N = 273) were in the recovery phase. Study 1 investigated changes in delusional dimensions, JTC and BF over eight weeks of antipsychotic treatment. Study 2 examined the factor structure and longitudinal relationship of conviction, JTC and BF over 12 months. Study 3 assessed moment-by-moment levels of delusional dimensions, BF and aberrant salience over two weeks using experience sampling methodology. -- Results: During the early phase of antipsychotic treatment, all delusional dimensions improved over eight weeks (Study 1), whereas only distress and disruption improved over two weeks (Study 3). BF and conviction were distinct factors (Study 2), and higher flexibility was consistently related to lower conviction. JTC was stable within the study periods, although higher rates of JTC during the acute phase suggest improvement across phases. JTC predisposed to the presence of delusions (Study 2) and was associated with higher and more variable conviction during treatment (Study 3). -- Conclusions: That JTC and BF contribute to the development and maintenance of delusions was largely confirmed using longitudinal data.
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