Abstract

In this chapter we review recent research in an area in which David Over has made an important contribution: reasoning in people with delusions. Delusional beliefs can be distinguished from the strong beliefs that everyone holds by the extra intensity with which they are held and their abnormally weak evidential basis, together with their tendency to disrupt an individual’s everyday functioning. However, non-clinical delusional ideation can also be found, where people tend to ‘jump to conclusions’ on the basis of apparently inadequate evidence compared to controls. Procedures used to assess this apparent bias may induce artificial behaviours that are normally not seen, through their demand characteristics and stressor properties. Delusion-prone people often respond more strongly than controls to such stressors, which may account for their tendency to reach the end-point of the experimental trials earlier. Furthermore, there is evidence that this tendency may not be irrational after all: the conclusion apparently jumped to may be fully justified on probabilistic grounds. Evans and Over’s (1996) dual rationality theory provides a framework for interpreting delusional reasoning: the aberrant belief may be a rational response to an anomalous experience or exaggerated emotional reaction.