REASONING AND DELUSIONS

Background Delusions are assumed to reflect disordered reasoning, but with little empirical support. We attempted to study this in 16 relati vely intelligent deluded patients and 16 normal volunteers. Method Sta ndard tests were used which required subjects to choose between logica lly fallacious and valid responses, both of which were plausible. The tests were: (a) conditional statements (if... then), (b) syllogisms (e .g. no A are C, some B are C, some C are not A), and (c) judgements of probability. All three tasks incorporated neutral and emotive content . Results Both normal and deluded subjects frequently made logical err ors. With conditionals, deluded subjects tended to endorse fallacies m ore often than normal controls and this was accentuated when the conte nt was emotive. Similarly, with syllogisms, the effect of emotional co ntent on the endorsement or unbelievable responses was increased sligh tly in the deluded group. Finally, the deluded patients showed a trend to be less prone to the conjunction fallacy than normals, suggesting less reliance on existing schema. Conclusions Differences in reasoning between deluded patients and controls are surprisingly small. Patient s are somewhat more prone to endorse invalid or fallacious responses, especially when emotive themes are involved.