PROBABILISTIC REASONING IN OBSESSIVE-COMPULSIVE AND DELUSIONAL DISORDERS

Background. Delusional disorder (DD) and obsessive-compulsive disorder (OCD) have been investigated in previous studies using probabilistic reasoning paradigms and abnormalities in each group have been reported . No study to date has compared results between these groups. This stu dy compares patients with these disorders with those who have both phe nomena. Methods. Thirty subjects with DD, 29 with OCD and 16 with obse ssive and delusional features were compared with 30 normal controls in a study of probabilistic reasoning using two different computer-based tasks involving a Bayesian paradigm. Results. Deluded subjects showed a 'jump to conclusions' reasoning style, but on a test that added a c onsequence to their choices did not differ from normals. OCD subjects deviated from Bayesian and control norms to a greater degree than did DD subjects. In subjects with mixed psychopathology, the presence of b oth phenomena appeared to 'normalize' these probability estimates. Con clusions. Our findings extend those of others but require cautious int erpretation as to the role of probabilistic reasoning in the genesis o f delusions or obsessions. Obsessionals in both the OCD and Mixed grou ps, showed substantial deviation from Bayesian norms, suggesting that obsessionality leads to a reasoning style that is less 'normal' than t hat of delusionals. Further work is required to investigate clinical c orrelates of these findings which provide modest support for the propo sal that the combination of obsessions and delusions confers greater f unctional advantages than simply having delusions or obsessions.