Inductive and deductive reasoning in obsessive-compulsive disorder

Background and objectives: This study examined the hypothesis that participants diagnosed with obsessive compulsive disorder (OCD) show a selective deficit in inductive reasoning but are equivalent to controls in deductive reasoning. Methods: Twenty-five participants with OCD and 25 non-clinical controls made inductive and deductive judgments about a common set of arguments that varied in logical validity and the amount of positive evidence provided (premise sample size). A second inductive reasoning task required participants to make forced-choice decisions and rate the usefulness of diverse evidence or non-diverse evidence for evaluating arguments. Results: No differences in deductive reasoning were found between participants diagnosed with OCD and control participants. Both groups saw that the amount of positive evidence supporting a conclusion was an important guide for evaluating inductive arguments. However, those with OCD showed less sensitivity to premise diversity in inductive reasoning than controls. The findings were similar for both emotionally neutral and OCD-relevant stimuli. Limitations: The absence of a clinical control group means that it is difficult to know whether the deficit in diversity-based reasoning is specific to those with OCD. Conclusions: People with OCD are impaired in some forms of inductive reasoning (using diverse evidence) but not others (use of sample size). Deductive reasoning appears intact in those with OCD. Difficulties using evidence diversity when reasoning inductively may maintain OCD symptoms through reduced generalization of learned safety information.