Inductive and deductive reasoning in obsessive-compulsive disorder

Abstract

This body of research investigated how individuals with high levels of obsessive-compulsive disorder (OCD) symptomatology or diagnosed OCD reason when presented with arguments that lead to certain conclusions (deductive reasoning) or probabilistic conclusions (inductive reasoning). Previous research has suggested a “spared deduction, impaired induction” account of OCD, where OCD patients are impaired on tasks of inductive reasoning but are unimpaired on most tasks of deductive reasoning. This thesis sought to test this account by utilising tasks adapted from the contemporary reasoning literature that allowed for a direct comparison between deductive and inductive reasoning. In numerous experiments, participants were presented with a common argument set and asked to make judgments about the deductive validity or inductive plausibility of the arguments. Deductive deficits in sensitivity to argument validity were noted in analogue samples high in OCD symptomatology and in individuals with diagnosed OCD on class-inclusion problems (Experiments 1, 2 and 8), but not when validity was manipulated via other logical forms (i.e., modus ponens, conjunctive syllogisms, or categorical syllogisms; Experiments 6 and 7). Further, individuals high in OCD symptoms were unimpaired in their use of background causal knowledge (Experiment 6) and sample size (premise monotonicity; Experiments 1 and 8) when making inductive judgments. However, both analogue samples high in OCD symptoms and an OCD patient group exhibited a reduced sensitivity to the implications of premise diversity in induction (Experiments 2, 3 and 8). This may be partially explained by differences in the way that individuals high and low in OCD symptoms perceive similarity between the premises (Experiment 5). Importantly, these deductive and inductive deficits were found across both OCD-neutral and OCD-relevant item content and were not due to group differences in general ability. These results challenge the “spared deduction, impaired induction” account of OCD. Cognitive therapies for OCD may be improved by incorporation of direct instruction about the implications of diverse evidence and how to distinguish between simple valid and invalid arguments.