Objectives. The aim of this study was to investigate the inductive and deductive reasoning abilities of people with obsessive-compulsive disorder (OCD). Following previous research, it was predicted that people with OCD would show different abilities on inductive reasoning tasks but similar abilities to controls on deductive reasoning tasks.

Design. A two-group comparison was used with both groups matched on a range of demographic variables. Where appropriate, unmatched variables were entered into the analyses as covariates.

Methods. Twenty-three people with OCD and 25 control participants were assessed on two tasks: an inductive reasoning task (the 20-questions task) and a deductive reasoning task (a syllogistic reasoning task with a content-neutral and content-emotional manipulation).

Results. While no group differences emerged on several of the parameters of the inductive reasoning task, the OCD group did differ on one, and arguably the most important, parameter by asking fewer correct direct-hypothesis questions. The syllogistic reasoning task results were analysed using both correct response and conclusion acceptance data. While no main effects of group were evident, significant interactions indicated important differences in the way the OCD group reasoned with content neutral and emotional syllogisms.

Conclusions. It was argued that the OCD group's patterns of response on both tasks were characterized by the need for more information, states of uncertainty, and doubt and postponement of a final decision.