Most research investigating reasoning in OCD has focused on decision making and the evaluation of probabilities and has concluded that people with OCD require more information and time before making a decision. Recent research has begun to focus on the differences between deductive reasoning - based on given assumptions and premises - and inductive reasoning in which additional information and prior knowledge is needed to reach a conclusion. Some researchers have argued that people with OCD have a distinctive inductive style of reasoning in which unpleasant outcomes are reasoned to be probable on the basis of irrelevant associations. A U.K. study of 48 people compared deductive and inductive reasoning in people with and without OCD. Deductive reasoning was tested using syllogisms (e.g. All Frenchmen like cheese, He does not like cheese, He is not a Frenchman) which the participants were asked to judge as valid or invalid while inductive reasoning was tested using a game of 20 questions. In the twenty questions game the participants with OCD asked fewer direct hypothesis questions, i.e. guessing at the right answer, than the other group although their performance was more or less the same in the deductive reasoning task. In both tasks the participants with OCD's responses were characterized by the need for more information, states of uncertainty, and doubt and postponement of a final decision.
Simpson, Jane ... [et al] - Reasoning in people with obsessive-compulsive disorder British Journal of Clinical Psychology 2007, 46, 397-411