The chicken or the egg: What drives OCD?

The prevailing conceptual model for Obsessive-Compulsive disorder (OCD) posits that obsessions drive compulsive rituals that serve to control or reduce obsessional distress. In recent years, an alternative hypothesis to explain the symptoms of OCD was suggested — the 'habit-driven’ hypothesis. According to this hypothesis, compulsions are the result of aberrant dysregulation of stimulus-response habit learning and obsessions are post hoc rationalizations of otherwise unexplained compulsive behaviors. In this article, we describe this hypothesis and briefly review data presented to support it. Next, we raise four questions about this hypothesis to explore how it fits the complex phenotype of OCD: (i) What are the deficits in the goal-directed system in OCD? (ii) How should we define and measure habits in humans? (iii) Are compulsions habits in the technical sense? and (iv) Are obsessions caused by compulsions? We conclude that how an imbalance in goal-directed versus habit behaviors might contribute to the complex phenotype of OCD is yet to be revealed.