Avoidance conditioning plays a critical role in obsessive-compulsive disorder (OCD). OCD is when an individual expresses impulses or thoughts that lead to repetitive actions in response to the obsession. Obsessions may include getting contaminated by touching something with your hands, so repetitive hand-washing serves as a way to "rid" oneself of such contaminations. Since the obsession tends to increase anxiety, the compulsion is drawn out to decrease anxiety. Avoidance plays a role when an OCD-sufferer, after a contamination episode, later avoids touching the initially contaminated item a second time. This is when phobias against, say, garbage can appear. According to the two-process theory, feeling contaminated by touching something is the classically conditioned response whereas the compulsive hand-washing is an operant response. This two-process theory does not explain every detail about OCD, as many people cannot pin-point the exact cause of their OCD symptoms. What is definite is that obsessive-compulsive disorder can result after a stressful period in one's life.