You may have had the experience of having an irresistible urge to go back to check whether you actually locked the door or turned off the oven, even when you’re pretty sure that you did. Or you may have been unable to resist engaging in some superstitious behavior, such as wearing your lucky shirt on a date or to a sporting event. In some people, such thoughts and actions spiral out of control and become a serious problem.

Karen, a 34-year-old with four children, sought treatment after several months of experiencing intrusive, repetitive thoughts in which she imagined that one or more of her children were having a serious accident. In addition, an extensive series of protective counting rituals hampered her daily routine. For example, when grocery shopping, Karen had the feeling that if she selected the first item (say, a box of cereal) on a shelf, something terrible would happen to her oldest child. If she selected the second item, some unknown disaster would befall her second child, and so on for the four children. For example, if she drank one cup of coffee, she felt compelled to drink four more to protect her children from harm. She acknowledged that her counting rituals were irrational, but she became extremely anxious when she tried to stop (Oltmanns, Neale, & Davison, 1991). Karen’s preoccupation with numbers extended to other activities, most notably, the pattern in which she smoked cigarettes and drank coffee.

Karen’s symptoms are typical of obsessive-compulsive disorder (OCD), a disorder in which repetitive, intrusive thoughts (obsessions) and ritualistic behaviors (compulsions) designed to fend off those thoughts interfere significantly with an individual’s functioning. Anxiety plays a role in this disorder because the obsessive thoughts typically produce anxiety, and the compulsive behaviors are performed to reduce it. In OCD, these obsessions and compulsions are intense, frequent, and experienced as irrational and excessive. Attempts to cope with the obsessive thoughts by trying to suppress or ignore them are of little or no benefit. In fact (as discussed in the Consciousness chapter), thought suppression can backfire, increasing the frequency and intensity of the obsessive thoughts (Wegner, 1989; Wenzlaff & Wegner, 2000). Despite anxiety’s role, in DSM–5, OCD is classified separately from anxiety disorders because this disorder and the anxiety disorders are believed to have distinct causes and to be maintained via different neural circuitries in the brain.

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Although 28% of adults in the United States report experiencing obsessions or compulsions at some point in their lives (Ruscio et al., 2010), only 2% will develop actual OCD (Kessler, Berglund, et al., 2005). Similar to anxiety disorders, rates of OCD are higher among women than men (Kessler et al., 2012). Although compulsive behavior is always excessive, it can vary considerably in intensity and frequency. For example, fear of contamination may lead to 15 minutes of hand washing in some individuals, whereas others may need to spend hours with disinfectants and extremely hot water, scrubbing their hands until they bleed.

The obsessions that plague individuals with OCD typically derive from concerns that could pose a real threat (such as contamination or disease), which supports preparedness theory. Thinking repeatedly about whether we’ve left a stove burner on when we leave the house makes sense, after all, if we want to return to a house that is not “well done.” The concept of preparedness places OCD in the same evolutionary context as phobias (Szechtman & Woody, 2006). However, as with phobias, fears that may have served an evolutionary purpose become distorted and maladaptive.

Researchers have not determined the biological mechanisms that may contribute to OCD (Friedlander & Desrocher, 2006), but one hypothesis implicates heightened neural activity in the caudate nucleus of the brain, a portion of the basal ganglia (discussed in the Neuroscience and Behavior chapter) known to be involved in the initiation of intentional actions (Rappoport, 1990). Drugs that inhibit the activity of the caudate nucleus can relieve some of the symptoms of obsessive-compulsive disorder (Hansen et al., 2002). However, this finding does not indicate that overactivity of the caudate nucleus is the cause of OCD. It could also be an effect of the disorder: People with OCD often respond favorably to psychotherapy and show a corresponding reduction in activity in the caudate nucleus (Baxter et al., 1992).