Schizophrenia and OCD co-occur with one another at a higher rate than would be expected by chance. It has been estimated that approximately 15% to 25% of people with schizophrenia have also OCD.
Both OCD and schizophrenia appear to affect both males and females equally, and both disorders begin around the end of adolescence. Interestingly, many people who develop both schizophrenia and OCD have reported that their OCD symptoms started first, often in their early teens; however, this is not always the case. The occurrence of OCD symptoms prior to schizophrenia poses a diagnostic challenge. Currently, there is no evidence that any particular type of OCD occurs consistently with any particular type of schizophrenia.
Although dysfunction of the serotonin system is thought to be a primary factor in each, a genetic link between OCD and schizophrenia has yet to be established. It should also be noted that it has been discovered that treatment of schizophrenia with atypical antipsychotics can sometimes cause the emergence of OCD symptoms. This, of course, complicates studying the link between OCD and schizophrenia.
Obsessions Versus Delusions
An additional difficulty in exploring the relationship between OCD and schizophrenia has been that symptoms can overlap in some cases.
As mentioned, delusions are a common symptom of schizophrenia. Delusions are false thoughts or beliefs that are held to be true despite very strong evidence to suggest that these beliefs are incorrect. Usually the person experiencing a delusion cannot recognize or be convinced of the irrationality of their thoughts.
People with OCD also experience irrational thoughts in the form of obsessions; however, the content of the obsessions seen in OCD is often quite different from the delusions observed in schizophrenia. While obsessions often relate to ideas around contamination, symmetry and/or aggressive or sexual impulses, delusions usually relate to beliefs around the possession of special powers, persecution or "special" connections to events or objects in the environment. As well, in the case of OCD, most people agree that their obsessions are irrational and are a product of their own mind.
However, some individuals with OCD have a very difficult time seeing that their obsessions are not grounded in reality. Indeed, although the content of obsessions and delusions are often quite different, this lack of insight can at times make it very difficult to distinguish between an obsession and a delusion. Research is ongoing to determine how to best tell the difference between delusions and obsessions.