Schizophrenia is a group of psychotic disorders characterized by positive and negative symptoms and associated with a deterioration in role functioning. Symptoms including delusions, prominent hallucinations, disorganized speech, and catatonic behavior result in a gross separation from reality. Since 400 B.C., there have been accounts of schizophrenia. Schizophrenia was formerly known by Kraeplin's term dementia praecox, and the term was used to describe individuals who exhibited symptoms that involved severe mental deterioration from an early age. Later, Eugen Bleuler coined the term "schizophrenia" to describe the splitting of mental associations, the disturbance that lies at the foundation of the disorder. Today, schizophrenia has an incidence rate (number of new cases reported) of one per ten thousand people per year and a lifetime prevalence rate of 1 percent. The age of onset ranges from late teens to mid-thirties, and there are gender differences: For men, the age of onset is early to mid-twenties. Women tend to have an age of onset in the late twenties and are more likely to exhibit more mood symptoms and have a better prognosis; yet, they also have a greater chance of being misdiagnosed. 60 to 70 percent of individuals diagnosed with the schizophrenia never marry, and about 10 percent end their life in suicide.

There are three common patterns of this primarily chronic disorder. The first one involves an acute onset, undulating course, and inter-episode recovery. This describes about 35 percent of the cases. The second common type, which describes about 17 percent of the cases reported, involves an acute onset, an undulating course, and increasing impairment over time. The final course, which is very common and describes about 24 percent of the cases, is similar to the individuals recorded by Kraeplin: there is a gradual onset, an insidious course, and steady impairment or deterioration.

According to the DSM-IV, a diagnosis for schizophrenia is made when an individual exhibits one of the characteristic symptoms, such as bizarre delusions or auditory hallucinations. The individual may also exhibit two of the secondary symptoms, such as delusions of grandeur, delusions of persecution, and referential delusions, or any sensory hallucination. Other secondary symptoms include disorganized speech, grossly disorganized or catatonic behavior, or negative symptoms. Whereas disorganized behavior and the psychotic symptoms of hallucinations and delusions are classified as positive symptoms, or symptoms that are present that should not be, negative symptoms consist of such things as affective flattening, elogia, and avolition. All of these indicate patterns of behavior that should be present but are actually absent. In addition to the presence of one primary characteristic or two secondary symptoms, the individual must exhibit some form of functional impairment. The symptoms must exist for a duration period greater than six months. If the symptoms are exhibited for less than a six-month period, the individual is diagnosed with schizophreniform disorder.

There are two types of schizophrenia: paranoid schizophrenia and disorganized schizophrenia. In the disorganized type, disorganized behavior is more prominent. The individual may have delusions and hallucinations, but it is harder to find any coherence in the symptoms. These individuals tend to express more inappropriate affect and are associated with an early, slow, insidious onset and worst outcome. In the paranoid type, delusions and hallucinations are more prominent and seem to surround a coherent theme. These individuals tend to be more angry, aloof, openly hostile, and more likely to possess a superior or patronizing manner. They do not necessarily have persecutory delusions and they are likely to have higher functioning and a better outcome.