This disorder, at some point in the illness, involves a psychotic phase (with delusions, hallucinations, or grossly bizarre/disorganized speech and behavior). This psychotic phase must last for at least one month (or less if successfully treated). Schizophrenia also causes impairment in social or vocational functioning which must last for at least 6 months. The psychotic phase is not due to a medical condition, medication, or illegal drug.

The life expectancy of individual with Schizophrenia is shorter than that of the general population for a variety of reasons. Suicide is an important factor, because approximately 10% of individuals with Schizophrenia commit suicide - and between 20% and 40% make at least one suicide attempt. There is an increased risk of assaultive and violent behavior. The major predictors of violent behavior are male gender, younger age, past history of violence, noncompliance with antipsychotic medication, and excessive substance use. However, it should be noted that most individuals with Schizophrenia are not more dangerous to others than those in the general population.

Alcoholism and drug abuse worsen the course of this illness, and are frequently associated with it. From 80% to 90% of individuals with Schizophrenia are regular cigarette smokers. Anxiety and phobias are common in Schizophrenia, and there is an increased risk of Obsessive-Compulsive Disorder and Panic Disorder. Schizotypal, Schizoid, or Paranoid Personality Disorder may sometimes precede the onset of Schizophrenia.

No laboratory test has been found to be diagnostic of this disorder. However, individuals with Schizophrenia often have a number of (non-diagnostic) neurological abnormalities. They have enlargement of the lateral ventricles, decreased brain tissue, decreased volume of the temporal lobe and thalamus, a large cavum septum pellucidi, and hypofrontality (decreased blood flow and metabolic functioning of the frontal lobes). They also have a number of cognitive deficits on psychological testing (e.g., poor attention, poor memory, difficulty in changing response set, impairment in sensory gating, abnormal smooth pursuit and saccadic eye movements, slowed reaction time, alterations in brain laterality, and abnormalities in evoked potential electrocephalograms).

Schizophrenia usually starts between the late teens and the mid-30s, whereas onset prior to adolescence is rare (although cases with age at onset of 5 or 6 years have been reported). Schizophrenia can also begin later in life (e.g., after age 45 years), but this is uncommon. Usually the onset of Schizophrenia occurs a few years earlier in men than women. The onset may be abrupt or insidious. Usually Schizophrenia starts gradually with a prepsychotic phase of increasing negative symptoms (e.g., social withdrawal, deterioration in hygiene and grooming, unusual behavior, outbursts of anger, and loss of interest in school or work). A few months or years later, a psychotic phase develops (with delusions, hallucinations, or grossly bizarre/disorganized speech and behavior). Individuals who have an onset of Schizophrenia later in their 20s or 30s are more often female, have less evidence of structural brain abnormalities or cognitive impairment, and display a better outcome. Schizophrenia usually persists, continuously or episodically, for a life-time. Complete remission (i.e., a return to full premorbid functioning) is uncommon. Some individuals appear to have a relatively stable course, whereas others show a progressive worsening associated with severe disability. The psychotic symptoms usually respond to treatment with antipsychotic medication, whereas the negative symptoms are less responsive to antipsychotic medication. Often the negative symptoms steadily become more prominent during the course of Schizophrenia.

The best outcomes are associated with early and persistent treatment with antipsychotic medication soon after the onset of Schizophrenia. Other factors that are associated with a better prognosis include good premorbid adjustment, acute onset, later age at onset, good insight, being female, precipitating events, associated mood disturbance, brief duration of psychotic symptoms, good interepisode functioning, minimal residual symptoms, absence of structural brain abnormalities, normal neurological functioning, a family history of Mood Disorder, and no family history of Schizophrenia.

The first-degree biological relatives of individuals with Schizophrenia have a risk for Schizophrenia that is about 10 times greater than that of the general population. Concordance rates for Schizophrenia are higher in monozygotic (identical) twins than in dizygotic (fraternal) twins. The existence of a substantial discordance rate in monozygotic twins also indicates the importance of environmental factors.

Antipsychotic medication shortens the duration of psychosis in Schizophrenia, and prevents recurrences (but psychotic relapses can still occur under stress).