Epidemiology of schizophrenia.

OBJECTIVE: To characterize the epidemiology of schizophrenia.
METHOD: Narrative literature review.
RESULTS: Each year 1 in 10,000 adults (12 to 60 years of age) develops schizophrenia. Based on a restrictive and precise definition of the diagnosis and using standardized assessment methods and large, representative populations, the incidence rates appear stable across countries and cultures and over time, at least for the last 50 years. Schizophrenic patients are not born into ecological and social disadvantage. The uneven distribution of prevalence rates is a result of social selection: an early onset leads to social stagnation, a late onset to descent from a higher social status. The main age range of risk for schizophrenia is 20 to 35 years. It is still unclear whether schizophrenia-like late-onset psychoses (for example, late paraphrenia) after age 60 should be classified as schizophrenia either psychopathologically or etiologically. In 75% of cases, first admission is preceded by a prodromal phase with a mean length of 5 years and a psychotic prephase of one year's duration. On average, women fall ill 3 to 4 years later than men and show a second peak of onset around menopause. Consequently, late-onset schizophrenias are more frequent and more severe in women than in men. The sex difference in age of onset is smaller in cases with a high genetic load and greater in cases with a low genetic load. Type of onset and core symptoms do not differ between the sexes. The most pronounced sex difference is the socially negative illness behaviour of young men.
CONCLUSIONS: Among the factors determining social course and outcome are level of social development at onset, the disorder itself (for example, genetic liability, severity of symptoms, and functional deficits), general biological factors (for example, estrogen), and sex- and age-specific illness behaviour.