Age of onset is the single most important characteristic of schizophrenia that could yield clues to its origin. To identify the age of onset, however, the onset of the pathological process must be determined. This process may have more than one component occurring at distinctly different times in the life of an individual. Nevertheless, many studies, using either the first appearance of psychosis or the age of first hospitalization, have found gender, familial, and other "age of onset" differences among patients with schizophrenia. These differences may aid in examining genetic mechanisms for schizophrenia.