Just as schizophrenic patients lack boundaries, so too does the concept of schizophrenia lack clear boundaries. In the absence one hallmark symptom of schizophrenia, the definition of the syndrome of schizophrenia has varied from an over-inclusive American approach (Bleuler, 1950) in which any gross impairment in the ability to function would be sufficient for diagnosis, to the more restrictive narrow European criteria (Kraeplin, 1919) which require the presence of hallucinations and or delusions. Generally speaking, the syndrome of schizophrenia is an amalgam of disordered thinking processes (e.g. poor abstract thinking and reasoning, impaired working memory, disorganized speech), perceptual disturbances (hallucinations, delusions), behavioral abnormalities (catatonia), social incapacity, and affective dysregulation.
Schizophrenia is a genetically transmitted disorder of brain development. It is debilitating, chronic, and may demonstrate subtle manifestations as early as the second trimester in utero (abnormal fingertip ridge count, Bracha, 1992), but does not make its full appearance as a psychosis until early adulthood. The genetically transmitted vulnerability probably is clinically manifested as a spectrum of disorders, since schizophrenia is related to schizotypal and schizoaffective disorders (Kendler, 1995), whose incidence is higher in families of schizophrenics than the general population. Schizophrenia itself directly affects anywhere between 0.2% to 1% of the population. This figure is remarkably stable across cultures, although there are population pockets of increased incidence. The risk of schizophrenia is higher in winter births, however the reasons for this are unknown.
Despite the very strong genetic component, it is clear that environmental factors are also involved in the manifestation of the disorder since identical twins share the disorder only 50% of the time.