What is schizophrenia? Changing perspectives in epidemiology.

The psychotic syndrome at the core of schizophrenia appears to be invariable across cultures. The risk of morbidity also seems to vary very little from country to country and over medium periods of time. Moreover, apart from gender differences in first onset, the cumulative lifetime risk is the same in females and males. A similar epidemiological pattern is only found in pathological conditions that are characterized by a precisely defined section of a psychopathological dimension with a continuous distribution in the population, e.g. severe mental retardation being the extreme section of normally distributed IQ values. The interpretation of schizophrenic psychosis as the extreme section of a psychopathological dimension or disposition that is almost evenly distributed in all populations is supported by the fact that milder psychiatric disorders occur more frequently before the onset of the psychosis and in close relatives of schizophrenic patients. The psychopathological heterogeneity of these disorders argues against the assumption of a manifest psychopathological dimension with a continuous transition from the schizophrenic psychosis to the "normal" schizothymic personality. More probable is a continuously distributed latent vulnerability to schizophrenia--with or without a threshold effect--which in severe degrees disposes to the uniform reaction pattern of the schizophrenia syndrome. Smaller degrees of vulnerability are associated with an increased risk for milder patterns of disturbances, which are also more strongly determined by environment and personality and therefore are rather heterogeneous. These assumptions lead to other epidemiological and genetic models than Kraepelin's early concept of a disease entity does.