Schizophrenia has been defined as an identifiable disorder based on phenomenologic classification. Support for this concept is derived from consistent observations of a low frequency of the disorder in general populations throughout the world but substantially higher frequency of occurrence in siblings of affected individuals.
The rates of concurrence in diagnosis for schizophrenia, surprisingly, vary in similar degree to those found for a series of physical disorders such as cervical cancer, emphysema and bronchitis, and electrocardiographic evaluations of cardiac disorder. The most recent findings from cross-national diagnostic studies of mental disorder uphold the need for a regular, more definitive classification system that is considered from both the service and research points of view.
Falek A, Moser HM. Classification in Schizophrenia. Arch Gen Psychiatry. 1975;32(1):59-67. doi:10.1001/archpsyc.1975.01760190061007