Abstract
Present clinical and research methods of differential diagnosis of schizophrenia and affective psychoses rely very heavily on presenting symptoms and signs, especially in acute psychosis. We have reviewed studies bearing on this issue, including studies of the phenomenology of psychotic illness, outcome, family history, response to treatment with lithium carbonate, and cross-national and historical diagnostic comparisons. We conclude that most so-called schizophrenic symptoms, taken alone and in cross section, have remarkably little, if any, demonstrated validity in determining diagnosis, prognosis, or treatment response in psychosis. In the United States, particularly, overreliance on such symptoms alone results in overdiagnosis of schizophrenia and underdiagnosis of affective illnesses, particularly mania. This compromises both clinical treatment and research.