The contemporary diagnoses of schizophrenia (sz)— Diagnostic and Statistical Manual of Mental Disorders,

ered as important scientific achievements. However, these algorithms were not a product of explicit conceptual anal-yses and empirical studies but defined through consensus with the purpose of improving reliability. The validity sta-tus of current definitions and of their predecessors remains unclear. The so-called ‘‘polydiagnostic approach’ ’ applies different definitions of a disorder to the same patient sample in order to compare these definitions on potential validity indicators. We reviewed 92 polydiagnostic sz studies published since the early 1970s. Different sz definitions show a considerable variation concerning frequency, concordance, reliability, outcome, and other validity measures. The DSM-IV and the ICD-10 show moderate reliability but both definitions appear weak in terms of concurrent validity, eg, with re-spect to an aggregation of a priori important features. The first-rank symptoms of Schneider are not associated with family history of sz or with prediction of poor out-come. The introduction of long duration criteria and exclu-sion of affective syndromes tend to restrict the diagnosis to chronic stable patients. Patients fulfilling the majority of definitions (core sz patients) do not seem to constitute a strongly valid subgroup but rather a severely ill subgroup. Paradoxically, it seems that a century after the introduc-tion of the sz concept, research is still badly needed, con-cerning conceptual and construct validity of sz, its essential psychopathological features, and phenotypic boundaries