Contradictory results in schizophrenia research are generally explained as being due to genetic heterogeneity and multiple factor heredity in relationship to manifold environmental factors. The book reports a short overview of all relevant twin studies on schizophrenic psychoses and provides data and case histories on a systematic twin study based on a polydiagnostic approach carried out by two independently working psychiatrists. In addition to the internationally applied operational diagnostic systems of DSM-III-R and ICD 10 Leonhards subclassification of schizophrenic psychoses was used. Up to now this sophisticated methodological approach is unique in the world.The data provide strong evidence that the spectrum of psychoses with schizophrenic and schizophrenia-like symptoms is not a continuum of diseases. At least in Leonhards three major groups of cycloid psychoses, unsystematic schizophrenias and systematic schizophrenias genetic, somatic and psycho-social factors play a completely different etiological role. Cycloid psychoses and systematic schizophrenias are predominantly caused by environmental factors. In unsystematic schizophrenias, however, genetic predisposition is the main etiological factor and environmental factors are subordinate.