A review of the recent literature on the differential diagnosis of schizophrenia suggests the illness has been misunderstood and inadequately defined on past clinical and research endeavors. The most common error has been undue emphasis on so-called "schizophrenic symptoms" (i.e., delusions, hallucinations, catatonia, Schneiderian FRSs) which, when present, have been used to justify this diagnosis. Such practices have contributed to a tendency to overdiagnose schizophrenia while underdiagnosing affective disorders, especially manic-depressive illness, manic type. An examination of the MMPI in general, and Scale 8 in particular, reveals that the instrument has never been successful in differentiating schizophrenics from other severely disturbed psychiatric patients. Moreover, the use of profile coding rules and other complex strategies have not improved the MMPI's effectiveness in this area. The present study hypothesized that these failures are largely due to the possibility that an unknown but significant number of the criterion "schizophrenics" used in the construction of Scale 8 were misdiagnosed affective disorders. Using carefully selected groups of schizophrenics, manics, and a reference group of general psychiatric inpatients (controls), three empirical MMPI scales were constructed: (a) a schizophrenia scale with items differentiating schizophrenics from controls; (b) a mania scale with items differentiating manics from controls; and (c) a mania-schizophrenia differentiation scale differentiating manics from schizophrenic&. All of the newly developed scales differentiated the criterion groups markedly better than any existing MMPI measures. This success was attributed to the use of restrictive diagnostic practices and profile invalidity criteria that enhanced the availability of meaningful information for scale development. The results were interpreted in terms of their support for the "positive" vs. "negative" distinction in classifying schizophrenia. Cross-validation issues and implications for future research were additionally discussed.
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