Logical reasoning deficits in schizophrenia and brain damage.

Von Domarus and Arieti have theorized that failure to reason by conventional logical rules is at the root of schizophrenic thought disorder, but the available research on this view is inconclusive. We compared the performance of schizophrenics (N = 100), brain-damaged patients (N = 50) and psychiatric controls (N = 50) on closely matched measures of Overexpansive and "Von Domarus" (similarity implies identity) syllogistic reasoning errors. Before the samples were matched for education and intelligence, the brain-damaged and schizophrenic patients made more Overexpansive errors than the controls, but the Von Domarus error difference was not significant. After matching, both differences were nonsignificant. The results indicate that inability to use syllogistic reasoning properly is probably not the root cause of schizophrenic thought disorder.