Several authors have investigated the presence of thought disorder in psychiatric patients using different reliable methods. Under the hypothesis of a genetic predisposition to thought disorder, the degree and quality of thought disorder have also been studied in populations at a high risk for psychosis, in particular for schizophrenia. As a result, an increasing incidence of throught disorder was detected in relatives of schizophrenics. To account for the thought disorder also found in normal subjects, researchers propose that thought disorder exists in normal subjects on a continuum with schizophrenic patients. In the following report, we evaluated the inherited component of thought disorder in normal subjects, using a sample of 25 normal twin pairs, 16 monozygotic and 9 dizygotic twin pairs. We applied the Thought Disorder Index (TDI) to assess disordered thinking, genetic estimates were made with classical methods, controlling for environmental sources of variability where possible. Our findings suggest a strong additive genetic component for the global TDI rating variable, with a heritability estimate approaching 80–90%. New approaches in neuropsychology and neuropsychiatry based on genetic methodologies should further define the cerebral physiology responsible for disordered thinkin