Over the past six years the Section on Twin and Sibling Studies, NIMH, has conducted intensive studies of 16 families with MZ twins discordant for schizophrenia. In addition, 3 families in which both twins were schizophrenic and 4 with no known psychiatric illness have been similarly investigated.

Each family is admitted to the Clinical Center, NIH, for two weeks of multi-disciplinary investigation. In the hope of shedding light on the question of neurologic findings in schizophrenic patients and their role in this disorder, detailed neurological examinations are performed on the twins by two neurologists. Patients with gross neurologic disturbance are screened out by our selection criteria. The examiners are therefore explicitly seeking minor deviations in neurologic status, rather than patterns of symptoms and signs leading to a specific neurologic diagnosis. Given this context, the neurologists recorded substantial numbers of signs. Yet, in none of the twins was there sufficient evidence to warrant a neurologic diagnosis.

The pairs of neurologic reports were subjected to a variety of procedures, in an attempt to quantify the results. For example, 11 of the first 13 index schizophrenic twins, as compared with 1 of 13 cotwin controls, were rated as having “probable” or “definite” neurologic abnormality, based upon the senior author's analysis of the number and type of signs recorded and the degree of agreement between the examiners. Significant group differences on number of signs reported were found between schizophrenics and nonschizophrenics, schizophrenic indexes and their cotwin controls, and schizophrenics and normals. In contrast, there were no significant group differences in the number of signs found between the nonschizophrenic cotwin controls and the normal twins.