Formal thought disorder in schizophrenics and their twins.

The present study was designed to determine whether formal thought disorder is less or more heritable than the global diagnosis of schizophrenia, and to examine relations among formal thought disorder and psychiatric diagnosis, stage of illness, and severity of psychiatric history. Interviews recorded in the Gottesman-Shields twin series (17 pairs of identical twins, 14 pairs of fraternal same-sex twins, and 12 unpaired twins) were assessed for the presence and severity of formal thought disorder, using a modified version of the Scale for the Assessment of Thought, Language, and Communication Disorders. No evidence of genetic influence on thought disorder was found. A factor analysis of the thought disorder scales revealed two interpretable factors reflecting verbosity and discontinuities in the form of speech. Scales with high loadings on the verbosity factor showed signs of familial influence and were significantly correlated with severity of psychiatric history. Scales with high loadings on the discontinuity factor were associated with the florid psychotic phase of illness. Despite numerous studies that have demonstrated a strong genetic component in the etiology of schizophrenia (see Gottesman & Shields, 1976, 1982; Kessler, 1980, for reviews), we do not yet know whether any specific behavioral manifestations of the disorder are genetically influenced. This difficulty has arisen, at least in pan, because schizophrenia is a disjunctive category. It is possible for two individuals to be diagnosed as schizophrenics even though they exhibit few, if any, common symptoms. The inability to specify a common underlying factor in schizophrenia has led some investigators to question whether the global diagnosis of schizophrenia is the