Thinking within the spectrum: schizophrenic thought disorder in six Danish pedigrees.

Formal thought disorder (FTD), a major symptom of schizophrenia, is known to aggregate in families. Our aim was to examine the specificity of FTD in the schizophrenia spectrum disorders and the hypothesized linear aggregation of FTD within pedigrees. Six individuals with a diagnosis of schizophrenia were identified in the Copenhagen High-Risk study and each pedigree was centered on one of the six original schizophrenic probands' nuclear families. The 329 pedigree members in the study were considered at risk for schizophrenia spectrum disorders because most were genetically related to the originating schizophrenic probands. The participants were administered the Copenhagen Interview of Functional Illness to determine diagnoses and the Thought Disorder Index (TDI) was used to assess FTD. Individuals with a schizophrenia diagnosis had higher global levels of FTD, exhibited more severe types of FTD, and had a qualitatively different type of FTD than did participants with other diagnoses or no mental illness. Individuals with Cluster A diagnoses exhibited more FTD and FTD similar in quality to participants with schizophrenia. These results support the construct of a spectrum of schizophrenia conditions. There was a generally high level of FTD in the pedigrees, in part due to assortative mating in this sample. However, there was no apparent pattern of linear aggregation of FTD within the families.