THOUGHT-DISORDER IN SCHIZOPHRENIC AND CONTROL ADOPTEES AND THEIR RELATIVES

Background: Previous research showed significantly elevated levels of thought disorder in the relatives of persons with schizophrenia, as we ll as in the persons with schizophrenia themselves. Comparisons of sch izophrenic and control adoptees and their respective relatives provide a method for minimizing the confounding of genetic and environmental sources of familial resemblance and for elucidating whether the elevat ed levels of thought disorder in persons with schizophrenia and their relatives reflect the influence of shared genetic factors, shared envi ronmental factors, or both. The present study provides the first such adoption-sample data on an operationally defined measure of thought di sorder. Methods: Speech samples elicited by standard interview questio ns from schizophrenic and control adoptees and their respective biolog ical and adoptive relatives were tape-recorded. Verbatim transcripts o f these speech samples were scored, while unaware of the personal or f amily diagnoses of the subjects, using the Thought Disorder Index (TDI ). Results: The mean TDI scores were significantly higher in schizophr enic than in control adoptees and in biological relatives of the schiz ophrenic adoptees than in the biological relatives of the control adop tees, whereas the respective groups of adoptive relatives did not diff er significantly. The differences were most marked for the samples of biological sibs and half sibs, which were larger and more representati ve than the samples of parents. Conclusion: Results suggest that the e levated TDI scores in the relatives of persons with schizophrenia that have been found in other studies reflect the operation of genes incre asing the liability for schizophrenia, rather than the rearing experie nces that were shared in common with schizophrenic probands.