Background. Diverse forms of thought disorder, as measured by the Thought Disorder Index
(TDI), are found in many conditions other than schizophrenia. Certain thought disorder categories
are primarily manifest during psychotic schizophrenic episodes. The present study examined
whether forms of thought disorder qualify as trait indicators of vulnerability to schizophrenia in
persons who are not clinically ill, and whether these features could be linked to genetic or
environmental risk or to genotype–environment interactions. The Finnish Adoptive Study of
Schizophrenia provided an opportunity to disentangle these issues.
Methods. Rorschach records of Finnish adoptees at genetic high risk but without schizophrenia-related clinical diagnoses (N = 56) and control adoptees at low genetic risk (N = 95) were blindly
and reliably scored for the Thought Disorder Index (TDI). Communication deviance (CD), a
measure of the rearing environment, was independently obtained from the adoptive parents.
Results. The differences in total TDI between high-risk and control adoptees were not statistically
significant. However, TDI subscales for Fluid Thinking and Idiosyncratic Verbalization were more
frequent in high-risk adoptees. When Rorschach CD of the adoptive rearing parents was introduced
as a continuous predictor variable, the odds ratio for the Idiosyncratic Verbalization component of
the TDI of the high-risk adoptees was significantly higher than for the control adoptees.
Conclusions. Specific categories of subsyndromal thought disorder appear to qualify as vulnerability
indicators for schizophrenia. Genetic risk and rearing-parent communication patterns significantly
interact as a joint effect that differentiates adopted-away offspring of schizophrenic mothers from
control adopted-away offspring.