Background. In the Finnish Adoptive Family Study of Schizophrenia, adoptee thinking disorders have been shown to be a joint effect of genetic liability for schizophrenia spectrum disorders and adoptive rearing-parent communication patterns. However, longitudinal predictions of clinical psychiatric disorders of the adoptees have not been reported.

Method. Adoptees (n=109) who had no DSM-III-R disorder at initial assessment (median age 18 years) were selected from the total sample of the Finnish Adoption Study of Schizophrenia. They were defined as at high versus low genetic risk based upon the lifetime diagnoses of their biological, adopting-away mothers – schizophrenia spectrum disorder versus no spectrum disorder. At initial assessment, adoptive rearing parents were independently evaluated from tape-recorded Rorschach protocols scored as manifesting either high or low Communication Deviance (CD), a composite index of communication patterns that distract and befuddle listeners. Adoptees were independently re-diagnosed after a median interval of 14 years and followed-up from national registers for an additional 7 years.

Results. The main effects of genetic liability (G) and CD of the adoptive parents (E), each taken separately, predicted significantly for psychiatric disorders of the adoptees as adults. However, when G, E, and their joint interaction effect were entered into the same logistic model, only the interaction effect was significant. The sample included seven adoptees with schizophrenia spectrum disorders, but a separate analysis to predict them was non-significant.

Conclusion. Genetic liability for schizophrenia spectrum disorder and an adoptive family rearing variable interact, predicting longitudinally and significantly to broadly defined adoptee psychiatric disorder.