Rosenthal and colleagues earlier compared the frequency of schizophrenia
spectrum disorders in two groups of persons adopted in infancy or early
childhood: those with a psychotic parent (index group) and those whose
biological parents had never had a psychiatric diagnosis or treatment
(control group). They found significantly more disorder in the index group.
Reanalysis of the original material using DSM-III and stricter exclusionary
criteria applied to the parents yielded three times as many schizophrenia
spectrum disorders in the index as in the control group. This difference
remained statistically significant, supporting the operation of genetic
factors in the transmission of the traits comprising the schizophrenic
spectrum of disorders.