A reconsideration of the Kety and associates study of genetic factors in the transmission of schizophrenia.

The author reviews a study by Kety and associates that reported a significantly greater prevalence of schizophrenic spectrum disorders among the biological relatives of schizophrenic adoptees than among those of nonschizophrenic adoptees. The principal statistical analysis of the Kety study used an incorrect sample size (306 rather than 66) and failed to weight each index and control case (family) equally. This violation of the independence assumption would allow a few families to disproportionately influence the outcome. The author argues that proper statistical analysis applied separately to available categories indicates that significant differences between the index and control groups occurred mainly in the half-sibling category; this result violates the principle that genetic effects increase with greater consanguinity. She concludes that Kety and associates' study raises more questions than it answers regarding the role of genetic factors in schizophrenia.