Background: Previous twin studies have supported a genetic contribution to the major categories of psychotic disorders, but few of these have employed operational diagnostic criteria, and no such study has been based on a sample that included the full range of functional psychotic disorders.

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In a group of schizophrenics of twin birth, no evidence of ventricular enlargement was found where there was a family history of major psychiatric disorder. Among those schizophrenics without such a family history, cerebral ventricular size was significantly increased (P less than 0.01), and there was also evidence of birth complications. Among normal control twins, those who reported complicated births had significantly larger ventricles.