Background: Our previous investigation of the prevalence of mental illness among the biological and adoptive relatives of schizophrenic adoptees in Copenhagen, Denmark, showed a significant concentration of chronic schizophrenia (5.6%) and what Bleuler called "latent schizophrenia" (14.8%) in the biological relatives of chronic schizophrenic adoptees, indicating the operation of heritable factors in the liability for schizophrenic illness.
Methods: We now report the results of a replication of that study in the rest of Denmark (the "Provincial Sample").
Results: In this sample, the corresponding prevalences were 4.7% and 8.2%. In the combined "National Sample" of adoptees with chronic schizophrenia, that disorder was found exclusively in their biological relatives and its prevalence overall was 10 times greater than that in the biological relatives of controls.
Conclusions: This study and its confirmation of previous results in the Copenhagen Study speak for a syndrome that can be reliably recognized in which genetic factors play a significant etiologic role. These findings provide important and necessary support for the assumption often made in family studies: observed familial clustering in schizophrenia is an expression of shared genetic factors.