The significance of genetic factors in the etiology of schizophrenia: results from the national study of adoptees in Denmark.

The prevalence of schizophrenic illness in the biological and adoptive relatives of schizophrenic adoptees has been examined for a total sample of adoptees in Denmark. The total sample was studied in two stages, beginning with the Copenhagen sample of adoptions granted by the courts in the city and county of Copenhagen, the results of which have been reported previously. The adoptions granted by the courts in the remainder of Denmark comprised the Provincial sample the preliminary results of which appear to confirm those obtained earlier. Chronic schizophrenia and milder syndromes described as latent, borderline, or uncertain schizophrenia were found in both samples to concentrate significantly in the biological relatives of schizophrenic adoptees as compared to their controls, but not in their adoptive relatives. These milder and marginal syndromes resembling schizophrenia occurring in the families of schizophrenic patients confirm the observations of Bleuler and others who succeeded him. Their presence in the biological families of schizophrenic adoptees indicates not only their familial but also their genetic relationship to schizophrenia.