Adoption studies in functional psychosis.

The scientific rationale of the adoptive methods in genetic studies has been discussed. Three strategies have been employed in psychiatric research. In the adoptee's study method, one examines the adopted away offspring of biological parents, known to be affected by the disorder. A higher prevalence of the disorder among adoptees born to affected biological parents indicates that genetic factors are important. In the adoptee's relatives method, one starts with adoptees known to be disturbed, and then continues by investigating their biological and adoptive families. Comparisons of prevalence rates between relatives of the biological and the rearing index and control families permits one to estimate the influence of the genetic factors. A third method is cross-fostering, which is rarely employed. Children of normals who are "cross-fostered" to adoptive parents who later became ill, are studied. Adoption studies in the field of schizophrenia and manic depressive illness are more specifically discussed. Taken all together, the family, twin and adoption studies support the genetic etiology in schizophrenia and manic depressive illness, although it must be admitted that we need replications to arrive at firm conclusions in the last group of disorder.