This study placed a focus on the incidence of schizophrenia in the biological and adoptive relatives of adoptees who were now schizophrenic adults. Hospital records of all persons who had been given up for non-family adoption at an early age in the Copenhagen area between 1923 and 1947 were analysed, and 33 cases were selected. 16 were chronic process schizophrenics, 7 had acute schizophrenic reactions and 10 were borderline schizophrenic. The selection process was completed by three independent groups so as to eliminate bias.
Biological and adoptive parents, siblings and half-siblings were identified, and hospital records were looked at. Differences in diagnoses were settled by discussions. The diagnoses were made by two separate groups who did not know whether they were diagnosing a biological or adoptive relative of an index or control subject.
Schizophrenic disorders occur significantly more often among the biological relatives of the index cases than among the biological relatives of the controls (p = 0.0072). 13 biological relatives of schizophrenic subjects were found to also be on the schizophrenic spectrum, whereas only 3 of the control group’s relatives were. This provides evidence for the hypothesis of genetic transmission. No significant evidence for the hypothesis of transmission from adoptive relatives was found.
There is a high likelihood that both the biological and adoptive relatives would have been screened through the adoption process, so therefore it is to be expected that there would be low number of relatives with schizophrenia. With only 16 process schizophrenics, the sample space is simply too small to give the heritability of such disorders the proper opportunity to express itself. By only relying on hospitalisations and not personal interviews, it is possible that many cases went unnoticed and undiagnosed. This is a study from 1972, so the results may be outdated or disproved at this point, but studies such as these have many ethical concerns and are less common these days. Similar to the previous study, the results are inconclusive and not completely reliable, but still provide evidence that schizophrenia stems from genetic contagion.