The adoption study design compares the prevalence of a disorder among adoptive and biological relatives of individuals with the disorder. The biological relatives of an individual who has been adopted are related genetically to the individual but have not shared any environmental influences. In contrast, adoptive relatives live in the same family environment but are biologically unrelated to the individual. Therefore, if a disorder is due to genetic factors, the biological relatives of individuals with the disorder should exhibit a higher rate of the disorder than the population base rate, whereas an elevated rate of the disorder among adoptive relatives would suggest that family environmental influences play a role in the etiology of the disorder.
The adoption design is quite elegant and has been helpful for some disorders, but two specific constraints have limited the utility of adoption studies. Most importantly, in societies in which adoption records are closed, it is often quite difficult to obtain information from the biological relatives of individuals who are adopted. Moreover, adoptive parents may not be representative of the overall population of parents due to the laudable desire of adoption agencies to place adopted children in an optimal environment with high-functioning parents with many available resources.
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