Adoption studies: historical and methodological critique.

The history of adoption studies and their use in separating heredity from environmental influences is reviewed. The adoptee separation paradigm became possible through changing social practices which formalized adoption procedures. In the earlier past of this century, the technique was used principally to investigate the relative importance of heredity and environment in the determination of IQ. It was not until the sixties that the technique was used to study the role of heredity in psychopathology. Genetic factors in alcoholism, criminality, personality disorders, antisocial personality, somatization disorder, affective disorder, hyperactivity and schizophrenia were assessed. The review analyses the potential interactions of confounding variables in such studies and how these can be controlled, and discusses the major methodological criticisms which have been raised. Although the predominant interest has been in the use of the technique to define genetic etiological factors in psychopathology, the paradigm is equally able to delineate precisely the role of environmental factors while controlling for heredity. With about 1 per cent of populations being adopted in Western countries, the further scope for such studies continues to hold promise.