For many, adoption studies offer a more convincing separation of the effects of genes and environment than do twin studies and, as we have already noted, adoption studies had an important role historically in turning the tide towards a more biological view of schizophrenia. However, because of the difficulties in carrying out adoption studies, they have been used much less than twin studies in investigating the genetics of behaviour. There are three principal designs, all of which have been used in studies of schizophrenia and which are summarized in Table 1.1.
Adoption studies have also had an important role in contributing to the evidence that genes play a part in antisocial behaviour, alcoholism and affective disorders. However, in the case of affective disorders, the pattern of findings has been less than clear-cut. Thus, in contrast to twin studies, all of which point to a genetic contribution, some of the adoption studies are inconclusive, and this seems at least in part a result of the fact that they have relied upon indirect sources of information rather than direct examination of the subject themselves .
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