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ABSTRACT. This review is a critical re-analysis of the Danish American Adoptees' Family Studies, by Kety and colleagues, which are perhaps the most frequently cited studies in support of a genetic predisposition for schizophrenia. As noted by previous reviewers and as examined in detail here, these studies, conducted primarily between 1968 and 1994, contain problems that include (a) their design, (b) the definition of schizophrenia (in particular, the concept of a schizophrenia spectrum) they used, (c) the validity of their method for counting second-degree relatives, (d) their questionable methods for counting diagnoses in certain statistical comparisons, (e) their likely violation of the assumption of independent observations (by individually counting relatives who grew up in the same families), and (f) their failure to study environmental variables. Most important, the author argues that the policies of Danish adoption agencies led index adoptees to be placed into environments inferior to those into which control adoptees were placed. He also concludes that methodological problems and likely environmental confounds call into question the investigators' conclusions that their studies furnished evidence supporting the genetic basis of schizophrenia. Given the problems associated with family, twin, and other adoption studies and the failure of molecular genetic studies to identify postulated genes, the entire body of evidence cited in support of a genetic predisposition for schizophrenia should be re-evaluated, and the debate over the existence of genetic factors in schizophrenia should be re-opened.
Key words: adaptive studies, genetics, Kety, psychiatric genetics, schizophrenia, selective placement
THE KETY AND ASSOCIATES Danish American schizophrenia adoption studies are perhaps the most widely cited evidence used in support of a genetic basis for schizophrenia.1 In spite of the acceptance of Kety and his colleagues' conclusions, this series of studies has been the subject of several critical analyses (Benjamin, 1976; Boyle, 1990; Breggin, 1991; Cassou, Schiff, & Stewart, 1980;
Joseph, 1998a, 1999b; Lewontin, Rose, & Kamin, 1984; Lidz, 1976; Lidz & Blatt, 1983; Lidz, Blatt, & Cook, 1981; Pam, 1995). This article's objective is to look closely at the key issues raised by these critiques to determine whether the conclusions reached by Kety and his associates are consistent with the evidence.
The Adoptees' Family method begins with a group of adoptees diagnosed with the condition in question (called index...