Don Jackson's "A critique of the literature on the genetics of schizophrenia": a reappraisal after 40 years

In 1960, Don Jackson published "A Critique of the Literature on the Genetics of Schizophrenia." Jackson's critique nearly relegated the classical twin method to obscurity as being hopelessly confounded by environmental factors. Jackson noted several trends in the schizophrenia twin data that were difficult to explain on genetic grounds. In fact, none of Jackson's 12 major points, examined in this article, have been satisfactorily answered by proponents of the twin method. The evidence in support of Jackson's most controversial claim-that the psychology of twinship itself might lead to a greater rate of schizophrenia among twins when compared with the single-born population--is inconclusive. However, although several leading twin researchers have dismissed Jackson's entire thesis on the basis of the inconclusiveness of this one claim, this idea was not central to Jackson's basic argument. The research undertaken for this article appears to confirm the validity of the most important points of Jackson's analysis and that therefore the classical twin method is of doubtful value as an indicator of genetic influences on schizophrenia and other diagnoses or psychological trait differences. Jackson's article demonstrated the implausibility of what came to be known as the MZ/DZ "equal environment assumption" of the twin method, implying that the twin method records nothing more than environmental differences between monozygotic and dizygotic twins.