A second example is multivariate genetic research which suggests that the genetic structure of common disorders differs substantially from current diagnostic classifications based on symptoms, both for psychopathology (Kendler, Prescott, Myers, & Neale, 2003) and learning disabilities (Plomin & Kovas, 2005). Genetic effects appear to be general, resulting in co-morbidity rather than heterogeneity, whereas environmental effects are specific to each disorder. For example, for psychopathology, a review of 23 twin studies and 12 family studies concludes that anxiety and depression are largely the same disorder genetically and what differentiates the disorders is non-shared environment (Middeldorp, Cath, Van Dyck, Boomsma, Middeldorp et al., 2005). Going beyond anxiety and depression, multivariate genetic research suggests two broad genetic categories of psychopathology: internalising and externalising disorders (Kendler et al., 2003). In other words, ‘generalist’ genes may affect several disorders within major domains such as internalising disorders, externalising disorders and learning disabilities, even though the same multivariate genetic research provides evidence for some specific genetic effects. These results suggest that generalist genes might be good targets for molecular genetic research within these domains (Davis, Kovas, Harlaar, Busfield, McMillan et al., 2008).
