Twin studies can provide insight into whether clinical heterogeneity may reflect differences in etiological risk factors. For example, alcohol dependence with comorbid drug dependence has been found to be a particularly heritable form of the disorder,19,20 and twin studies have suggested a genetic influence on typical versus atypical forms of major depression.21 Changing genetic influence across Inhibitors,research,lifescience,medical development Another active area of research is the clarification of how genetic and environmental influences may change across development. A recent meta-analysis

examined published studies with at least two heritability time points across adolescence and young adulthood for eight different behavioral domains. These analyses revealed significant cross-time Inhibitors,research,lifescience,medical heritability increases for externalizing behaviors, anxiety symptoms, depressive symptoms, IQ, and social attitudes, and nonsignificant increases for alcohol consumption and nicotine initiation.

The only domain that showed no evidence of heritability changes across time was attention-deficit/hyperactivity Inhibitors,research,lifescience,medical disorder.22 Similarly, in a large study of >11 000 pairs of twins from four countries, the heritability of general cognitive ability was found to increase significantly and linearly from 41% in childhood (9 years) to 55% in adolescence (12 years) and to 66% in young adulthood (17 years).23 The robust finding Inhibitors,research,lifescience,medical of increases in the importance of genetic influences across development likely reflects, in part, active gene-environment correlation, as individuals increasingly select and create their own experiences based on their genetic propensities. In addition to changes in the relative magnitude of importance of genetic and environmental influences, another

dynamic change is that different genes Inhibitors,research,lifescience,medical may be acting at different time points. This is nicely illustrated in recent analyses of alcohol use problems, as assessed at five time points from ages 19 to 28 in the Dutch Twin Registry (Kendler et al, in preparation). Kendler and colleagues found strong innovation and attenuation of genetic factors across this age range – indicating that some genetic influences on alcohol problems that were evident at age 19 declined in importance Cilengitide across time, while new genetic influences became important starting at ages 21 and 23. Thus, although the overall heritability of alcohol problems remained fairly stable, it appeared that different genetic factors were important at different timepoints. In analyses in the TCHAD Swedish study which followed twins from ages 9 to 20 across four waves of assessment, large changes were seen in the genetic risk factors for fears and phobias24 and for symptoms of anxiety and depression,25 with particularly pronounced evidence for genetic innovation at puberty. These analyses suggest that genetic influences of many psychiatric and substance use disorders are likely to be developmentally dynamic.