and functional magnetic resonance imaging studies may improve the characterization of biological pathways between genes, brain, and behavior. This improvement may in turn help to identify more homogenous clinical subgroups that differ in their responses to treatment. In addition, although the research discussed in this review might be thought to be focused on identifying, and maximizing, heritable variance, it does, of course, follow that the nonheritable variance can be refined in a similar fashion. This, too, can help target future research, by portioning the environmental variance into shared and nonshared environmental factors, indicating where epidemiologic studies should be directed. Better still, multivariate twin studies have the potential to clarify the role of putative environmental risk factors by assessing whether they affect ADHD behaviors directly or moderate the influence of genetic or other environmental sources of variation. Further, multivariate SEM of twin data reveals environmental causes of covariation, indicating where clinicians can direct target treatment in, for example, addressing comorbid oppositional and ADHD behaviors. This is particularly valuable where twin studies establish that the same behaviors in different settings (e.g., across tasks or situations) do not share the same environmental factors. The validation of increased ADHD correlates in unaffected relatives of
