An interesting question is whether the heritability values reported in healthy individuals are similar in individuals with neurodevelopmental disorders. The difficulty of amassing a suitable sample of affected individuals and family members necessary for this type of analysis is considerable, and only a few studies have attempted to assess this. The limited data available suggests that relative contributions of genetic and environmental factors may be different for some brain structures in affected twins. The most common study design is comparison of MZ twins who are discordant for the disorder with healthy pairs. Because the genetic contribution is hypothesized to be the same within MZ twin pairs, increased variation within the discordant pair suggests the impact of additional environmental or disease-related factors, whereas brain differences found in both twins of the affected pair compared with the healthy twins imply a genetic risk factor independent of disease state. Studies have reported increased environmental contributions to the volumes of the lateral ventricles (Suddath, Christison, Torrey, Casanova, & Weinberger, 1990; van Haren et al., 2004) and the hippocampus (van Erp et al., 2004) in
