By comparing similarities between monozygotic (MZ) twins, who share ~100% of the same genes, and dizygotic (DZ) twins, who share ~50% of the same genes, we can estimate relative contributions of genetic and nongenetic influences on trajectories of brain development. To pursue this question, we are conducting a longitudinal neuroimaging study of twins and currently have acquired ~600 scans from 90 MZ and 60 DZ twin pairs. Structural equation modeling (SEM) is used to assess age × gene × environment interactions and other epistatic phenomena that challenge conventional interpretation of twin data. SEM describes the interacting effects as (A) additive genetic, (C) shared environmental, or (E) unique environmental factors (Neale and Cardon, 1992). For most brain structures examined, additive genetic effects (i.e., “heritability”) are high and shared environmental effects are low (Wallace et al., 2006). Additive genetic effects for total cerebral and lobar volumes (including GM and WM sub-compartments) ranged from 0.77 to 0.88; for the caudate, 0.80; and for the corpus callosum, 0.85. The cerebellum has a distinctive heritability profile with an additive genetic effect of only 0.49, although
