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 twins discordant for schizophrenia.Hulshoff Pol and colleagues (2004) studied gray and white matter volumes, and found that gray matter was different between the affected twin and their sibling, whereas white matter was decreased in both twins compared to controls, suggesting that white matter differences may be a clearer correlate of genetic vulnerability. A study of twins discordant for autism also found several structures to be similarly affected in both twins, whereas cerebellar volumes were only abnormal in the affected individuals (Kates et al., 2004). The recent findings of epistatic modifications and copy number variation within MZ twin pairs may make interpretation of brain differences in discordant pairs more complex.
