Heritability estimates based on studies of clinically ascertained twin samples (Bailey et al., 1995; Folstein and Rutter, 1977; Hallmayer et al., 2011; Steffenburg et al., 1989) vary widely, from 38% to 90%, but it is clear that genes play a major role in ASD. Early twin studies observed 80–90% concordance for ASDs in monozygotic (MZ) twins and 5–15% concordance in dizygotic (DZ) twins and siblings. Two recent studies report somewhat higher rates of concordance in DZ twins (31%), suggesting, that the contribution of shared environmental factors could be greater than had been previously estimated (Hallmayer et al., 2011; Rosenberg et al., 2009).
