modifications (when not inherited) or sporadic structural change in DNA, which can occur in one member of an MZ pair but not the other, are not captured in a twin study. Reviews of the evidence from twin and adoption studies report that the heritability of alcohol use disorders averages 0.5 to 0.6 (Schuckit, 2009), with estimates ranging from 0.3 to 0.8 for other substances (Agrawal & Lynskey, 2006, 2008; Tsuang, Bar, Harley, & Lyons, 2001; Vink, Willemsen, & Boomsma, 2005).
