While the heritability of the binary phenotype of dependence on any substance was similar in this sample (h2=0.54) to the most common heritability estimate (h2=0.50) reported for any of the four substances from twin studies (Goldman et al., 2005), the heritability for the quantitative phenotype was much higher (h2=0.86). This is consistent with one prior twin study of a latent genetic factor (h2=0.81) underlying alcohol and drug problems as well as measures of impulsivity and conduct problems (Krueger et al., 2002) but significantly higher than some others (e.g. h2=0.40) (Button et al., 2009). Although this high heritability should not be over-interpreted (Goldman et al., 2005), it is possible that the use of a multi-variable quantitative phenotype, utilizing the pattern of endorsement of the 7 DSM-IV criteria across all four substances, captured valuable genetic information across the vulnerability spectrum.
