Our results were consistent with many of the critical findings from the previous personal interview-based twin studies of multiple forms of DA. Most importantly, in accord with the previous findings from the Vietnam Era and VATSPSUD studies (Kendler et al., 2003, 2007; Tsuang et al., 1998), we found that the large majority of genetic variation to DA of individual substance classes was non-specific. In some instances, the results were strikingly similar. For example, in the VATSPSUD study with both males and females (Kendler et al., 2007), the total heritability of cannabis abuse/dependence was estimated at 71% (versus 70% in this report) and the proportion of genetic variance that was substance-specific was estimated at 17% (versus 24%) in this report. The Vietnam era study estimated that the proportion of genetic variance which was substance specific for cannabis, stimulants and sedative abuse, was respectively, 33%, 27% and 19% (Tsuang et al., 1998). These estimates were all moderately higher than our parallel figures of 24%, 8% and 16%.
