Despite mounting evidence that both use of and dependence on alcohol and cannabis are influenced by heritable factors, there is relatively little information on the extent to which heritable influences on these phenotypes overlap across the two substances. Results from one such study, which examined use and problem use of alcohol, cannabis and cigarettes in an adolescent twin sample, revealed a genetic correlation of 0.62 between problem alcohol use and problem cannabis use (Young et al., 2006). An investigation of cannabis dependence symptoms and AD symptoms along with conduct disorder in an all-male adult twin sample identified a common genetic factor that accounted for 44.7% of variance in AD symptoms, but only 7.6% of variance in cannabis dependence symptoms (True et al., 1999). Another study using this same sample examined AD and cannabis dependence in combination with nicotine dependence and found that 42.4% of the variance in AD and 33.7% of the variance in cannabis dependence was attributable to a shared genetic factor (Xian et al., 2008). By contrast, when examined in combination with cocaine, caffeine, and nicotine dependence in
