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 a study of male and female adult twins, AD and cannabis dependence loaded on two different – but correlated (r = 0.82) – genetic factors, one encompassing the illicit substances, the other comprised of the licit substances (Kendler et al., 2007). Taken together, the literature suggests that there are some common genetic contributions to alcohol and cannabis-related phenotypes, but differences in samples and measurements of outcomes across studies do not allow for clear conclusions to be drawn about the magnitude of those overlapping influences. (Differences in the manifestation of genetic risk by developmental stage or gender, for example, could explain variations in findings, as might the shift in estimates of overlapping influences specific to alcohol and cannabis that can result from the addition of other phenotypes to genetic models).
