The proportion of genetic variance attributable to various sources was calculated using unstandardized path coefficients for the four phenotypes. (Coefficients for each pathway and the method for apportioning variance are reported in Figure 3, which is included in the appendix.) A total of 90.9% of genetic variance in alcohol consumption was attributable to a source common to all four phenotypes. For cannabis dependence symptoms, 37.5% of genetic variance was attributable to influences common to all four phenotypes, 5.5% to influences shared by alcohol consumption, cannabis dependence symptoms, and cannabis use (but not AD symptoms), and 57.0% to influences shared with cannabis use. For cannabis use, the genetic variance was distributed as follows: 38.6% to influences common to all four phenotypes, 7.5% to influences shared by alcohol consumption, cannabis dependence symptoms, and cannabis use (but not AD symptoms), 50.0% to influences shared by cannabis use and cannabis dependence symptoms (but not AD symptoms or alcohol consumption), and 3.9% to phenotype-specific influences.
