Fig. 1 depicts one of two quadrivariate Cholesky models (Neale & Cardon, 1992; Loehlin, 1996) that were fitted to quantify the extent to which (a) the phenotypic covariation between MDD and AUD was explained by SR and CON, and (b) the genetic and environmental covariation between MDD and AUD was explained by additive genetic and unique environmental factors due to SR and CON. In this first model, factor A1 accounts for all of the genetic variation in SR (the variable entered first) and the genetic variation in CON (path a12), AUD (a13), and MDD (a14) that is explained by SR. Factor A2 then accounts for the residual genetic variation (i.e. after accounting for variation in SR) in CON (a22), and any residual genetic variation in AUD (a23) and MDD (a24) that is due to CON. Factor A3 accounts for any residual genetic variation (i.e. after accounting for variation in SR and CON) in AUD (a33), and any residual genetic variation in MDD (a34) due to AUD. Finally, factor A4 accounts for any genetic variation in the risk for MDD, unexplained
