and nicotine dependence. Importantly, these studies specified that a proportion of the genetic influences on nicotine dependence was in common with and contingent on exposure to nicotine use (39). When such a contingency was imposed on both cannabis and tobacco involvement, Neale and colleagues (40) found that, while there was considerable genetic overlap (RG=0.82) between cannabis and tobacco initiation, after accounting for this genetic overlap, the residual genetic covariance between cannabis and tobacco progression was modest (RG=0.38). In a sample of adolescent twins, after accounting for that initial genetic overlap, Huizink and colleagues (41) also found no further evidence for shared genetic influences on cannabis and tobacco progression. Both these studies concluded that shared genetic factors have the strongest influence at earlier stages of cannabis and tobacco use while genes specific to each drug are of particular importance at later stages.
