Genetic influences play a role in adolescent alcohol, cigarette and marijuana use [13,14]. Studies into the aetiology of substance use and abuse that do not take genetic influences into account may present an incomplete picture. The twin method is based on comparisons of monozygotic (MZ) twins, who share 100% of their genes, with dizygotic (DZ) twins, who share on average 50% of their genes. In the basic (ACE) model, variation can arise from three sources: (1) additive genetic effects (a2), (2) common environmental effects that are shared by twins and make them more similar (c2) and (3) unique environmental effects that are experienced by only one twin in a pair (e2) [15].
