Several recent studies have investigated the underlying aetiology of both initiation and progression to heavier/problem use of cigarettes, alcohol and marijuana in adolescence. Consistent with adult-based findings, most studies found a greater role for the common environment on initiation of substance use and greater genetic influence on heavier/problem use [13,14,19,20]. However, some studies reported greater genetic influences on initiation, dependent upon the substance investigated and gender (e.g. [13,14]). To date, twin studies of adolescent substance use have treated initiation and progression as independent constructs. Little is known about the underlying relationship between initiation and progression of substance use and the aetiology specific to progression in this age group.
