For the current study, data were used from an intensive subsample of FT12, consisting of 1) a sample randomly drawn from the 1983 cohort, with the exclusion of those living in the most remote rural areas of Finland (13%), 2) a random sample from the 1984–1987 cohorts (59%), and 3) all remaining twins from the 1984–1987 cohorts at elevated familial risk for alcohol problems based on parental reports of alcohol use (28%) [17]. Previously it was shown that the modest sample enrichment for familial alcohol use did not systematically affect the genetic and environmental variance components for various behavioural variables, including measures of drinking frequency, smoking initiation, and behaviour problems [17]. The intensive subsample included 1035 twin pairs who were invited to complete a full psychiatric face-to-face interview in early adolescence (age 14) and young adulthood (age 19–26). At age 14, 90% of the twins completed the assessment [17,22] and at age 19–26, 73% of the twins studied at age 14 were retained. Attrition was not selective on early adolescent conduct problems or substance use in early or mid-adolescence (all p-values >0.05).
