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Aims To determine (1) the prospective associations of conduct problems during early adolescence with tobacco, alcohol and cannabis use in young adulthood and (2) to what extent these associations are due to overlapping genetic versus environmental influences. Design A prospective twin study using biometric twin modelling. Setting Finland. Participants A total of 1847 Finnish twins (943 males and 904 females) were interviewed in early adolescence, 73% of whom ( n = 1353, 640 males and 713 females) were retained in young adulthood. Measurements Symptom counts of conduct disorder (CD) criteria were obtained from a semi-structured clinical interview in early adolescence [age 14-15 years, mean = 14.2, standard deviation (SD) = 0.15]. Frequency of alcohol, tobacco and cannabis use was obtained from a semi-structured clinical interview in young adulthood (age 19.9-26.6 years, mean = 22.4, SD = 0.7). Findings We found modest to moderate phenotypical correlations ( r = 0.16-0.35) between early adolescent CD symptoms and substance use in young adulthood. In males, the phenotypical correlations of CD symptoms with all three substance use variables are explained largely by overlapping genetic influences. In females, overlapping shared environmental influences predominantly explain the phenotypical correlation between CD symptoms and tobacco and cannabis use. Conclusions Conduct disorder symptoms in early adolescence appear to moderately predict substance use in early adulthood. In males, genetic influences seem to be most important in explaining the relationship between conduct disorder symptoms and substance use whereas in females, shared environmental influences seem to be most important. [ABSTRACT FROM AUTHOR]

Aims To control for familial confounds, we studied the association between adolescent physical activity and later smoking in twin siblings discordant for their baseline physical activity. Design and measurements In this prospective population-based twin study, we asked whether persistent physical activity/inactivity in adolescence (assessed at 16, 17 and 18.5 years) predicted questionnaire-reported daily smoking at ages 22–27. Twins who, on the three baseline questionnaires, consistently reported frequent leisure physical activity (more than three times weekly) were classified as persistent exercisers, those who exercised less than three times monthly were called persistently inactive, others were occasional exercisers. Setting Finland. Participants A total of 4240 individuals, including 1870 twin pairs. Findings In analyses of individual twins, compared to persistent activity, persistent physical inactivity predicted increased risk of daily smoking (age- and sex-adjusted odds ratio 5.53, 95% confidence interval 3.88–7.88, P < 0.001). The risk remained elevated even after excluding all those who had smoked 50 cigarettes or more life-time at baseline and adjusted for educational level in adolescence. In within-pair analyses compared to the active members of discordant twin pairs, the physically inactive co-twins had increased risk of future daily smoking (sex-adjusted odds ratio 3.39, 95% confidence interval 1.56–7.39, P = 0.002). Conclusions Persistent physical inactivity in adolescence relates to adult smoking, even after familial factors are taken into account. [ABSTRACT FROM AUTHOR]