Twin studies represent an adequate method to test for common aetiology by determining the extent to which genetic and environmental factors are shared by two or more traits. For both conduct problems and substance use, the contribution of genetic and environmental factors has been demonstrated. With regard to conduct problems, moderate heritability has been shown, with reported estimates ranging from .31 to .61 [9–11]. Heritability estimates of substance use have been found to vary across stages of substance use involvement, with strongest genetic influences for heavier stages of substance use [12–13]. For persistent smoking and lifetime cannabis use, meta-analyses point towards moderate heritability, with estimates of .46 in females and .59 in males for persistent smoking [14] and of .40 in females and .48 in males for cannabis use [13]. For level of alcohol consumption, genetic factors also appear to have a moderate influence, with estimates around .50 [15–16].
