Familial Risk for Substance Use was operationalized as a dichotomous variable representing co-twin early use of the corresponding substance, which was derived from the categorization of the co-twin’s reported use/age at first use of the substance as early, average, late, or never. An interaction term between co-twin early use and zygosity, which is an indicator of the extent to which familial influences are attributable to genetic factors, was also included in analyses.
