We were primarily interested in examining the association between first substance use and problematic use, which is contingent on having used a substance. Thus, respondents who had never used a substance were coded as missing for both their age of initiation and the symptom count for that substance. Despite this, all abstainers contributed to the multivariate analyses. Multivariate models included all twin pairs where at least one twin reported having used at least one substance, as well as singletons who had used at least one substance. Maximum likelihood procedures were employed to account for missing data.
