The ES was designed to extend the MTFS by oversampling 11-year-old twins likely to develop substance use disorders by virtue of being high on an index of externalizing behavior. Like the MTFS twins, ES twins were located through Minnesota state birth records (1988–1994). However, roughly half of the ES sample was selected after the mothers in those families completed a screening interview that established that at least one member of the twin pair was high on the externalizing dimension. The final ES sample included 300 MZ and 199 like-sex DZ twin pairs and their parents. The scheduling of intake and follow-up assessments of the ES twins parallels that for the younger MTFS cohort, with assessments targeted at ages 11, 14 (93.2% participation), 17 (in progress) and 20 (in progress). Among families eligible for participation in ES, 75.4% completed an intake assessment and participation rates did not vary by screening status. Differences in socioeconomic and demographic factors between participating and eligible but non-participating families were generally minimal (Keyes et al., 2009). In addition, 48 pairs of MZ twins aged 14–16 at
