The history of this sample, including ascertainment and data collection, has been described in detail elsewhere (Meyer et al. 1996; Hewitt et al. 1997). In brief, the 1412 twin families (2824 children) who were interviewed and used for the subsequent analyses are members of a sequential cohort and were followed prospectively at approximately 15-month intervals over four waves of data collection (Maes et al. 2007). Twins and their parents were ascertained through the Virginia public and private school systems in 1987 and 1988. The first wave of data collection took place between March 1990 and March 1992 and twins in this cohort ranged in age from 8-17 years. As the study progressed, twins over the age of 17 were considered too old for inclusion and were aged out of the sample.
