Written informed assent/consent was obtained prior to data collection, and all study protocols were approved by an institutional review broad. The twins were invited to participate in follow-up assessments every 3–5 years: Cohort 1 was assessed at ages 11, 14, 17, 20, 24, and 29; Cohort 2 at ages 17, 20, 24, and 29; and Cohort 3 at ages 11, 14, 17, and 20. Retention rates were excellent, with approximately 90% of each sample retained across follow-up assessment. Bias due to attrition on the variables included in this study has been shown to be minimal.23–25
