A number of correlated risk-factors were included as control variables in survival analyses. Control variables were selected based on review of the literature and for consistency with prior research [19,27]. Among family background risks, we examined maternal age at twins’ birth, maternal educational attainment, and offspring religious attendance during childhood. Dummy variables for twins’ birth occurring before mother’s age 20, between ages 20–24, and on/after age 35 were coded, with ages 25–34 comprising the reference group. Maternal education was coded from mother self-report and, if missing, twin report at Wave 5 (the only wave for which parental education was reported by twins). Dummy variables for mothers who dropped out of high school and those attending at least some college were computed, with reference to mothers completing high school only. Weekly or more religious attendance between the ages of 6–13 years was coded from twin self-report at Wave 4 and, if missing, parent report.
