Parental separation prior to twins’ age 18 was coded from parent and Waves 1, 3, 4, and 5 twin interviews. Consistent with previous work [16,19], parental separation was defined as change in marital and/or cohabitation status for reasons of relationship dissolution. Twins’ age at separation was coded from year parents’ marriage ended or, if missing, age last lived with both parents. Age at separation in families where parental separation occurred prior to twins’ first birthday was coded as a fraction of a year.
