Twin parents or biological co-parents completed the Child Behavior Checklist for Ages 6–18 (CBCL; Achenbach, 2001) reporting on up to three age-eligible offspring. The CBCL is a widely used parent report measure of child behavior problems, with well-established psychometrics (Achenbach & Rescorla, 2001; Achenbach, 1991). Parents reported whether a given behavior described his or her child's behavior within the previous six months using a likert rating of 0 = “not true”, 1 = “somewhat or sometimes true,” or 2 = “very true or often true” for each item. For offspring ages 19–24, parents provided a retrospective report of offspring behavior at age 18. Scores from internalizing and externalizing scales of the CBCL were analyzed. Internalizing consists of items from anxious/depressed, withdrawn/depressed, and somatic complaints subscales, with rule-breaking and aggressive behavior subscales comprising externalizing. Scores from a combined index of problem behavior, total problems, were also analyzed and include items from all subscales, including social problems, thought problems, and attention problems subscales. Given significant positive skew observed of raw CBCL scores, log-transformations of internalizing, externalizing, and total problems scales were analyzed.
