At age 14, externalizing behavior was measured using the Behavioral Problems scale from the Teacher Form of the Multidimensional Peer Nomination Inventory (Pulkkinen, Kaprio, & Rose, 1999). This scale tapped behavioral problems related to three factors: hyperactivity-impulsivity (seven items such as “is hyperactive” and “is talkative”); aggression (six items such as “goes around telling people’s secrets to others” and “hurts other kids when angry”); and inattention (four items such as “tends to ignore instructions” and “is conscientious with homework”). Teachers were asked how much each statement applied to each twin in their classroom on a four-point scale anchored 0 (does not apply) to 3 (applies in a pronounced way). Reliability for the scale was high (α = 0.95). Items were reverse scored as needed and averaged for the present analyses. Externalizing data were available for all participants in the subsample.
