Behavior problem data were collected from 1,411 same-sex twin pairs (357 female MZ, 343 male MZ, 337 female DZ, and 374 male DZ) at age 12 using the MPNI (Pulkkinen et al., 1999). The MPNI was developed during the course of the Finnish twin studies for use by teachers, parents, and peers as assessors of behavioral and emotional problems of the twins. This study used teacher reports of behavior problems. The behavior problems measure is the composite of 17 items comprising 3 subscales: aggression (6 items), hyperactivity–impulsivity (7 items), and inattention (4 items). Items were presented in the format of statements (e.g. “hurts other kids when they’re angry…”), and the teacher was asked to rate each item on a 4-point scale where 0 = does not apply, 1 = applies sometimes, but not consistently, 2 = certainly applies, but not in a pronounced way, and 3 = applies in a pronounced way. The scores on the 3 subscales are averaged to yield a composite behavior problems score. Scores ranged from 0 to 3 with a mean (M) of 0.88 and
