birth cohorts reached age 11, and most twins (89.4%) returned their questionnaires before the end of that year (mean age at response 11.4 years, SD = 0.3). We obtained teacher ratings during winter/spring of the year each cohort turned age 12, by requesting the classroom teacher for all twins enrolled into FT12 to complete a teacher version of the Multidimensional Peer Nomination Inventory (MPNI) (Pulkkinen et al., 1999). Ratings were completed by 93% of teachers. In Finnish culture, twin children are seldom placed in separated classrooms; in nearly 90% of the twin pairs in this study, both co-twins were in the same class and, therefore, were rated by the same teachers (Happonen et al., 2002). Finnish children do not yet change classes for different school subjects at this age; accordingly, the classroom teacher would teach the majority of the students’ classes. Additionally, it is common for classroom teachers to transition with their class of students across the elementary school years; accordingly, teachers generally know their students quite well. All twins were sent a follow-up questionnaire within 3 months of the date they reached age 14 (mean age at response 14.1 years, SD = 0.1). Of the 2,567 twin pairs who
