Assessment of educational achievement was based on teacher-reported school performance at the first two study waves and on self-reported information on ongoing and completed education at the last two waves. Specifically, when the twins were 12 and 14 years their teachers responded to questionnaire items on school performance, including grade point average (GPA) in the latest report. Finnish schools use a uniform grading system of 7 numbered grades (4 to 10, from failure to highest). Five ordered categories for GPA were used in the teachers’ questionnaire: (1) 9.0 or over, (2) 8.0 – 8.9, (3) 7.0 – 7.9, (4) 6.0 – 6.9, and (5) Below 6. For analyses, this scale was reversed so that higher values denoted better performance.
