Objective:

To study genetic and environmental factors affecting body mass index (BMI) and BMI phenotypic correlations across adolescence. Design:

Prospective, population-based, twin cohort study. Participants and methods:

We used twin modeling in 2413 monozygotic and same-sex and opposite-sex dizygotic Finnish twin pairs born in 1983–1987 and assessed using self-report questionnaires at 11–12, 14 and 17 years of age. Results:

Heritability of BMI was estimated to be 0.58–0.69 among 11–12- and 14-year-old boys and girls, 0.83 among 17-year-old boys and 0.74 among 17-year-old girls. Common environmental effects shared by siblings were 0.15–0.24 among 11–12- and 14-year-old boys and girls but no longer discernible at 17 years of age. Unique environmental effects were 0.15–0.23. Additive genetic factors explained 90–96% of the BMI phenotypic correlations across adolescence, whereas unique environmental factors explained the rest. Common environment had no effect on BMI phenotypic correlations. Conclusions:

The genetic contribution to BMI is strong during adolescence, and it mainly explains BMI phenotypic correlations across adolescence. Common environmental factors have an effect on BMI during early adolescence, but that effect disappears by late adolescence.