Introduction: The aim of this study was to investigate the relative influence of genetic and environmental factors on children's leisure time exercise behavior through the classic twin design.
Methods: Data were taken from The Netherlands Twin Register. The twins were 7 (n = 3966 subjects), 10 (n = 3562), and 12-yr-olds (n = 8687), with longitudinal data for 27% of the sample. Parents were asked to indicate the children's regular participation in leisure time exercise activities, including frequency and duration. Resemblance between monozygotic and dizygotic twins for weekly MET-hours spent on exercise activities was analyzed as a function of their genetic relatedness.
Results: Average weekly MET-hours increased with age for both boys (age 7 yr: 14.0 (SD = 11.8); age 10 yr: 22.6 (SD = 18.7); age 12 yr: 28.4 (SD = 24.9)) and girls (age 7 yr: 9.7 (SD = 9.5); age 10 yr: 15.3 (SD = 15.1); age 12 yr: 19.3 (SD = 19.8)). Around 13% of boys and girls across all age groups did not participate in any regular leisure time exercise activities. Tracking of exercise behavior from age 7 to 12 yr was modest (0.168 < r < 0.534). For boys, genetic effects accounted for 24% (confidence interval, 18%-30%) of the variance at age 7 yr, 66% (53%-81%) at age 10 yr, and 38% (32%-46%) at age 12 yr. For girls, this was 22% (15%-30%), 16% (9%-24%), and 36% (30%-43%), respectively. Environmental influences shared by children from the same family explained 71%, 25%, and 50% of the variance in boys (age 7, 10, and 12 yr) and 67%, 72%, and 53% in girls. The shared environment influencing exercise behavior was partially different between boys and girls.
Conclusion: Our results stress the important role of shared environment for exercise behavior in young children.