The main purpose of this study was to investigate the relative influence of genetic and environmental factors on children’s participation in leisure time exercise activities. Average weekly MET hours spent on exercise activities in young Dutch twins doubled from age 7 to 12 but this was mainly due to those who were already active increasing their MET hours further. 13% of boys and girls of all ages were inactive in that they did not participate in any regular leisure time exercise activities. In accordance with previous findings, boys were more active than girls (e.g., 3
). For boys, additive genetic effects accounted for 23.7%, 65.7% and 38.3% of the variance in exercise behavior at ages 7, 10 and 12. For girls, this was 22.1%, 16.3% and 36.1%. Within all three age groups, shared environmental factors explained the largest part of the variance (70.5%, 24.6%, 50.1% for boys and 67.3%, 72.3%, 53.4% for girls).The correlation between shared environmental factors influencing exercise behavior in boys and girls (rcdos) was less than unity, suggesting that boys and girls in the same family do not receive the same level of familial support.
The important role of shared environmental factors for children’s regular exercise behavior is consistent with results of smaller-sized twin studies that focused on total physical activity rather than leisure time exercise activities (19
). Fisher et al. (19
) measured time spent in moderate and vigorous physical activity (MVPA) by accelerometry in a sample of 234 9–12 year old twins. Shared environmental factors accounted for 61% of the variance, with the remaining 39% being explained by unique environmental effects. No genetic influence was found. Franks et al. (20
) measured physical activity energy expenditure in 200 4–10 year old twins using respiratory gas exchange and doubly labeled water with very similar results (shared environment: 69%, unique environment: 31%). Plomin and Foch (27
) investigated one-week pedometer counts in a sample of 174 7.6-year old twins (SD= 1.6 years). Again, the shared environment was by far the most important contributor to physical activity (MZ correlation: .99, DZ correlation: .94).
As previously outlined, the shared environment is made up of all environmental factors that twins share. Thus, the strong shared environmental impact in the present study may be explained by factors such as the neighborhood and recreational environment, school and common friends. These factors may all be related to (accessibility of) exercise opportunities. However, as parents often act as gatekeepers to children’s leisure time activities (5
), parenting behavior may be one of the more prominent shared environmental influences on children’s exercise behavior. Their support of their children’s exercise behavior depends on their attitudes regarding these activities (2
) which may vary across families. In a recent review, Beets et al. (5
) identified four categories of parental influence on their children’s physical activity. Parents may or may not provide tangible support by organizing transportation to exercise location and pay for sport clubs and equipment (instrumental support
) and by being physically present during their children’s exercise activities or even coach/participate themselves (conditional support
). They may also provide intangible support to increase children’s self-efficacy and attitudes towards physical activity by encouragement and praise (motivational support
) or by providing advice, suggestions and information about (the benefits of) being active (informational support
). This theory predicts that parental influence on their children’s exercise activities should wane when the children get older and become less dependent on others for transportation, and less willing to imitate their parents’ behavior or adopt their attitudes (25
). The decrease in common environmental influences from age 7 to age 12 is entirely compatible with this prediction and continues during adolescence as has been shown by Van der Aa et al. (38
). The important role of tangible support is further supported by the finding that around two thirds of the twin pairs had at least one type of exercise activity in common (age7: 69.5%, age10: 65.9%, age12: 61.5%), which is much higher than could be expected based on the frequency of each of the types of exercise activities (approximately 20%). It is likely more convenient for parents to organize transportation and cheer their children at a single exercise location as opposed to handling two locations.
As the environmental correlation between DOS twin pairs was not unity for ages 7 and 12, (some of) the shared environmental influences must be qualitatively different for boys and girls. Given the parents’ influential role, a look at their differential treatment of sons and daughters with regard to exercise activities is warranted. Although the findings are not unanimous, boys tend to receive more parental support than girls (5
). In addition, mother-daugther and father-son correlations for physical activity are generally higher than opposite-sex correlations (21
) indicating a sex-specific influence of parents on their children. Accordingly, Edwardson and Gorely (16
) found a positive association between fathers’ explicit modeling and their son’s moderate-to-vigorous physical activity, but no association for girls. Anderson et al. (2
) reported that parents deemed boys’ participation in team sports to be more important than girls’ participation – for higher educated parents, this bias was also apparent for individual activities - and that boys are more similar to their parents with regard to the value placed on being active (“parent-child attitude congruence”). Parents may not only provide more support to their sons – they may also differ in the initial choice of which type of activity their sons and daughters should participate in. This may be a main reason why girls in our study engaged predominantly in individual exercise activities, whereas boys participated in all kinds of activities, including the more vigorous team sports (see and 25). Accordingly, the percentage of opposite-sex siblings that share at least one activity dropped to 46.5% which is clearly lower than that for same-sex siblings (75.0%).
The relative contribution of genetic factors was much larger in 10 year old boys compared to 7 and 12 year olds. This pattern was not seen in girls. As we used identical parental surveys, the difference cannot be attributed to a change in study methods. Also there are no major changes in the educational system at this age (high school starts at least 2 years later). One possible explanation is that most clubs, whether in team sports (e.g. soccer) or individual sports (e.g. tennis), increasingly start selecting for ability around this age. The amount of training is usually larger in the ‘first teams’ compared to the lower ranked teams. As exercise abilities are strongly heritable (10
), this may have boosted heritability of participation in these types of activities in boys, who may be more sensitive to their relative ranking among peers than girls. However, it is unclear why this effect has dissipated at age 12. Replication in larger samples is needed before drawing definitive conclusions.
After numerous studies using adolescent and adult twin data (e.g., 35
) this study is the first to investigate the relative contribution of genes and the environment on exercise behavior in children younger than 12 years old. Our findings fit the existing literature rather well as shown in which summarizes the results of all existing twin studies on leisure time exercise behavior. The figure includes the twin studies that were listed by Stubbe and De Geus (36
), extended with additional studies (12
) and the present one.
Figure 1 Summary of (previous) study results: Top panel shows the relative influence of genes (A), the shared environment (C), and the unique environment (E) -indicated as percentages - on leisure time exercise behavior across the lifespan for males, bottom panels (more ...)
From childhood onward, the heritability of exercise behavior increases to a peak during late adolescence and then decreases again to reach stable proportions in adulthood. The substantial impact of shared environmental influences is only found in children. Our group (13
) has hypothesized that the heritability of leisure time exercise behavior reflects three major sources: individual differences in a homeostatic need for activity, exercise ability, and acute psychological effects of exercise (also see 11
). Personality, itself a heritable trait, may be a fourth important determinant of stable individual differences in exercise participation (14
A limitation of the present study is the reliance on parental ratings of leisure time exercise behavior. Subjective ratings by the parents may tend to overestimate the actual exercise behavior of the children. However, the correlations between mothers’ and fathers’ ratings were high and the results were remarkably comparable to similar studies that used objective measurements of general physical activity, to which leisure time exercise activities make an important contribution (19
). Our use of a fixed list of the most common exercise activities performed by Dutch children probably helped to increase the reliability of parental reporting. It should be noted, however, that by focusing on these structured and well-defined exercise activities, we have ignored an important other contribution to children’s leisure-time physical activity, namely active play. Active play probably contributes to overall leisure time physical activity in different proportions across different age groups, with less opportunity for play in the 12-year olds once they enter high school. How this affects the heritability/environmentability of participation in regular structured exercise activities remains uncharted. A specific limitation of using twins, although in general the best design to estimate heritability, is that the findings may not generalize to families with siblings of different ages or a single child. As twins have the same age, it could be argued that the role of tangible support (a shared environmental factor) is greater, as it is more convenient for the parents to handle the twins as a pair, than would be for siblings with larger age differences. To balance these limitations this study had a very large sample size, estimated heritability in groups with a confined age range, and deliberately focused on participation in well-defined leisure time exercise activities, which are easier to assess in a standardized way than overall physical activity.
Our analyses confirmed the important role of shared environmental factors for children’s exercise behavior that gradually give way to genetic influences when they reach early adolescence. The shared family environment is likely to be an important target for the development of successful interventions on childhood exercise behavior, but family-based strategies may become less useful in adolescence.