In summary, there is moderate quantitative evidence that children with siblings exhibit more MVPA than only children, with qualitative synthesis reporting mixed results between the groups on LPA and sedentary behaviour. In the meta-analysis, children with siblings had a slight overall increase in MVPA per day, an average five more minutes MVPA/day than only children. There may be a possible dose-response, with more siblings leading to more child MVPA and less sedentary behaviour. For each additional sibling, there were 3.13 more minutes of MVPA per day. In the longitudinal studies, there was no consistent direction on sibling’s influence on change in physical activity, and the sole intervention study hypothesised that siblings may influence the efficacy of their efforts to reduce SB. When evaluating demographics, children with siblings continued to have higher MVPA than did only children, regardless of sex or age. It may be important to evaluate the role of siblings in facilitating physical activity or sedentary behaviour in young children to ensure that all children are physically active.
Modes of physical activity
As a part of this investigation, the authors focused on objectively measured physical activity. However, as addressed above, physical activity can also be measured by sports participation, active commuting and qualitative analysis (interviews and focus groups). When comparing all included articles to studies that operationalised physical activity through methods other than accelerometer and pedometer, the authors found evidence that children with siblings had more MVPA and that siblings influence children’s sedentary opportunities. Articles retrieved on sports participation reiterated the sibling’s positive influence,59 60 with most signifying that sibling’s existing sports participation positively influences the child’s involvement in sports.13 61 62
China is a country with a larger one-child population. In studies from China, there are conflicting results between only children and children with siblings in sports participation63 64
5and sedentarybehaviour.65Authors note the priority of scholastic work, and the higher parental expectations of academic achievement for only children in China driving this difference.64A few articles mentioned that sibling coparticipation in sports and activity may breed rivalry or jealousy between the children66and forced interaction for the child,67accordingly creating negative feelings towards physical activity.
In qualitative studies, children identified siblings’ preferences for sedentary activities as a deterrent to being active68 or forgoing physical activity due to their incompatible skill levels in the activity69; noting that the sibling was not at the same developmental level as they were in sport, in most cases citing that the younger sibling was not at the same level. Parents also felt that older siblings who engage in more screen time have the potential to be a negative influence on the sedentary behaviours of younger siblings.61 Children with siblings participate in more active transportation70 71 and time in open space, such as parks,72 with few studies reporting no association with sibling status.73–75 Studies with no association in active transport theorised the age of the sibling as the explanation.73–75 The lack of association between siblings and active transport may relate to the barriers mentioned in qualitative analysis, that younger siblings may not have the same ability level and may be seen more as a burden than enabler of active transport. The sibling influence on physical activity methods, modalities of sports participation and active commuting confirms the increase in MVPA and reveals that sibling demographics and preferences may create the mixed results in sedentary behaviour.
Hypothesised mechanism of action
Across studies, it was hypothesised that siblings in the home steer children away from solitary pursuits, decrease sedentary behaviour and raise MVPA.43 45 47 50 This finding may support the sex differences, in which females with siblings shift their free time from sedentary pursuits to active pursuits in the presence of a sibling.52 54 Siblings may also facilitate activity through involvement in active transport40 and sport participation by serving as a facilitator and supervision during activity, which also decreases sedentary behaviour, especially in adolescents.42 There was no consensus about siblings’ influence in afterschool or weekend physical activity,49 51 54 indicating there is no single day or period that encompasses the sibling’s influence. In contrast, if the siblings prefer sedentary activities or are unmotivated to be active, this peer influence may encourage their siblings to participate in sedentary behaviour instead of MVPA.56
As for family structure and parenting practices, one-parent households may force a sibling to serve as a facilitator of physical activity while providing supervision.46 In single parent or dual working households, only children may also be encouraged to participate in sedentary activities, since these activities are easier to monitor and frequently considered safer than outdoor or active opportunities.45 46 Choosing these sedentary activities can appease short-term needs, but have long-term impacts on physical activity. Further, only children spend more time in childcare relative to children with siblings and that care may be provided by grandparents. Grandparents may not provide as many physical activity opportunities as a sibling due to the grandparent’s potential limited mobility and participation in activities together and may prefer television.45 Similarly, grandparents may be more likely to indulge in the child’s desires to engage in sedentary behaviour through TV watching. Family structure and dynamics may also be different across cultures and influence the role a sibling may play in caregiving, as evidenced by the difference in sibling influence on physical activity by ethnic groups in McMinn.44 The dose response in number of siblings with decreased sedentary behaviour and increased MVPA may be due to an increased likelihood of having a sibling with positive physical activity preferences.41 Instead of having one sibling to shape activity preferences, there are multiple children to guide activity preferences, create opportunities and provide supervision of such activities. Further, with more children in the household, there may be less restrictive parenting and a greater chance of siblings serving as caregivers, which may lead to more physical activity opportunities.
The strengths of the current study include the use of objective measures and comparison of intensities between groups. Objective measures help elucidate the influence of the sibling on movement throughout the day and activity intensities, instead of subjective or participation measures. The use of the meta-analytic approach allows a quantitative difference between groups to be determined and referenced to other physical activity standards. Subgroup analysis and dose-response analysis were conducted to determine the difference on subpopulations of young children to better evaluate the sibling influence. The investigation into the mechanism and complexity of sibling relationships aid in contextualising the entire sibling influence on physical activity. The results from this current meta-analysis may provide a partial explanation for higher obesity rates in only children compared with children with siblings.28 76 77 Last, this review was conducted using the PRISMA guidelines with a rigorous abstraction and assess risk of bias across all studies, which allows for reproducibility and assessment of study quality.
A limitation of this review is the heterogeneity in defining sibling status and physical activity; not all studies could be compared. Therefore, the meta-analysis may not include all potential estimates of siblings on physical activity. Further, the focus of this paper was on energy balance-related health behaviours and did not address the influence of siblings on underweight or malnutrition, which is explored in less developed nations.78 The authors acknowledge that these same behaviours may lead to other health outcomes in only children and children with siblings. The articles retrieved for the meta-analysis were all from developed countries (eg, Canada, USA and UK), where siblings may play a different role in family demands than they do in other developing counties. Further, non-significant results were not presented in most articles. Thus, we could not compare non-significant results in the quantitative portion of this analysis. In many cases, the study measured objective physical activity, but did not record the number of siblings. In visually reviewing the funnel plots, there was no evidence of publication bias. Finally, few studies addressed siblings as an antecedent to physical activity and leading to more physical activity.41 50 Instead, siblings were analysed as a consequence of physical activity or in posthoc analysis.
Future studies should examine the impact of siblings on children’s preferences and opportunities for physical activity. Verloigne noted that children with siblings were less likely to reduce their sedentary time in the intervention, since the children’s respective siblings were not motivated to change their sedentary time.56 Other studies have indicated that the sibling plays a major role in family life style changes for weight loss.79 Most family interventions solely include the parent80; consequently, in clinical practice it may be important to include the entire family for encouraging physical activity and reducing sedentary behaviours. Only children may need encouragement for peer physical activity through sports participation, since the at home peer influence may not be available. Parents of only children may also need to take a more active role in promoting and facilitating physical activity in the home. Last, it is important to consider other familial factors, such as family structure and grandparent involvement, when creating physical activity opportunities.
In conclusion, this systematic review and meta-analysis suggests that children with siblings have a healthier physical activity pattern with more physical activity and potentially less sedentary behaviour. We found a potential dose response, with more siblings leading to more MVPA, with no differences by age or sex of the child. Proposed mechanisms include siblings serving as peer models, encouraging sports participation and additional supervision in physical activity. Sibling involvement in physical activity opportunities may be mediated by the sibling’s age and established preference. Considering the influence of the sibling in preference and involvement may lead to more impactful physical activity changes. Overall, all children benefit from physical activity, and siblings may enable more physical activity during childhood.