Interpreting study findings
This systematic review and meta-analysis has updated the evidence base regarding the potential health benefits of volunteering. By removing adult age and language filters, trials and cohort studies deemed ineligible by earlier reviews [
11,
12] were included. Furthermore, volunteering interventions were systematically described and the impact on health outcomes of factors such as volunteering intensity and duration, and volunteers’ characteristics (e.g. age, gender) were summarised.
Heterogeneous findings were observed in the five trials [
30,
35,
37,
54,
58,
62] investigating the health effects of intergenerational volunteering among older adults, with benefits reported for some elements of physical activity and cognitive function. No significant effects were observed for depression, self-rated health or self-esteem. However, all studies recruited small samples that were likely to be underpowered to detect important between-group differences, and this was exacerbated by sample attrition.
Most cohort studies recruited large samples with lengthy follow-ups, thus being at low risk of bias. Meta-analysis of five studies [
9,
26,
38,
49,
50] identified a 22% reduction (CI: 10% to 34%) in mortality among volunteers compared to non-volunteers. Vote counting failed to identify any consistent beneficial effects of volunteering on either physical functional ability or self-rated health. For mental health, volunteering had a favourable effect on depression, life satisfaction and wellbeing. With the possible exception of wellbeing [
62], the limited trial evidence did not support findings from observational studies.
Conflicting results from studies exploring the influence of volunteering type and intensity on the magnitude of observed health benefits prevented any clear evidence being synthesised.
Several limitations should be acknowledged. While meta-analysis of survival data was undertaken, analysis of the remaining physical and mental health outcomes was restricted to vote counting [
23] due to heterogeneous trial interventions and study methods of both trial and observational studies. The generalisability of the evidence reviewed here is also limited. Indeed, most studies were based in the USA where there is a strong history of volunteering and a wide disparity in health, and involved samples of community dwelling people aged 50 years or over. The relevance of the current findings on a nation where health inequalities and volunteering are less prevalent may be questionable. Unfortunately, many studies based outside the USA reported cross-sectional data that were excluded at the study eligibility stage of the review. Reassuringly, the estimates of the prevalence of volunteering from observational studies is consistent with other sources [
3,
5], which found the prevalence of volunteering is generally higher in the USA compared with European cohorts, and that older people may be less likely to volunteer than their younger counterparts [
64].
A key challenge remains in unpacking the theoretical mechanisms by which volunteers accrue specific health benefits. This poses an interesting hypothesis that different health benefits are accrued in different and potentially antagonistic ways. For example, the tentative effect of volunteering on physical activity [
35,
53,
57,
58] may simply be explained by the increase in the number of trips out of the house, for whatever reason [
65,
66]. Here, in terms of dosage, more volunteering would have greater effects on physical activity and associated physical health outcomes. However, it emerges the opposite may be true for mental health; i.e. less volunteering may be more beneficial. Although people tend to volunteer for altruistic reasons [
9], if reciprocity is not experienced, then the positive impact of volunteering on quality of life is negated [
56,
63]. The importance of reciprocity was highlighted in the cross sectional analysis of the English Longitudinal Study of Ageing (ELSA); retired people who engaged in either paid work or volunteering experienced greater levels of wellbeing compared to those retirees who engaged in caring [
8]. Similar trends were found in employed and/or volunteering older caregivers (aged 60 years or above) who reported better self-rated health compared to those older caregivers who did neither activity [
67]. However, there may be a fine line between volunteering enough to experience mental health benefits (e.g. up to ten hours a month) and spending too much time volunteering so that it becomes another commitment [
31]. If volunteering becomes a burden, this may lead to ‘burnout’ and possibly giving up volunteering [
9,
54]. An individual’s life history also influences the impact of volunteering. The small number of observational studies that stratified analysis by age found that older people may be more likely to experience reduced functional dependency and fewer depressive symptoms through volunteering compared with their younger counterparts [
40,
42,
48], although one study found no such benefit [
45].
Another key challenge is to explain why volunteering has such a significant impact on survival given the lack of robust changes in physical and mental health outcomes. Selection effects driven by unknown confounders cannot be conclusively ruled out when interpreting the survival data from observational studies. Similarly reverse causality cannot be completely discounted as the volunteers are often from more affluent backgrounds and in better health than non-volunteers [
6,
8,
63]. To limit such effects, meta-analysis pooled mortality risk data after adjustment for baseline between-group differences in socio-demographic, economic, lifestyle and physical and mental health status. While such adjustments strongly mediated survival, a significant effect remained. Social integration also mediated the relationship between survival and volunteering status [
38,
50]. Since people reporting stronger social relationships have a reduced risk of mortality [
68], the social aspects of volunteering may contribute to the observed survival differences. Taken together, this review suggests that bio-social and cultural factors may influence both a willingness to engage in volunteering, as well as the benefits that might accrue.
This review aimed to identify evidence regarding the health benefits of formal volunteering undertaken on a sustained and regular basis. Although unproblematic when considering trial eligibility, many cohort studies failed to fully describe how volunteering status was defined or measured. This is unsurprising given the nature of these large, population cohort studies; volunteering is often only one of many social activities assessed. An inclusive approach was adopted to maximise the evidence available. Tighter study inclusion criteria would not only result in many observational studies being omitted from this analysis, but might substantially change the findings.
Implications for health inequalities, practice and research
The State of the World’s Volunteerism Report 2011 [
14], the Policy Agenda for Volunteering in Europe (PAVE) [
13], the CNCS Strategic Plan 2011–2015 [
7] and the UK government policy [
15] advocate the uptake of volunteering as a method of improving civic engagement, with the added potential of improving participants’ health and wellbeing [
16]. Alongside more traditional health promotion goals, such as reducing physical inactivity and excess weight in adults, the new Public Health Framework for England (2013–16) [
69] includes self-reported wellbeing and improving health-related quality of life for older people as indicators. In this review, the potential for advocating volunteering as a public health promotion intervention to improve physical and mental health outcomes was explored.
Many uncertainties remain that preclude clear recommendations for practice. For example, it is unclear what type or dose of volunteering activity is associated with the greatest health improvement, for which outcomes and for whom. While the underlying causal mechanisms cannot be explained due to the potential for reverse causation and selection bias, synthesis of observational data suggests that people who choose to volunteer are at a lower risk of mortality, and may experience some benefits in terms of physical and mental health. With the lack of experimental evidence, this could be interpreted as proof of no public health benefits arising through volunteering roles. However, given the methodological limitations of trial evidence (e.g. small selected sample sizes), it must equally be acknowledged that such evidence cannot conclusively rule out the potential for volunteering as a public health intervention. If it is accepted that volunteering may result in health benefits, perhaps the key challenge to practitioners is how to achieve wider participation amongst socially-disadvantaged groups [
7,
8,
35,
57] at the greatest risk of experiencing health inequalities. Socially-inclusive volunteering interventions, such as the Experience Corps Program [
35], require careful planning and partnership working with the voluntary sector, to ensure that barriers to participation for disadvantaged groups are identified and removed. While having the potential to be a low cost, sustainable intervention, service commissioners must recognise that the infrastructure required to improve community engagement is not cost free.