Volunteering in the Community: Potential Benefits for Cognitive Aging

Volunteering in the Community: Potential Benefits for Cognitive Aging Abstract Objectives This review aims to advance understanding of the potential benefits of volunteering in the community for older adults’ cognitive functioning by taking an in-depth look at the relevant evidence to date. Method This review describes the main pathways through which volunteering could plausibly benefit cognitive functioning and critically examines research that has specifically investigated links between volunteering and cognition. Fifteen articles that assessed in adults aged ≥ 55 years the relationship between volunteering (predictor) and cognitive functioning (outcome) were identified via literature database searches. Results On balance, evidence from the small number of relevant studies to date supports the idea that volunteering can protect against cognitive aging with respect to global functioning and at least some specific cognitive domains. Studies that used robust designs and assessed domain-specific cognitive functioning produced the largest effect sizes. Discussion To help advance the field, this review puts forward recommendations for future research, with an emphasis on the need for robust study designs and specific investigations into the nature and extent of the cognitive benefits of volunteering. Through that work, researchers can determine how a simple and accessible activity like volunteering can best be used to help reduce the burden of age-related cognitive decline. Cognition, Executive function, Successful aging As the population ages and the health system burden posed by age-related cognitive decline and concomitant loss of independence increases (World Health Organization, 2015), there is a need to identify accessible, population-level interventions that could promote healthy brain aging. Based on the idea that keeping the brain active, engaging in physical activity, and remaining socially active can maximize the brain’s potential functioning (discussed in Hertzog, Kramer, Wilson, & Lindenberger, 2008), researchers have suggested that formal volunteering (termed here “volunteering”), defined as unpaid, noncompulsory work done through an organization and for the benefit of people outside the person’s household (International Labour Organization, 2011), could provide a simple and accessible means by which to improve older adults’ cognitive functioning (e.g., Anderson et al., 2014). The purpose of this article is to review the current state of the evidence relating to this hypothesis and to recommend directions for future research. Several recent reviews have covered the relatively large body of evidence relating to the benefits of volunteering for older adults’ physical health, mental health, and quality of life (e.g., Anderson et al., 2014; Cattan, Hogg, & Hardill, 2011; Jenkinson et al., 2013; von Bonsdorff & Rantanen, 2011). However, with the exception of a brief summary in Anderson et al. (2014), previous reviews have largely omitted discussion of potential cognitive benefits, presumably due to the dearth of relevant research. In light of a number of recent studies that measured cognitive outcomes, a review focused specifically on the impact of volunteering on cognition is now warranted to help advance this emerging field. This review first considers the plausibility of the hypothesis that volunteering can improve older adults’ cognitive functioning by describing, and summarizing the evidence for, the potential pathways through which such an effect could occur. It then critically reviews research that has directly examined the relationship between volunteering and performance on global or domain-specific cognitive tests, before concluding with a summary of the current state of the evidence and recommendations for future research. Is it Plausible that Volunteering can Improve Older Adults’ Cognitive Functioning? The main theoretical model for explaining how volunteering could lead to improvements in older adults’ health, including their cognitive functioning, began as a description of the effects of a specific intergenerational volunteer program (Fried et al., 2004). It was later adapted and extended to include other types of volunteering (Anderson et al., 2014). Fried et al.’s (2004) and Anderson et al.’s (2014) models are broad, and the specific details in each differ, but both are based on the hypothesis that volunteering first increases physical, social, and cognitive activity (to varying degrees, depending on the type of volunteering undertaken and associated demands), and in turn, those activity increases lead to improved health outcomes via a range of physiological and psychological mechanisms. Figure 1 shows a simplified version of these models. It focuses on cognitive functioning, rather than on other functional outcomes, and posits that increases in physical, social, and cognitive activity lead to improved cognitive functioning via improvements in mental and neurological health. The model treats mental and neurological health as distinct constructs, even though they of course overlap. Mental health is defined as psychological wellbeing, including concepts such as depression, anxiety, and self-efficacy. Neurological health is defined as the physical structure and physiological functioning of the brain. Note that Figure 1 shows only the main theoretical pathways through which volunteering could plausibly benefit cognitive functioning (i.e., intermediate physiological and psychological mechanisms are not shown), and depicts only the forward direction from volunteering to cognitive functioning. However, many of the effects are likely to be bidirectional, and the interactions between variables more complex than illustrated. Figure 1. View largeDownload slide Main theoretical pathways through which volunteering could improve cognitive functioning in older adulthood. Figure 1. View largeDownload slide Main theoretical pathways through which volunteering could improve cognitive functioning in older adulthood. There is evidence to support the hypothesis that volunteering increases older adults’ physical, social, and cognitive activity. Regarding physical activity levels, cross-sectional research has revealed a positive correlation with volunteering (Klinedinst & Resnick, 2014), and longitudinal studies show that physical activity levels increase more over time among volunteers than nonvolunteers (Pillemer, Fuller-Rowell, Reid, & Wells, 2010; Tan et al., 2009). Intervention studies show that engaging in a volunteering program leads to greater increases in physical activity relative to a control condition (Fried et al., 2004; Parisi et al., 2015; Tan, Xue, Li, Carlson, & Fried, 2006, but only in those with low baseline activity). Regarding social activity, intervention studies have shown that volunteering increases older adults’ social interactions (Fried et al., 2004; Parisi et al., 2015), although this was not the case in Parisi et al.’s (2015) more conservative intention-to-treat analyses that modeled outcomes for those lost to follow-up. Regarding cognitive activity, qualitative studies have found that participants in volunteering programs report learning new skills and knowledge through their participation (e.g., Chen, 2016). In addition, intervention studies have shown that volunteers were more likely than controls to maintain or increase their frequency of engagement in cognitive activities (Fried et al., 2004; Parisi et al., 2015; Sakurai et al., 2016). There is also evidence that physical (reviewed in Guiney & Machado, 2013), social (Bourassa, Memel, Woolverton, & Sbarra, 2017; Hertzog et al., 2008), and cognitive (reviewed in Hertzog et al., 2008) activity benefit older adults’ cognitive functioning. It could therefore be inferred that volunteering-driven increases in those activities might lead to improved cognition. While research has not yet fully explained the exact mechanisms through which these effects might occur, they are likely due to improvements in mental and neurological health. For example, in older adults, physical (Motl et al., 2005) and social (Hong, Hasche, & Bowland, 2009) activity are linked to reductions in depressive symptoms, and fewer depressive symptoms predict better cognitive functioning (Shimada et al., 2014). Older adults’ participation in other leisure activities that include social and cognitive but not physical components is also associated with better mental wellbeing (Lampinen, Heikkinen, Kauppinen, & Heikkinen, 2006). In terms of neurological health, which underlies effective cognitive functioning, higher physical activity levels have been linked to better cerebrovascular health (Bailey et al., 2013) and improvements in the structural integrity of the brain (reviewed in Guiney & Machado, 2013). Moreover, animal studies show that living in enriched environments involving engagement in physical, social, and cognitive activities promotes neurological health through processes such as neurogenesis, synaptogenesis, and angiogenesis (reviewed in Valenzuela, Breakspear, & Sachdev, 2007). In summary, although causal evidence has not yet been established for all pathways in Figure 1, the above findings provide sufficient evidence to support the idea that volunteering could plausibly bring about improvements in cognitive functioning through those pathways. Is there any Direct Evidence that Volunteering Benefits Cognitive Functioning? In this section, we examine studies that directly investigated links between volunteering and cognitive functioning in older adults (summarized in Table 1). To obtain relevant articles, we entered the search terms (volunteer* OR productive activit* OR social activit*) AND (cognit* OR brain) AND (older adults OR senior OR age* OR aging) into PubMed, MedLine, PsycInfo, and Web of Science. Further articles were obtained by searching the reference lists of eligible studies. We included all English language peer-reviewed articles published prior to June 2017 that assessed the relationship between volunteering (predictor) and cognitive functioning (outcome) or the impact of volunteering on cognition (via randomized control trials) in adults aged 55 years and over. In line with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Moher, Liberati, Tetzlaff, Altman, & PRISMA Group, 2009), Figure 2 depicts the process for identifying eligible articles and the number of articles excluded at each stage of the search process. In brief: eligible articles were identified by searching the titles and abstracts first, but if eligibility was not clear, the full text was examined. Articles were excluded after reviewing the full text if: (a) volunteering was not assessed (note that several abstracts made general reference to “social,” “productive,” or “leisure” activities, but it was not clear until reading the full text whether volunteering was one of those activities); (b) the specific relationship between volunteering and cognitive functioning could not be determined as only the combined effect of volunteering with other activities was reported; (c) cognitive functioning was used only as a covariate to determine the impact of volunteering on another outcome (e.g., depressive symptoms); (d) cognitive functioning was not assessed (one abstract referred to “cognition” but the full-text review indicated the definition to be thought processes rather than brain function); or (e) only past volunteering behavior was assessed (e.g., whether or not older adult participants had volunteered in mid-life). Fifteen articles (summarized in Table 1) were included in the final set. Table 1. Summary of Studies Examining Volunteering-Cognition Links in Older Adults Study  Design  Volunteering type  Volunteering status or amount  Follow-up duration  N (control n)  Age (years)  Outcome measure  Sig.  Effect size (subgroup)  Global cognitive functioning              Schwingel et al. (2009)  CS  Any  Current (yes/no)  None  2,716  55+  Cognitive performance (score)  *  d = 0.13  Shmotkin et al. (2003)  CS  Any  Current (yes/no)  None  1,343  75–94  Cognitive impairment (score)  *  d = -0.61  Okura et al. (2017)  CS  Any  Current (yes/no)  None  5,076  65+  Cognitive impairment (yes/no)  *  d = -0.10  Herrera et al. (2011)  CS  Any  Days/week  None  226  60+  Cognitive performance (score)  ns ns  β = 0.03 (Latino)β = -0.08 (Caucasian)  Lee et al. (2016)  CS  Any  Regular (yes/no)  None  2,401  65+  Cognitive impairment (yes/no)  *  d = -0.23  Schwingel et al. (2009)  LG  Any  Current (yes/no)  2 years  1,754  55+  Cognitive performance (score)  ns  -  Hsu et al. (2007)  LG  Any  Current (yes/no)  6 years  2,310  60+  Cognitive impairment (yes/no)  ns ns  d = -0.59 (women)d = -0.45 (men)  Kim et al. (2016)  LG  Any  Current (yes/no)  6 years  2,495  65+  Cognitive decline from baseline (yes/no)  ns  d = -0.12  Tomioka et al. (2016)  LG  Any  Current (yes/no)  3 years  6,083  65+  Cognitive decline from baseline (yes/no)  * ns  d = -0.23 (women) d = -0.03 (men)  Hughes et al. (2013)  LG  Any  Frequency categories  3 years  816a  65+  Severe cognitive impairment (yes/no)  *  -  Infurna et al. (2016)  LG  Any  Past year (yes/no)  14 years  13,262  60+  Cognitive impairment (yes/no)  *  d = -0.14  Griep et al. (2017)  LG  Any  Hours/week  5 years  1,001  65+  Cognitive complaints (score)Dementia medication (yes/no)  * *  r2 = 0.47 d = -0.50  George & Singer (2011)  RCT  Intergenerational  1 hr/fortnight  5 months  15 (7)b  M = 85.7  Cognitive performance (score)  ns  g = 0.57  Domain-specific cognitive functioning              Carlson et al. (2008)  RCT  Intergenerational  15 hr/week  4–8 months  128 (58)  60+  Task switching  †  -  Visual memory  †  -  Verbal memory  ns  -  38 (16)c  60+  Task switching  *  -  Visual memory  †  -  Verbal memory  *  -  Carlson et al. (2009)  RCT  Intergenerational  15 hr/week  4–8 months  17 (9)c  60+  Attentional control Flanker taskPFC activation  * *  d > 1 d > 1  Carlson et al. (2015)  RCT  Intergenerational  15 hr/week  2 years  111 (53)  60+  Cortical volume  ns *  - (women) d > 1 (men)  Hippocampal volume  ns *  d = 0.66 (women) d > 1 (men)  Study  Design  Volunteering type  Volunteering status or amount  Follow-up duration  N (control n)  Age (years)  Outcome measure  Sig.  Effect size (subgroup)  Global cognitive functioning              Schwingel et al. (2009)  CS  Any  Current (yes/no)  None  2,716  55+  Cognitive performance (score)  *  d = 0.13  Shmotkin et al. (2003)  CS  Any  Current (yes/no)  None  1,343  75–94  Cognitive impairment (score)  *  d = -0.61  Okura et al. (2017)  CS  Any  Current (yes/no)  None  5,076  65+  Cognitive impairment (yes/no)  *  d = -0.10  Herrera et al. (2011)  CS  Any  Days/week  None  226  60+  Cognitive performance (score)  ns ns  β = 0.03 (Latino)β = -0.08 (Caucasian)  Lee et al. (2016)  CS  Any  Regular (yes/no)  None  2,401  65+  Cognitive impairment (yes/no)  *  d = -0.23  Schwingel et al. (2009)  LG  Any  Current (yes/no)  2 years  1,754  55+  Cognitive performance (score)  ns  -  Hsu et al. (2007)  LG  Any  Current (yes/no)  6 years  2,310  60+  Cognitive impairment (yes/no)  ns ns  d = -0.59 (women)d = -0.45 (men)  Kim et al. (2016)  LG  Any  Current (yes/no)  6 years  2,495  65+  Cognitive decline from baseline (yes/no)  ns  d = -0.12  Tomioka et al. (2016)  LG  Any  Current (yes/no)  3 years  6,083  65+  Cognitive decline from baseline (yes/no)  * ns  d = -0.23 (women) d = -0.03 (men)  Hughes et al. (2013)  LG  Any  Frequency categories  3 years  816a  65+  Severe cognitive impairment (yes/no)  *  -  Infurna et al. (2016)  LG  Any  Past year (yes/no)  14 years  13,262  60+  Cognitive impairment (yes/no)  *  d = -0.14  Griep et al. (2017)  LG  Any  Hours/week  5 years  1,001  65+  Cognitive complaints (score)Dementia medication (yes/no)  * *  r2 = 0.47 d = -0.50  George & Singer (2011)  RCT  Intergenerational  1 hr/fortnight  5 months  15 (7)b  M = 85.7  Cognitive performance (score)  ns  g = 0.57  Domain-specific cognitive functioning              Carlson et al. (2008)  RCT  Intergenerational  15 hr/week  4–8 months  128 (58)  60+  Task switching  †  -  Visual memory  †  -  Verbal memory  ns  -  38 (16)c  60+  Task switching  *  -  Visual memory  †  -  Verbal memory  *  -  Carlson et al. (2009)  RCT  Intergenerational  15 hr/week  4–8 months  17 (9)c  60+  Attentional control Flanker taskPFC activation  * *  d > 1 d > 1  Carlson et al. (2015)  RCT  Intergenerational  15 hr/week  2 years  111 (53)  60+  Cortical volume  ns *  - (women) d > 1 (men)  Hippocampal volume  ns *  d = 0.66 (women) d > 1 (men)  Note: CS = Cross-sectional; LG = Longitudinal; PFC = prefrontal cortex; RCT = Randomized control trial; Sig. = Statistical significance; - = Insufficient information. * = significant (p < .05); † = marginally significant (p < .1); ns = not significant. Longitudinal and RCT sample sizes are those at follow-up. aAll had mild cognitive impairment at baseline; bAll had mild to moderate dementia; cSubset of full sample who had impaired Trail Making Test-B performance at baseline. View Large Figure 2. View largeDownload slide PRISMA flowchart depicting the article identification process. Figure 2. View largeDownload slide PRISMA flowchart depicting the article identification process. In the discussion below, the included articles are grouped according to whether the authors used measures of (a) global (n = 12) or (b) domain-specific (n = 3) cognitive functioning, as evidence suggests that some domains may be more sensitive to change than others (e.g., in relation to physical activity levels; reviewed in Guiney & Machado, 2013). Where possible, effect sizes are discussed in the text and presented in Table 1. These were either extracted directly from the cited article or converted from the published data to Cohen’s d using the Campbell Collaboration effect size calculator (Wilson, 2017). Global Cognitive Functioning Several studies have tested the idea that volunteering benefits older adults’ global cognitive functioning, as assessed by standardized cognitive impairment tests (e.g., the Mini-Mental State Examination [MMSE]; Folstein, Folstein, & McHugh, 1975). To date, five cross-sectional studies have investigated the association between volunteering participation and global cognitive performance, after controlling for other variables that could independently explain the relationship (e.g., age, education, socioeconomic status, physical health, and lifestyle factors). Three of those studies used large mixed-sex samples of older adults and found significant links between volunteering and cognition. Specifically, current volunteers scored better than nonvolunteers on global cognitive tests (Schwingel, Niti, Tang, & Ng, 2009; Shmotkin, Blumstein, & Modan, 2003), and had a lower likelihood of meeting criteria for cognitive impairment (Okura et al., 2017). Interestingly, Schwingel et al. (2009) found that age moderated the relationship between volunteering and cognition such that a significant link emerged only among those aged at least 62 years, possibly indicating that the benefits of volunteering for cognition increase with age. This contention is supported by the observation of a larger effect size in Shmotkin et al.’s (2003) study with the “oldest-old” (75–94-year-olds), but more research would be needed to confirm it. In two other cross-sectional studies, which included older women only, the results were inconsistent. In one study, there was no evidence for a relationship between volunteering frequency and global cognitive performance (Herrera et al., 2011). In the other, women who regularly engaged in volunteering activities were less likely than nonvolunteers to receive an MMSE score that indicated possible cognitive impairment (Lee et al., 2016). One explanation for this discrepancy is the considerably smaller sample size in the first study (see Table 1), although it could also be that the different methodological approach produced different effects. Whereas Herrera et al. (2011) utilized a self-complete postal survey, Lee et al. (2016) employed trained nurses to collect data face-to-face, which may have produced more accurate estimates of global cognitive functioning. Furthermore, while Herrera et al. (2011) assessed the linear relationship between volunteering frequency and cognitive test scores, all other cross-sectional studies assessed the effect of volunteer status (yes/no). It may be that doing any volunteering is more closely linked to global cognitive functioning than the specific amount or frequency, but more research would be needed to test this possibility. The majority of the cross-sectional studies to date have shown significant positive links between volunteering and measures of global cognitive functioning, but the effect sizes were mostly small. Moreover, the implications of these studies are limited by their cross-sectional nature. Despite the authors controlling for potentially confounding variables, there may be other important covariates that they did not take into account. It is also not possible to say that volunteering protected or improved older adults’ cognitive functioning. Indeed, the direction of causation may be the reverse, whereby those with better cognitive functioning are more likely to volunteer, but note that this argument goes against findings from longitudinal analyses showing that cognitive functioning at baseline did not predict later volunteering engagement (Cramm & Nieboer, 2015; Griep et al., 2017; Han & Hong, 2012). Longitudinal studies that follow older adult cohorts for several years and adjust for baseline differences between volunteers and nonvolunteers in cognitive functioning and other sociodemographic, health, and lifestyle factors partly address the limitations of cross-sectional data. To date, five longitudinal studies have assessed the relationship between baseline volunteering engagement and cognitive performance at follow-up without adjusting for changes in volunteering behavior in the intervening period. Three showed no statistically significant relationship between volunteering status (current volunteers vs nonvolunteers) and cognition, as indicated by mean MMSE scores (Schwingel et al., 2009), the likelihood of meeting the criteria for cognitive impairment (Hsu, 2007), or the likelihood of showing cognitive decline from baseline to follow up (Kim, Arai, & Kim, 2016). However, it should be noted that Schwingel et al. (2009) reported a weak positive trend (p = .15), and that Hsu et al.’s (2007) results indicated medium-sized effects. The other two studies showed small but significant prospective benefits of volunteering for cognition. In the first, current volunteers were less likely than nonvolunteers to show a decline in cognitive performance between baseline and follow-up, although the effect was significant in women only (Tomioka, Kurumatani, & Hosoi, 2016). In the second study, conducted with people who had mild cognitive impairment, participants who volunteered more frequently at baseline were less likely to progress to severe cognitive impairment over the course of the study (Hughes, Flatt, Fu, Chang, & Ganguli, 2013). However, this effect was not adjusted for covariates, as volunteering-cognition relationships were not the main topic of interest. Given that those who progressed to severe cognitive impairment were also older and had lower levels of education, it is possible that those factors explain the apparently protective effect of volunteering. Together, these longitudinal studies provide only limited support for the idea that volunteering benefits global cognitive functioning. Many were not statistically significant and the results typically indicated small effect sizes. However, these studies are limited by the assumption that volunteering participation at baseline remained consistent over the study duration. Evidence shows that older adults’ volunteering participation changes over time (Han & Hong, 2012; Infurna, Okun, & Grimm, 2016), and that the relationship between baseline volunteering engagement and cognitive functioning at follow-up weakens with longer intervals (Griep et al., 2017). Thus, more accurate assessments of volunteering-cognition relationships should be derived from those studies that take changes in volunteering engagement into account. Two longitudinal studies have taken into account changes in volunteering behavior over time (Griep et al., 2017; Infurna et al., 2016). In Infurna et al. (2016), a very large cohort of older adults with no evidence of cognitive impairment at baseline was followed-up via face-to-face interviews eight times over 14 years. Initial descriptive analyses showed that the more time points at which participants reported past-year volunteering, the lower their likelihood of meeting the criteria for cognitive impairment. For example, of those who reported no volunteering at baseline, the risk for future cognitive impairment was highest among those who continued to not volunteer, and lowest among those who reported volunteering at three or more subsequent time points. Of those who reported volunteering at baseline, the risk for future cognitive impairment was highest among those who did not report volunteering at any other time and lowest among those who reported volunteering at four or more subsequent time points. Statistical modeling that adjusted for confounding factors confirmed that maintenance of volunteering over time was associated with a small but significant reduction in risk for meeting cognitive impairment criteria over the study period. In Griep et al.’s (2017) study, a large sample of older adults was followed up three times over 5 years. Self-administered postal survey responses were used to determine sociodemographic characteristics, volunteering status, and cognitive complaints, while government administration data were used to determine whether participants had been prescribed medications to treat dementia. Among those who reported volunteering at all time points, number of hours spent volunteering each week at baseline was associated with fewer cognitive complaints at follow-up and a lower likelihood of receiving a prescription for dementia medications (both measures indicated medium effect sizes). No such relationship emerged among those who reported volunteering at some but not all time points, indicating that continuous volunteering was more beneficial than transient volunteering. Taken together, the Infurna et al. (2016) and Griep et al. (2017) studies provide good evidence that continued volunteering over time protects against future cognitive decline, although it could still be argued that other coinciding factors not accounted for, such as health-related behaviors, might explain the better cognitive outcomes in continuous volunteers. Longitudinal studies provide more robust evidence than cross-sectional ones, but only well-designed randomized control trials allow causal inference. Currently, there is only one small-sample randomized control trial that has examined the effect of a volunteering intervention on global cognitive functioning (George & Singer, 2011). In that study, older adults from an assisted living facility and with a diagnosis of mild to moderate dementia were randomly assigned to either an intergenerational volunteering program or an educational seminar group. Those in the intervention group visited local schools and engaged in reading, writing, and singing activities with the children, while those in the control group attended regular seminars about healthy aging. Mean MMSE scores declined from baseline to follow up in both groups, and there was no significant effect of the intervention on the magnitude of the decline. Although these findings suggest no benefit of volunteering for cognitive functioning in neurologically-impaired older adults, the notably small sample size would have severely limited the chances of detecting a significant effect—an argument supported by the observation of a medium effect size favoring the intervention group. Furthermore, attending the educational seminars may have increased cognitive activity in the control group, which would have reduced any relative increase in the volunteers. In sum, most cross-sectional studies to date show small but significant positive relationships between volunteering and global cognitive functioning. Whereas longitudinal studies that did not take into account changes in volunteering engagement between baseline and follow-up reported inconsistent and often small cognitive effects, those that did adjust for such changes showed larger, more robust effects. The only relevant randomized control trial to date indicated a medium-sized albeit nonsignificant effect of volunteering on global cognition, possibly indicating that it was underpowered. Thus, on balance, current evidence on the link between volunteering and global cognitive functioning appears promising, although it is clear that additional research is needed to addresses the limitations of existing studies. In particular, there is a dearth of sophisticated longitudinal studies that adjust for changes in volunteering engagement over time (as well as other covariates) and sufficiently-powered randomized control studies. Moreover, since the only randomized control trial to date assessed the impact of volunteering on institutionalized older adults with dementia, there is currently no intervention data relevant to nondemented, community-dwelling older adults. Finally, it should be noted that with the exception of three studies (Griep et al., 2017; Herrera et al., 2011; Hughes et al., 2013), all observational research to date used only generic measures of volunteering status (i.e., whether or not the person is a current volunteer or regular volunteer) and thus little is known about the potential impact of volunteering type, frequency, or duration on global cognition. Domain-specific Cognitive Functions Research shows that healthy aging adversely and disproportionately affects higher-order cognitive functions (e.g., Bierre, Lucas, Guiney, Cotter, & Machado, 2017; Murman, 2015), which are not selectively tapped by measures of global cognitive impairment. Thus, in this section, we review studies that utilized more specific cognitive tests to evaluate the impact of volunteering on particular cognitive domains known to decline with healthy aging. No cross-sectional or prospective cohort studies have assessed the association between volunteering participation and performance on domain-specific cognitive tests; however, three articles have reported on relevant data from two randomized control trials (Carlson et al., 2009; Carlson et al., 2015; Carlson et al., 2008). Both trials were designed to evaluate the impact of Experience Corps, which is an ongoing, intensive intergenerational program in which older adults volunteer in local schools for 15 hr each week to assist with children’s reading and literacy skills, library administration, and promoting prosocial behavior in the playground (for a detailed description of the program, see Glass et al., 2004). Carlson et al. (2008) reported on the impact of the first Experience Corps trial, in which participants completed at baseline and follow-up three tests that assessed specific cognitive constructs: (a) Trail Making Test-B, which is a paper-and-pencil instrument that taps task switching by requiring participants to efficiently switch between two sets of instructions in order to complete the test as quickly as possible (Reitan, 1958); (b) Rey-Osterreith Complex Figure Delayed Recall, which taps visual memory by requiring participants to reproduce a complex drawing they viewed 15 min earlier (Lezak, 1995); and (c) immediate and delayed word list recall, which tap verbal memory. At follow-up, the results of the main analysis showed nonsignificant trends for greater improvements in the intervention relative to the control group on Trail Making Test-B and Rey-Osterreith Complex Figure Delayed Recall. Specifically, the group by time interactions approached significance, in the context of no group differences at baseline and significant differences favoring the intervention group at follow-up. No significant effects were observed for either the immediate or delayed word recall tests. In a secondary analysis, for which the sample was restricted to those whose baseline Trail Making Test-B performance indicated impairment, significant intervention effects emerged for task switching and verbal memory, while a marginally-significant intervention effect emerged for visual memory. Although the article contained insufficient information to enable calculation of standardized effect sizes, the numerical effects in the secondary analysis were relatively large (e.g., for delayed word recall, a 40% improvement in the intervention group versus a 12% decline in controls). Carlson et al. (2009) also reported on the impact of the first Experience Corps trial on a subset of participants who had shown impaired functioning at baseline (as defined above) and who met inclusion criteria for undergoing functional magnetic resonance imaging (fMRI). In that study, fMRI scans were conducted while participants completed a flanker task, which requires attentional control as participants must attend to target stimuli while ignoring distractors (Machado, Guiney, & Struthers, 2013). The comparison of performance changes from baseline to follow up showed large and significant benefits of the intervention, evidenced by greater improvements in the intervention group relative to controls. In addition, during performance of the flanker task, the intervention group showed greater increases in activation in areas of the prefrontal cortex used for the control of attention. Thus, this substudy provided evidence that volunteering benefits an important cognitive function that is adversely impacted by adult aging (Machado, Devine, & Wyatt, 2009). Carlson et al. (2015) reported on the impact of the second Experience Corps trial, in which participants completed at baseline and follow-up a verbal memory test and underwent MRI scanning to assess hippocampal and cortical volume. The overall results showed a medium-sized but nonsignificant intervention effect on hippocampal and cortical volumes at the 2-year follow-up. Sex-stratified analyses revealed large, significant intervention effects on hippocampal and cortical volume in males, and a medium-sized, nonsignificant intervention effect on hippocampal volume in females (no effect size for cortical volume in women was reported). Results for baseline to follow-up change in verbal memory test performance for the intervention relative to the control group were not reported, but the authors did note that, in the intervention group only, change in memory performance from baseline to follow-up was moderately correlated with change in cortical volume (r = .39), irrespective of sex, which suggests that structural improvements supported better verbal memory. In sum, the limited current evidence indicates a positive impact of volunteering on specific aspects of older adults’ cognitive functioning, particularly attentional control. There were also some promising trends for task switching, verbal memory, and visual memory, as well as indications that the cognitive benefits of volunteering might be greater for those with impaired cognitive functioning (Carlson et al., 2008). Moreover, the evidence to date indicates that volunteering can have a positive impact on brain structure (particularly in males; Carlson et al., 2015) and functional activation in areas of the brain that support attentional control (Carlson et al., 2009). However, there are several limitations associated with these studies. First, they all assessed only one type of volunteering (an intergenerational program in schools), making it unclear whether the observed domain-specific cognitive benefits extend to other types of volunteering. Second, despite robust numerical trends, not all positive findings were statistically significant, which in at least some cases may reflect inadequate sample size. Third, women made up the majority of participants in all three studies, which could be important given that some studies have found gender differences in volunteering-cognition effects (e.g., Tomioka et al., 2016). Fourth, both Experience Corps trials used relatively inactive control groups, leaving open the possibility that the observed cognitive improvements were due to some other aspect of being in the intervention group (e.g., more interaction with program co-ordinators) rather than the volunteering itself. Finally, only a handful of the cognitive functions known to decline with aging have been assessed. Others, including inhibitory control over prepotent responses, response planning, problem-solving, working memory, and spatial memory, have yet to be investigated. Conclusions and Recommendations for Future Research In this review, we first presented evidence that it is at least plausible that volunteering can positively impact older adults’ cognitive functioning through increased physical, social, and cognitive activity, and consequent improvements in mental and neurological health. Second, we reviewed the studies to date that investigated whether volunteering in older adulthood benefits global and domain-specific aspects of cognitive functioning, many of which provided at least some evidence for positive effects of volunteering. Interestingly, studies that used robust designs (e.g., sophisticated longitudinal studies or randomized control trials) and those that assessed domain-specific cognitive functioning produced the largest effect sizes. In contrast, most of the observational studies assessing the relationship between volunteering and global cognitive functioning reported relatively small effects. Overall, the evidence is promising, but it is clear that more research is needed to advance the field beyond its current state of infancy. Based on the limitations of the existing evidence, there are several straightforward recommendations for future volunteering-cognition research. First, as conclusions from several studies are limited by the results not reaching significance despite positive trends, future studies need to take greater care to ensure adequate power to detect potential volunteering effects. Second, to allow causal inference, researchers should prioritize randomized control trials over other study designs, although longitudinal studies that take into account changes in volunteering over time as well as other covariates will also help to advance the field. Third, given that all current randomized control data relate to intergenerational volunteering, future trials should seek to test the effects of other specific types of volunteering, or of volunteering in general. Fourth, more studies assessing the effects of volunteering on domain-specific cognitive functions are needed. In terms of the specific research questions to be answered, there is a need for studies that test whether the cognitive benefits gained through volunteering (a) occur equally in healthy, at-risk, and clinical populations, given suggestions that those of more advanced age or with impaired functioning might benefit most (Carlson et al., 2008; Schwingel et al., 2009); (b) are domain-specific, global, or both; (c) occur irrespective of gender, in light of some gender-specific findings (Carlson et al., 2015; Tomioka et al., 2016); and (d) depend on the amount or type of volunteering undertaken, given that most studies to date utilized only broad measures of volunteering that do not take these factors into account. As more evidence emerges, researchers can begin to address wider questions, including testing the specific mechanisms through which volunteering might improve cognition and investigating whether volunteering-cognition effects are more pronounced in those with low initial levels of physical, social, and/or cognitive activity. By determining the ways in which volunteering benefits cognition and the extent of those benefits, researchers can elucidate how this simple and accessible activity can best be used to help slow age-related cognitive decline. Funding This research was supported by Brain Research New Zealand. Conflict of Interest None reported. References Anderson, N. D., Damianakis, T., Kröger, E., Wagner, L. M., Dawson, D. R., Binns, M. A.,… Cook, S. L.; BRAVO Team. ( 2014). The benefits associated with volunteering among seniors: A critical review and recommendations for future research. 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Volunteering in the Community: Potential Benefits for Cognitive Aging

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Abstract

Abstract Objectives This review aims to advance understanding of the potential benefits of volunteering in the community for older adults’ cognitive functioning by taking an in-depth look at the relevant evidence to date. Method This review describes the main pathways through which volunteering could plausibly benefit cognitive functioning and critically examines research that has specifically investigated links between volunteering and cognition. Fifteen articles that assessed in adults aged ≥ 55 years the relationship between volunteering (predictor) and cognitive functioning (outcome) were identified via literature database searches. Results On balance, evidence from the small number of relevant studies to date supports the idea that volunteering can protect against cognitive aging with respect to global functioning and at least some specific cognitive domains. Studies that used robust designs and assessed domain-specific cognitive functioning produced the largest effect sizes. Discussion To help advance the field, this review puts forward recommendations for future research, with an emphasis on the need for robust study designs and specific investigations into the nature and extent of the cognitive benefits of volunteering. Through that work, researchers can determine how a simple and accessible activity like volunteering can best be used to help reduce the burden of age-related cognitive decline. Cognition, Executive function, Successful aging As the population ages and the health system burden posed by age-related cognitive decline and concomitant loss of independence increases (World Health Organization, 2015), there is a need to identify accessible, population-level interventions that could promote healthy brain aging. Based on the idea that keeping the brain active, engaging in physical activity, and remaining socially active can maximize the brain’s potential functioning (discussed in Hertzog, Kramer, Wilson, & Lindenberger, 2008), researchers have suggested that formal volunteering (termed here “volunteering”), defined as unpaid, noncompulsory work done through an organization and for the benefit of people outside the person’s household (International Labour Organization, 2011), could provide a simple and accessible means by which to improve older adults’ cognitive functioning (e.g., Anderson et al., 2014). The purpose of this article is to review the current state of the evidence relating to this hypothesis and to recommend directions for future research. Several recent reviews have covered the relatively large body of evidence relating to the benefits of volunteering for older adults’ physical health, mental health, and quality of life (e.g., Anderson et al., 2014; Cattan, Hogg, & Hardill, 2011; Jenkinson et al., 2013; von Bonsdorff & Rantanen, 2011). However, with the exception of a brief summary in Anderson et al. (2014), previous reviews have largely omitted discussion of potential cognitive benefits, presumably due to the dearth of relevant research. In light of a number of recent studies that measured cognitive outcomes, a review focused specifically on the impact of volunteering on cognition is now warranted to help advance this emerging field. This review first considers the plausibility of the hypothesis that volunteering can improve older adults’ cognitive functioning by describing, and summarizing the evidence for, the potential pathways through which such an effect could occur. It then critically reviews research that has directly examined the relationship between volunteering and performance on global or domain-specific cognitive tests, before concluding with a summary of the current state of the evidence and recommendations for future research. Is it Plausible that Volunteering can Improve Older Adults’ Cognitive Functioning? The main theoretical model for explaining how volunteering could lead to improvements in older adults’ health, including their cognitive functioning, began as a description of the effects of a specific intergenerational volunteer program (Fried et al., 2004). It was later adapted and extended to include other types of volunteering (Anderson et al., 2014). Fried et al.’s (2004) and Anderson et al.’s (2014) models are broad, and the specific details in each differ, but both are based on the hypothesis that volunteering first increases physical, social, and cognitive activity (to varying degrees, depending on the type of volunteering undertaken and associated demands), and in turn, those activity increases lead to improved health outcomes via a range of physiological and psychological mechanisms. Figure 1 shows a simplified version of these models. It focuses on cognitive functioning, rather than on other functional outcomes, and posits that increases in physical, social, and cognitive activity lead to improved cognitive functioning via improvements in mental and neurological health. The model treats mental and neurological health as distinct constructs, even though they of course overlap. Mental health is defined as psychological wellbeing, including concepts such as depression, anxiety, and self-efficacy. Neurological health is defined as the physical structure and physiological functioning of the brain. Note that Figure 1 shows only the main theoretical pathways through which volunteering could plausibly benefit cognitive functioning (i.e., intermediate physiological and psychological mechanisms are not shown), and depicts only the forward direction from volunteering to cognitive functioning. However, many of the effects are likely to be bidirectional, and the interactions between variables more complex than illustrated. Figure 1. View largeDownload slide Main theoretical pathways through which volunteering could improve cognitive functioning in older adulthood. Figure 1. View largeDownload slide Main theoretical pathways through which volunteering could improve cognitive functioning in older adulthood. There is evidence to support the hypothesis that volunteering increases older adults’ physical, social, and cognitive activity. Regarding physical activity levels, cross-sectional research has revealed a positive correlation with volunteering (Klinedinst & Resnick, 2014), and longitudinal studies show that physical activity levels increase more over time among volunteers than nonvolunteers (Pillemer, Fuller-Rowell, Reid, & Wells, 2010; Tan et al., 2009). Intervention studies show that engaging in a volunteering program leads to greater increases in physical activity relative to a control condition (Fried et al., 2004; Parisi et al., 2015; Tan, Xue, Li, Carlson, & Fried, 2006, but only in those with low baseline activity). Regarding social activity, intervention studies have shown that volunteering increases older adults’ social interactions (Fried et al., 2004; Parisi et al., 2015), although this was not the case in Parisi et al.’s (2015) more conservative intention-to-treat analyses that modeled outcomes for those lost to follow-up. Regarding cognitive activity, qualitative studies have found that participants in volunteering programs report learning new skills and knowledge through their participation (e.g., Chen, 2016). In addition, intervention studies have shown that volunteers were more likely than controls to maintain or increase their frequency of engagement in cognitive activities (Fried et al., 2004; Parisi et al., 2015; Sakurai et al., 2016). There is also evidence that physical (reviewed in Guiney & Machado, 2013), social (Bourassa, Memel, Woolverton, & Sbarra, 2017; Hertzog et al., 2008), and cognitive (reviewed in Hertzog et al., 2008) activity benefit older adults’ cognitive functioning. It could therefore be inferred that volunteering-driven increases in those activities might lead to improved cognition. While research has not yet fully explained the exact mechanisms through which these effects might occur, they are likely due to improvements in mental and neurological health. For example, in older adults, physical (Motl et al., 2005) and social (Hong, Hasche, & Bowland, 2009) activity are linked to reductions in depressive symptoms, and fewer depressive symptoms predict better cognitive functioning (Shimada et al., 2014). Older adults’ participation in other leisure activities that include social and cognitive but not physical components is also associated with better mental wellbeing (Lampinen, Heikkinen, Kauppinen, & Heikkinen, 2006). In terms of neurological health, which underlies effective cognitive functioning, higher physical activity levels have been linked to better cerebrovascular health (Bailey et al., 2013) and improvements in the structural integrity of the brain (reviewed in Guiney & Machado, 2013). Moreover, animal studies show that living in enriched environments involving engagement in physical, social, and cognitive activities promotes neurological health through processes such as neurogenesis, synaptogenesis, and angiogenesis (reviewed in Valenzuela, Breakspear, & Sachdev, 2007). In summary, although causal evidence has not yet been established for all pathways in Figure 1, the above findings provide sufficient evidence to support the idea that volunteering could plausibly bring about improvements in cognitive functioning through those pathways. Is there any Direct Evidence that Volunteering Benefits Cognitive Functioning? In this section, we examine studies that directly investigated links between volunteering and cognitive functioning in older adults (summarized in Table 1). To obtain relevant articles, we entered the search terms (volunteer* OR productive activit* OR social activit*) AND (cognit* OR brain) AND (older adults OR senior OR age* OR aging) into PubMed, MedLine, PsycInfo, and Web of Science. Further articles were obtained by searching the reference lists of eligible studies. We included all English language peer-reviewed articles published prior to June 2017 that assessed the relationship between volunteering (predictor) and cognitive functioning (outcome) or the impact of volunteering on cognition (via randomized control trials) in adults aged 55 years and over. In line with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Moher, Liberati, Tetzlaff, Altman, & PRISMA Group, 2009), Figure 2 depicts the process for identifying eligible articles and the number of articles excluded at each stage of the search process. In brief: eligible articles were identified by searching the titles and abstracts first, but if eligibility was not clear, the full text was examined. Articles were excluded after reviewing the full text if: (a) volunteering was not assessed (note that several abstracts made general reference to “social,” “productive,” or “leisure” activities, but it was not clear until reading the full text whether volunteering was one of those activities); (b) the specific relationship between volunteering and cognitive functioning could not be determined as only the combined effect of volunteering with other activities was reported; (c) cognitive functioning was used only as a covariate to determine the impact of volunteering on another outcome (e.g., depressive symptoms); (d) cognitive functioning was not assessed (one abstract referred to “cognition” but the full-text review indicated the definition to be thought processes rather than brain function); or (e) only past volunteering behavior was assessed (e.g., whether or not older adult participants had volunteered in mid-life). Fifteen articles (summarized in Table 1) were included in the final set. Table 1. Summary of Studies Examining Volunteering-Cognition Links in Older Adults Study  Design  Volunteering type  Volunteering status or amount  Follow-up duration  N (control n)  Age (years)  Outcome measure  Sig.  Effect size (subgroup)  Global cognitive functioning              Schwingel et al. (2009)  CS  Any  Current (yes/no)  None  2,716  55+  Cognitive performance (score)  *  d = 0.13  Shmotkin et al. (2003)  CS  Any  Current (yes/no)  None  1,343  75–94  Cognitive impairment (score)  *  d = -0.61  Okura et al. (2017)  CS  Any  Current (yes/no)  None  5,076  65+  Cognitive impairment (yes/no)  *  d = -0.10  Herrera et al. (2011)  CS  Any  Days/week  None  226  60+  Cognitive performance (score)  ns ns  β = 0.03 (Latino)β = -0.08 (Caucasian)  Lee et al. (2016)  CS  Any  Regular (yes/no)  None  2,401  65+  Cognitive impairment (yes/no)  *  d = -0.23  Schwingel et al. (2009)  LG  Any  Current (yes/no)  2 years  1,754  55+  Cognitive performance (score)  ns  -  Hsu et al. (2007)  LG  Any  Current (yes/no)  6 years  2,310  60+  Cognitive impairment (yes/no)  ns ns  d = -0.59 (women)d = -0.45 (men)  Kim et al. (2016)  LG  Any  Current (yes/no)  6 years  2,495  65+  Cognitive decline from baseline (yes/no)  ns  d = -0.12  Tomioka et al. (2016)  LG  Any  Current (yes/no)  3 years  6,083  65+  Cognitive decline from baseline (yes/no)  * ns  d = -0.23 (women) d = -0.03 (men)  Hughes et al. (2013)  LG  Any  Frequency categories  3 years  816a  65+  Severe cognitive impairment (yes/no)  *  -  Infurna et al. (2016)  LG  Any  Past year (yes/no)  14 years  13,262  60+  Cognitive impairment (yes/no)  *  d = -0.14  Griep et al. (2017)  LG  Any  Hours/week  5 years  1,001  65+  Cognitive complaints (score)Dementia medication (yes/no)  * *  r2 = 0.47 d = -0.50  George & Singer (2011)  RCT  Intergenerational  1 hr/fortnight  5 months  15 (7)b  M = 85.7  Cognitive performance (score)  ns  g = 0.57  Domain-specific cognitive functioning              Carlson et al. (2008)  RCT  Intergenerational  15 hr/week  4–8 months  128 (58)  60+  Task switching  †  -  Visual memory  †  -  Verbal memory  ns  -  38 (16)c  60+  Task switching  *  -  Visual memory  †  -  Verbal memory  *  -  Carlson et al. (2009)  RCT  Intergenerational  15 hr/week  4–8 months  17 (9)c  60+  Attentional control Flanker taskPFC activation  * *  d > 1 d > 1  Carlson et al. (2015)  RCT  Intergenerational  15 hr/week  2 years  111 (53)  60+  Cortical volume  ns *  - (women) d > 1 (men)  Hippocampal volume  ns *  d = 0.66 (women) d > 1 (men)  Study  Design  Volunteering type  Volunteering status or amount  Follow-up duration  N (control n)  Age (years)  Outcome measure  Sig.  Effect size (subgroup)  Global cognitive functioning              Schwingel et al. (2009)  CS  Any  Current (yes/no)  None  2,716  55+  Cognitive performance (score)  *  d = 0.13  Shmotkin et al. (2003)  CS  Any  Current (yes/no)  None  1,343  75–94  Cognitive impairment (score)  *  d = -0.61  Okura et al. (2017)  CS  Any  Current (yes/no)  None  5,076  65+  Cognitive impairment (yes/no)  *  d = -0.10  Herrera et al. (2011)  CS  Any  Days/week  None  226  60+  Cognitive performance (score)  ns ns  β = 0.03 (Latino)β = -0.08 (Caucasian)  Lee et al. (2016)  CS  Any  Regular (yes/no)  None  2,401  65+  Cognitive impairment (yes/no)  *  d = -0.23  Schwingel et al. (2009)  LG  Any  Current (yes/no)  2 years  1,754  55+  Cognitive performance (score)  ns  -  Hsu et al. (2007)  LG  Any  Current (yes/no)  6 years  2,310  60+  Cognitive impairment (yes/no)  ns ns  d = -0.59 (women)d = -0.45 (men)  Kim et al. (2016)  LG  Any  Current (yes/no)  6 years  2,495  65+  Cognitive decline from baseline (yes/no)  ns  d = -0.12  Tomioka et al. (2016)  LG  Any  Current (yes/no)  3 years  6,083  65+  Cognitive decline from baseline (yes/no)  * ns  d = -0.23 (women) d = -0.03 (men)  Hughes et al. (2013)  LG  Any  Frequency categories  3 years  816a  65+  Severe cognitive impairment (yes/no)  *  -  Infurna et al. (2016)  LG  Any  Past year (yes/no)  14 years  13,262  60+  Cognitive impairment (yes/no)  *  d = -0.14  Griep et al. (2017)  LG  Any  Hours/week  5 years  1,001  65+  Cognitive complaints (score)Dementia medication (yes/no)  * *  r2 = 0.47 d = -0.50  George & Singer (2011)  RCT  Intergenerational  1 hr/fortnight  5 months  15 (7)b  M = 85.7  Cognitive performance (score)  ns  g = 0.57  Domain-specific cognitive functioning              Carlson et al. (2008)  RCT  Intergenerational  15 hr/week  4–8 months  128 (58)  60+  Task switching  †  -  Visual memory  †  -  Verbal memory  ns  -  38 (16)c  60+  Task switching  *  -  Visual memory  †  -  Verbal memory  *  -  Carlson et al. (2009)  RCT  Intergenerational  15 hr/week  4–8 months  17 (9)c  60+  Attentional control Flanker taskPFC activation  * *  d > 1 d > 1  Carlson et al. (2015)  RCT  Intergenerational  15 hr/week  2 years  111 (53)  60+  Cortical volume  ns *  - (women) d > 1 (men)  Hippocampal volume  ns *  d = 0.66 (women) d > 1 (men)  Note: CS = Cross-sectional; LG = Longitudinal; PFC = prefrontal cortex; RCT = Randomized control trial; Sig. = Statistical significance; - = Insufficient information. * = significant (p < .05); † = marginally significant (p < .1); ns = not significant. Longitudinal and RCT sample sizes are those at follow-up. aAll had mild cognitive impairment at baseline; bAll had mild to moderate dementia; cSubset of full sample who had impaired Trail Making Test-B performance at baseline. View Large Figure 2. View largeDownload slide PRISMA flowchart depicting the article identification process. Figure 2. View largeDownload slide PRISMA flowchart depicting the article identification process. In the discussion below, the included articles are grouped according to whether the authors used measures of (a) global (n = 12) or (b) domain-specific (n = 3) cognitive functioning, as evidence suggests that some domains may be more sensitive to change than others (e.g., in relation to physical activity levels; reviewed in Guiney & Machado, 2013). Where possible, effect sizes are discussed in the text and presented in Table 1. These were either extracted directly from the cited article or converted from the published data to Cohen’s d using the Campbell Collaboration effect size calculator (Wilson, 2017). Global Cognitive Functioning Several studies have tested the idea that volunteering benefits older adults’ global cognitive functioning, as assessed by standardized cognitive impairment tests (e.g., the Mini-Mental State Examination [MMSE]; Folstein, Folstein, & McHugh, 1975). To date, five cross-sectional studies have investigated the association between volunteering participation and global cognitive performance, after controlling for other variables that could independently explain the relationship (e.g., age, education, socioeconomic status, physical health, and lifestyle factors). Three of those studies used large mixed-sex samples of older adults and found significant links between volunteering and cognition. Specifically, current volunteers scored better than nonvolunteers on global cognitive tests (Schwingel, Niti, Tang, & Ng, 2009; Shmotkin, Blumstein, & Modan, 2003), and had a lower likelihood of meeting criteria for cognitive impairment (Okura et al., 2017). Interestingly, Schwingel et al. (2009) found that age moderated the relationship between volunteering and cognition such that a significant link emerged only among those aged at least 62 years, possibly indicating that the benefits of volunteering for cognition increase with age. This contention is supported by the observation of a larger effect size in Shmotkin et al.’s (2003) study with the “oldest-old” (75–94-year-olds), but more research would be needed to confirm it. In two other cross-sectional studies, which included older women only, the results were inconsistent. In one study, there was no evidence for a relationship between volunteering frequency and global cognitive performance (Herrera et al., 2011). In the other, women who regularly engaged in volunteering activities were less likely than nonvolunteers to receive an MMSE score that indicated possible cognitive impairment (Lee et al., 2016). One explanation for this discrepancy is the considerably smaller sample size in the first study (see Table 1), although it could also be that the different methodological approach produced different effects. Whereas Herrera et al. (2011) utilized a self-complete postal survey, Lee et al. (2016) employed trained nurses to collect data face-to-face, which may have produced more accurate estimates of global cognitive functioning. Furthermore, while Herrera et al. (2011) assessed the linear relationship between volunteering frequency and cognitive test scores, all other cross-sectional studies assessed the effect of volunteer status (yes/no). It may be that doing any volunteering is more closely linked to global cognitive functioning than the specific amount or frequency, but more research would be needed to test this possibility. The majority of the cross-sectional studies to date have shown significant positive links between volunteering and measures of global cognitive functioning, but the effect sizes were mostly small. Moreover, the implications of these studies are limited by their cross-sectional nature. Despite the authors controlling for potentially confounding variables, there may be other important covariates that they did not take into account. It is also not possible to say that volunteering protected or improved older adults’ cognitive functioning. Indeed, the direction of causation may be the reverse, whereby those with better cognitive functioning are more likely to volunteer, but note that this argument goes against findings from longitudinal analyses showing that cognitive functioning at baseline did not predict later volunteering engagement (Cramm & Nieboer, 2015; Griep et al., 2017; Han & Hong, 2012). Longitudinal studies that follow older adult cohorts for several years and adjust for baseline differences between volunteers and nonvolunteers in cognitive functioning and other sociodemographic, health, and lifestyle factors partly address the limitations of cross-sectional data. To date, five longitudinal studies have assessed the relationship between baseline volunteering engagement and cognitive performance at follow-up without adjusting for changes in volunteering behavior in the intervening period. Three showed no statistically significant relationship between volunteering status (current volunteers vs nonvolunteers) and cognition, as indicated by mean MMSE scores (Schwingel et al., 2009), the likelihood of meeting the criteria for cognitive impairment (Hsu, 2007), or the likelihood of showing cognitive decline from baseline to follow up (Kim, Arai, & Kim, 2016). However, it should be noted that Schwingel et al. (2009) reported a weak positive trend (p = .15), and that Hsu et al.’s (2007) results indicated medium-sized effects. The other two studies showed small but significant prospective benefits of volunteering for cognition. In the first, current volunteers were less likely than nonvolunteers to show a decline in cognitive performance between baseline and follow-up, although the effect was significant in women only (Tomioka, Kurumatani, & Hosoi, 2016). In the second study, conducted with people who had mild cognitive impairment, participants who volunteered more frequently at baseline were less likely to progress to severe cognitive impairment over the course of the study (Hughes, Flatt, Fu, Chang, & Ganguli, 2013). However, this effect was not adjusted for covariates, as volunteering-cognition relationships were not the main topic of interest. Given that those who progressed to severe cognitive impairment were also older and had lower levels of education, it is possible that those factors explain the apparently protective effect of volunteering. Together, these longitudinal studies provide only limited support for the idea that volunteering benefits global cognitive functioning. Many were not statistically significant and the results typically indicated small effect sizes. However, these studies are limited by the assumption that volunteering participation at baseline remained consistent over the study duration. Evidence shows that older adults’ volunteering participation changes over time (Han & Hong, 2012; Infurna, Okun, & Grimm, 2016), and that the relationship between baseline volunteering engagement and cognitive functioning at follow-up weakens with longer intervals (Griep et al., 2017). Thus, more accurate assessments of volunteering-cognition relationships should be derived from those studies that take changes in volunteering engagement into account. Two longitudinal studies have taken into account changes in volunteering behavior over time (Griep et al., 2017; Infurna et al., 2016). In Infurna et al. (2016), a very large cohort of older adults with no evidence of cognitive impairment at baseline was followed-up via face-to-face interviews eight times over 14 years. Initial descriptive analyses showed that the more time points at which participants reported past-year volunteering, the lower their likelihood of meeting the criteria for cognitive impairment. For example, of those who reported no volunteering at baseline, the risk for future cognitive impairment was highest among those who continued to not volunteer, and lowest among those who reported volunteering at three or more subsequent time points. Of those who reported volunteering at baseline, the risk for future cognitive impairment was highest among those who did not report volunteering at any other time and lowest among those who reported volunteering at four or more subsequent time points. Statistical modeling that adjusted for confounding factors confirmed that maintenance of volunteering over time was associated with a small but significant reduction in risk for meeting cognitive impairment criteria over the study period. In Griep et al.’s (2017) study, a large sample of older adults was followed up three times over 5 years. Self-administered postal survey responses were used to determine sociodemographic characteristics, volunteering status, and cognitive complaints, while government administration data were used to determine whether participants had been prescribed medications to treat dementia. Among those who reported volunteering at all time points, number of hours spent volunteering each week at baseline was associated with fewer cognitive complaints at follow-up and a lower likelihood of receiving a prescription for dementia medications (both measures indicated medium effect sizes). No such relationship emerged among those who reported volunteering at some but not all time points, indicating that continuous volunteering was more beneficial than transient volunteering. Taken together, the Infurna et al. (2016) and Griep et al. (2017) studies provide good evidence that continued volunteering over time protects against future cognitive decline, although it could still be argued that other coinciding factors not accounted for, such as health-related behaviors, might explain the better cognitive outcomes in continuous volunteers. Longitudinal studies provide more robust evidence than cross-sectional ones, but only well-designed randomized control trials allow causal inference. Currently, there is only one small-sample randomized control trial that has examined the effect of a volunteering intervention on global cognitive functioning (George & Singer, 2011). In that study, older adults from an assisted living facility and with a diagnosis of mild to moderate dementia were randomly assigned to either an intergenerational volunteering program or an educational seminar group. Those in the intervention group visited local schools and engaged in reading, writing, and singing activities with the children, while those in the control group attended regular seminars about healthy aging. Mean MMSE scores declined from baseline to follow up in both groups, and there was no significant effect of the intervention on the magnitude of the decline. Although these findings suggest no benefit of volunteering for cognitive functioning in neurologically-impaired older adults, the notably small sample size would have severely limited the chances of detecting a significant effect—an argument supported by the observation of a medium effect size favoring the intervention group. Furthermore, attending the educational seminars may have increased cognitive activity in the control group, which would have reduced any relative increase in the volunteers. In sum, most cross-sectional studies to date show small but significant positive relationships between volunteering and global cognitive functioning. Whereas longitudinal studies that did not take into account changes in volunteering engagement between baseline and follow-up reported inconsistent and often small cognitive effects, those that did adjust for such changes showed larger, more robust effects. The only relevant randomized control trial to date indicated a medium-sized albeit nonsignificant effect of volunteering on global cognition, possibly indicating that it was underpowered. Thus, on balance, current evidence on the link between volunteering and global cognitive functioning appears promising, although it is clear that additional research is needed to addresses the limitations of existing studies. In particular, there is a dearth of sophisticated longitudinal studies that adjust for changes in volunteering engagement over time (as well as other covariates) and sufficiently-powered randomized control studies. Moreover, since the only randomized control trial to date assessed the impact of volunteering on institutionalized older adults with dementia, there is currently no intervention data relevant to nondemented, community-dwelling older adults. Finally, it should be noted that with the exception of three studies (Griep et al., 2017; Herrera et al., 2011; Hughes et al., 2013), all observational research to date used only generic measures of volunteering status (i.e., whether or not the person is a current volunteer or regular volunteer) and thus little is known about the potential impact of volunteering type, frequency, or duration on global cognition. Domain-specific Cognitive Functions Research shows that healthy aging adversely and disproportionately affects higher-order cognitive functions (e.g., Bierre, Lucas, Guiney, Cotter, & Machado, 2017; Murman, 2015), which are not selectively tapped by measures of global cognitive impairment. Thus, in this section, we review studies that utilized more specific cognitive tests to evaluate the impact of volunteering on particular cognitive domains known to decline with healthy aging. No cross-sectional or prospective cohort studies have assessed the association between volunteering participation and performance on domain-specific cognitive tests; however, three articles have reported on relevant data from two randomized control trials (Carlson et al., 2009; Carlson et al., 2015; Carlson et al., 2008). Both trials were designed to evaluate the impact of Experience Corps, which is an ongoing, intensive intergenerational program in which older adults volunteer in local schools for 15 hr each week to assist with children’s reading and literacy skills, library administration, and promoting prosocial behavior in the playground (for a detailed description of the program, see Glass et al., 2004). Carlson et al. (2008) reported on the impact of the first Experience Corps trial, in which participants completed at baseline and follow-up three tests that assessed specific cognitive constructs: (a) Trail Making Test-B, which is a paper-and-pencil instrument that taps task switching by requiring participants to efficiently switch between two sets of instructions in order to complete the test as quickly as possible (Reitan, 1958); (b) Rey-Osterreith Complex Figure Delayed Recall, which taps visual memory by requiring participants to reproduce a complex drawing they viewed 15 min earlier (Lezak, 1995); and (c) immediate and delayed word list recall, which tap verbal memory. At follow-up, the results of the main analysis showed nonsignificant trends for greater improvements in the intervention relative to the control group on Trail Making Test-B and Rey-Osterreith Complex Figure Delayed Recall. Specifically, the group by time interactions approached significance, in the context of no group differences at baseline and significant differences favoring the intervention group at follow-up. No significant effects were observed for either the immediate or delayed word recall tests. In a secondary analysis, for which the sample was restricted to those whose baseline Trail Making Test-B performance indicated impairment, significant intervention effects emerged for task switching and verbal memory, while a marginally-significant intervention effect emerged for visual memory. Although the article contained insufficient information to enable calculation of standardized effect sizes, the numerical effects in the secondary analysis were relatively large (e.g., for delayed word recall, a 40% improvement in the intervention group versus a 12% decline in controls). Carlson et al. (2009) also reported on the impact of the first Experience Corps trial on a subset of participants who had shown impaired functioning at baseline (as defined above) and who met inclusion criteria for undergoing functional magnetic resonance imaging (fMRI). In that study, fMRI scans were conducted while participants completed a flanker task, which requires attentional control as participants must attend to target stimuli while ignoring distractors (Machado, Guiney, & Struthers, 2013). The comparison of performance changes from baseline to follow up showed large and significant benefits of the intervention, evidenced by greater improvements in the intervention group relative to controls. In addition, during performance of the flanker task, the intervention group showed greater increases in activation in areas of the prefrontal cortex used for the control of attention. Thus, this substudy provided evidence that volunteering benefits an important cognitive function that is adversely impacted by adult aging (Machado, Devine, & Wyatt, 2009). Carlson et al. (2015) reported on the impact of the second Experience Corps trial, in which participants completed at baseline and follow-up a verbal memory test and underwent MRI scanning to assess hippocampal and cortical volume. The overall results showed a medium-sized but nonsignificant intervention effect on hippocampal and cortical volumes at the 2-year follow-up. Sex-stratified analyses revealed large, significant intervention effects on hippocampal and cortical volume in males, and a medium-sized, nonsignificant intervention effect on hippocampal volume in females (no effect size for cortical volume in women was reported). Results for baseline to follow-up change in verbal memory test performance for the intervention relative to the control group were not reported, but the authors did note that, in the intervention group only, change in memory performance from baseline to follow-up was moderately correlated with change in cortical volume (r = .39), irrespective of sex, which suggests that structural improvements supported better verbal memory. In sum, the limited current evidence indicates a positive impact of volunteering on specific aspects of older adults’ cognitive functioning, particularly attentional control. There were also some promising trends for task switching, verbal memory, and visual memory, as well as indications that the cognitive benefits of volunteering might be greater for those with impaired cognitive functioning (Carlson et al., 2008). Moreover, the evidence to date indicates that volunteering can have a positive impact on brain structure (particularly in males; Carlson et al., 2015) and functional activation in areas of the brain that support attentional control (Carlson et al., 2009). However, there are several limitations associated with these studies. First, they all assessed only one type of volunteering (an intergenerational program in schools), making it unclear whether the observed domain-specific cognitive benefits extend to other types of volunteering. Second, despite robust numerical trends, not all positive findings were statistically significant, which in at least some cases may reflect inadequate sample size. Third, women made up the majority of participants in all three studies, which could be important given that some studies have found gender differences in volunteering-cognition effects (e.g., Tomioka et al., 2016). Fourth, both Experience Corps trials used relatively inactive control groups, leaving open the possibility that the observed cognitive improvements were due to some other aspect of being in the intervention group (e.g., more interaction with program co-ordinators) rather than the volunteering itself. Finally, only a handful of the cognitive functions known to decline with aging have been assessed. Others, including inhibitory control over prepotent responses, response planning, problem-solving, working memory, and spatial memory, have yet to be investigated. Conclusions and Recommendations for Future Research In this review, we first presented evidence that it is at least plausible that volunteering can positively impact older adults’ cognitive functioning through increased physical, social, and cognitive activity, and consequent improvements in mental and neurological health. Second, we reviewed the studies to date that investigated whether volunteering in older adulthood benefits global and domain-specific aspects of cognitive functioning, many of which provided at least some evidence for positive effects of volunteering. Interestingly, studies that used robust designs (e.g., sophisticated longitudinal studies or randomized control trials) and those that assessed domain-specific cognitive functioning produced the largest effect sizes. In contrast, most of the observational studies assessing the relationship between volunteering and global cognitive functioning reported relatively small effects. Overall, the evidence is promising, but it is clear that more research is needed to advance the field beyond its current state of infancy. Based on the limitations of the existing evidence, there are several straightforward recommendations for future volunteering-cognition research. First, as conclusions from several studies are limited by the results not reaching significance despite positive trends, future studies need to take greater care to ensure adequate power to detect potential volunteering effects. Second, to allow causal inference, researchers should prioritize randomized control trials over other study designs, although longitudinal studies that take into account changes in volunteering over time as well as other covariates will also help to advance the field. Third, given that all current randomized control data relate to intergenerational volunteering, future trials should seek to test the effects of other specific types of volunteering, or of volunteering in general. Fourth, more studies assessing the effects of volunteering on domain-specific cognitive functions are needed. In terms of the specific research questions to be answered, there is a need for studies that test whether the cognitive benefits gained through volunteering (a) occur equally in healthy, at-risk, and clinical populations, given suggestions that those of more advanced age or with impaired functioning might benefit most (Carlson et al., 2008; Schwingel et al., 2009); (b) are domain-specific, global, or both; (c) occur irrespective of gender, in light of some gender-specific findings (Carlson et al., 2015; Tomioka et al., 2016); and (d) depend on the amount or type of volunteering undertaken, given that most studies to date utilized only broad measures of volunteering that do not take these factors into account. As more evidence emerges, researchers can begin to address wider questions, including testing the specific mechanisms through which volunteering might improve cognition and investigating whether volunteering-cognition effects are more pronounced in those with low initial levels of physical, social, and/or cognitive activity. By determining the ways in which volunteering benefits cognition and the extent of those benefits, researchers can elucidate how this simple and accessible activity can best be used to help slow age-related cognitive decline. Funding This research was supported by Brain Research New Zealand. Conflict of Interest None reported. References Anderson, N. D., Damianakis, T., Kröger, E., Wagner, L. M., Dawson, D. R., Binns, M. A.,… Cook, S. L.; BRAVO Team. ( 2014). The benefits associated with volunteering among seniors: A critical review and recommendations for future research. 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The Journals of Gerontology Series B: Psychological Sciences and Social SciencesOxford University Press

Published: Mar 1, 2018

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