How Is Volunteering Associated With Reduced Mortality? A Mediator-Wide Approach Julia S. Nakamura 1 , Koichiro Shiba 2, 3 , Baoyi Shi 4 , Rachel S. Leong 1 , Tyler J. VanderWeele 2, 5, 6 , and Eric S. Kim 1, 2, 7 1 Department of Psychology, University of British Columbia 2 Human Flourishing Program, Institute for Quantitative Social Science, Harvard University 3 Department of Epidemiology, Boston University School of Public Health 4 Department of Biostatistics, Columbia University Irving Medical Center 5 Department of Epidemiology, Harvard T.H. Chan School of Public Health 6 Department of Biostatistics, Harvard T.H. Chan School of Public Health 7 Lee Kum Sheung Center for Health and Happiness, Harvard T.H. Chan School of Public Health Objective: Volunteering has been repeatedly associated with reduced mortality in older adults, yet research examining the mechanisms explaining this association remains limited. We evaluated potentially modifiable mediators, and combinations of mediators, that may underlie the volunteering–mortality association. Method: We used prospective data from 9,962 participants in the Health and Retirement Study (2006– 2018), a national, diverse, and longitudinal cohort of U.S. adults aged .50. We evaluated associations between volunteering at baseline (2008/2010), mediators at Wave 2 (2010/2012), and mortality between Waves 3 and 4 (2010–2016 for Cohort A, 2012–2018 for Cohort B). Results: After adjusting for demo- graphic confounders and mediators in the prebaselinewave (2006/2008), we observed evidence of mediation for those who volunteered ≥100 hr/year (vs. 0 hr/year) through combined physical health factors (propor- tion mediated [PM] = 49.56%, p = .004) and social factors (PM = 90.76%, p = .017) as well as through increased contact with friends (PM = 25.34%, p = .015) and helping friends/neighbors/relatives (PM = 25.12%, p = .018). However, there was less evidence of mediation through other proposed mediators. Conclusions: With further research, these results inform basic science, interventions, and policies by iden- tifying potential mechanisms, which might become modifiable features of the volunteering experience, to promote longevity in our rapidly aging population. Public Significance Statement While volunteering has been repeatedly associated with a reduced mortality risk in older adults, research examining why volunteering is associated with reduced mortality risk remains limited. We identified several mechanisms underlying the volunteering–mortality association (e.g., improved social factors). With further research, our findings identify several mechanisms, or modifiable features of a volunteering experience, that policymakers and interventionists may use to enhance volunteering’s ability to bolster health. Wendy C. Birmingham served as action editor. Julia S. Nakamura https://orcid.org/0000-0002-6015-4146 Tyler J. VanderWeele and Eric S. Kim share senior authorship. This work was supported by a grant from the Michael Smith Foundation for Health Research and the Canadian Institutes of Health Research. We would like to acknowledge and thank the Health and Retirement Study, which is conducted by the Institute for Social Research at the University of Michigan, with grants from the National Institute on Aging (U01AG09740) and the Social Security Administration. We would also like to thank the Vanier Canada Graduate Scholarships pro- gram—Canadian Institutes of Health Research. We thank Denny Zhang, Sofie Jensen, Ching Zhang, Swara Aman, Michelle Lin, Isaac Ng, Bita Jokar, Gurveer Palia, Marisa Nelson, and Ramit Seth for their con- tributions to this article. Tyler J. VanderWeele reports receiving personal fees from Flerish Inc. and Flourishing Metrics. Data are publicly avail- able through the Health and Retirement Study (https://hrsdata.isr.umich .edu/data-products/public-survey-data). Documentation, code, and other materials are available upon request. Julia S. Nakamura served as lead for formal analysis, project administra- tion, and writing–original draft and contributed equally to conceptualization and funding acquisition. Koichiro Shiba served as lead for formal analysis and methodology and contributed equally to supervision. Baoyi Shi served as lead for formal analysis and methodology. Rachel S. Leong contributed equally to writing–original draft and served in a supporting role for project administration. Tyler J. VanderWeele served as lead for conceptualization, methodology, and supervision and served in a supporting role for formal analysis. Eric S. Kim served as lead for conceptualization, funding acquisi- tion, and supervision and contributed equally to formal analysis and project administration. Julia S. Nakamura, Koichiro Shiba, Baoyi Shi, Rachel S. Leong, Tyler J. VanderWeele, and Eric S. Kim contributed equally to writing–review and editing. Correspondence concerning this article should be addressed to Julia S. Nakamura, Department of Psychology, University of British Columbia, 2136 West Mall, Vancouver, BC V6T 1Z4, Canada. Email: jnakamura@ psych.ubc.ca Health Psychology © 2025 American Psychological Association 2025, Vol. 44, No. 5, 518–527 ISSN: 0278-6133 https://doi.org/10.1037/hea0001429 518 This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.