In the famous Tecumseh study of almost three thousand people by psychology researcher James House (et al. 1988) at the University of Michigan, an interesting tidbit of information emerged. While this study focused on a different topic—the health benefits of social support—one of the variables they uncovered was that volunteer activities were among the most powerful predictors of reduced morbidity and mortality in men. In fact, they discovered that, more than any other single activity, doing regular volunteer work correlated with better health and longevity. The men who did no volunteer work were two and a half times as likely to die during the time period of the study as the men who did volunteer work, even just a couple of hours a week.
Reference: House J, Landis K, and Umberson D. Social relationships and health. Science, 1988;241:540-545.