Stephen Meier and Alois Stutzer. "Is Volunteering Rewarding in Itself?" Economica 75(297):39–59.
Volunteering constitutes one of the most important pro-social activities. Following Aristotle, helping others is the way to higher individual wellbeing. This view contrasts with the selfish utility maximizer, who avoids helping others. The two rival views are studied empirically. We find robust evidence that volunteers are more satisfied with their life than non-volunteers. The issue of causality is studied from the basis of the collapse of East Germany and its infrastructure of volunteering. People who lost their opportunities for volunteering are compared with people who experienced no change in their volunteer status.
That's a clever research design.