Why do people volunteer, or why are they altruistic? I have followed the open source movement, and beyond the sense of doing this for the common good, there certainly is an element of showing off skills, be it just for brownie points or for signaling to potential employers. But such signals are not relevant in other areas of volunteering.
Take volunteer firefighters, who Jeffrey Carpenter and Caitlin Knowles Myers study. They have data on time spent volunteering, altruism measures from the standard dictator game as well as other behavorial measures and conclude that both altruism and social image matter. And the more people get compensation by money, the less the social images matters.
These results are hardly surprising. Volunteer firefighters quite obviously like their work and their utensils. Just see how shiny they keep their trucks and take every opportunity to parade them. But that is good, and this is a way to show to others how altruistic they are, similarly to Shriners in their miniature cars. And once paid, firefighters seem to consider their job like any other, so image is less important to them.
That said, firefighters enjoy quite some social prestige, which is repeatedly reinforced by authorities and media. This applies also to the police force and teachers. I have always wondered whether this was a way to compensate them for low pay. From this study, it seems firefighters are certainly internalizing this trade-off.