As far as I know, researchers in neurology and the game theory types aren't talking much to one another, which is a shame. I have in hand a fascinating article called "The Neural Basis of Altruistic Punishment" that is causing me to rethink many of my assumptions about how to apply the Prisoner's Dilemma in my daily business life.
The gist of this article, which can be found readily in full text on the Web, is that people derive a sense of satisfaction that can be empirically observed and measured in the dorsal striatum from punishing violations of social norms. This sense of satisfaction can seem more desirable than avoiding economic costs. I am tired enough tonight that the deeper implications of this seem too obvious to explain.
It also makes me wonder if the converse is also true. Do we derive a sense of dissatisfaction from observing violations of social norms that we are unable to punish? This might explain why I've never really liked Rigoletto.