"It is a widely accepted principle of distributive justice that goods
should be distributed to individuals according to their contribution,
i.e., people should receive equal pay for equal work (equity principle)...
Recently, neuroscientific studies have begun to address neural processes
underlying social and economic phenomena like e.g., reactions to norm violations,
status concerns, and reactions to unfair behavior.
These studies have convergingly identified brain regions that are important for these aspects of social behavior.
One consistent finding is that activations of the dopaminergic mesolimbic (“reward”) system, especially the
nucleus accumbens (NAcc) do not exclusively reflect material self-interest, but also social aspects"
http://www.frontiersin.org/Human_Neuroscience/10.3389/fnhum.2012.00165/abstract