From Scientific American:
The ultimatum game brings out conflicting impulses in human beings. In the game, a researcher offers two players a set amount of money and explains that if they agree on how to divvy it up they will keep that money for themselves. If they don’t, neither will get anything. One player then offers the other a split. Our thirst for fairness dictates that most players will reject a patently unfair division–such as offering only $4 out of a total of $20. Yet, self interest would argue that even $4 is better than nothing, which is what will otherwise result. Brain imaging studies have shown that the prefrontal cortex is engaged when players ponder an offer and now new research finds that damping down activity in that region can set free our selfish side.
More here.