Research findings show that proposers make surprisingly fair offers in the ultimatum and dictator games, an observation that contradicts predictions of classical game theory. The present functional magnetic resonance imaging (fMRI) study examines brain activities of proposers that contribute to fair and unfair behaviors. We hypothesized that egoistic and altruistic motives of proposers affect fairness differentially in the two games. fMRI analysis revealed that the 28% of fair offers in the present ultimatum game were related to enhanced activity in prefrontal areas, in particular, in regions involved in reward and theory of mind. This corroborates the idea that egoistic motives are primarily responsible for fair offers in this game, which we denote as strategic fairness. Fair offers in the dictator game, however, were related to increased activity in the dorsal anterior cingulate cortex and the posterior cingulate cortex. This supports the idea that altruistic motives primarily drive fair offers in the dictator game, denoted here as altruistic fairness.