In a paper in Nature; the authors Tricomi, Rangel, Camerer, and O’Doherty used fMRI experiments to reveal that the brain is wired for egalitarianism.

Activity rose in rich people when their poor colleagues got money. In fact, it was greater in that case than when they got money themselves, which means the “rich” people’s neural activity was more egalitarian than their subjective ratings were. Whereas in “poor” people, the vmPFC and the ventral striatum only responded to getting money, not to seeing the rich getting even richer.

Neuroskeptic provides some perspective:

Notice that this is essentially a claim about psychology, not neuroscience, even though the authors used neuroimaging in this study. They started out by assuming some neuroscience – in this case, that activity in the vmPFC and the ventral striatum indicates reward i.e. pleasure or liking – and then used this to investigate psychology, in this case, the idea that people value equality per se, as opposed to the alternative idea, that “dislike for unequal outcomes could also be explained by concerns for social image or reciprocity, which do not require a direct aversion towards inequality.”

(Guang Puang ping:  Marciano Siniscalchi.)