This may be highly counter-intuitive because we culturally associate this sense of justice with idealism or, as Che Guevara put it, with being driven by superior feelings of love for fellow humans.
Actually it has nothing to do with it but with greater rational function, brain scans show.
Keith J. Yoder & Jean Decety, The Good, the Bad, and the Just: Justice Sensitivity Predicts Neural Response during Moral Evaluation of Actions Performed by Others. Journal of Neuroscience 2014. Pay per view → LINK [doi:10.1523/JNEUROSCI.4648-13.2014]
A copy of the study has been uploaded by co-author J. Decety to academia.edu.
Complementary source: press release.
From the latter:
During the behavior-evaluation exercise, people with high justice sensitivity showed more activity than average participants in parts of the brain associated with higher-order cognition. Brain areas commonly linked with emotional processing were not affected.
The conclusion was clear, Decety said: “Individuals who are sensitive to justice and fairness do not seem to be emotionally driven. Rather, they are cognitively driven.”
According to Decety, one implication is that the search for justice and the moral missions of human rights organizations and others do not come primarily from sentimental motivations, as they are often portrayed. Instead, that drive may have more to do with sophisticated analysis and mental calculation.
This may explain why there is a well known positive correlation between intelligence and leftism: fairness is simply more rational.