Psychologists from the University of Chicago have linked high "justice sensitivity"—how individuals react to experiences of injustice and unfairness—to lower emotional sensitivity. For the study, published in The Journal of Neuroscience, researchers relied on both brain scans and questionnaires that assessed justice sensitivity and cognitive and emotional empathy.
"We were interested [in]… the contribution of emotion and cognition in moral judgment," said lead researcher Jean Decety.
Justice is obviously a very broad concept—social justice differs from vigilante justice differs from criminal justice. But at its most basic, justice is simply "the quality of being just, impartial, or fair," according to Merriam-Webster. Emotions are anything but impartial, so it's not surprising that highly emotional people would perhaps fail at assessing justness. But it's strange that they may be less concerned with justice or its execution.* A deep concern for fairness seems to me like a pretty emotional proposition.
Nonetheless, brain scans administered while study participants viewed videos of "good" and "bad" behavior showed people with high justice sensitivity also had less activity in parts of the brain linked with emotional processing. Meanwhile, they showed greater than average activity in parts of the brain linked to higher-order cognitive processing such as planning, decision making, abstract reasoning, interpreting social cues, and distinguishing between different perspectives.
"Individuals who are sensitive to justice and fairness do not seem to be emotionally driven," said Decety. He noted that this goes against conventional wisdom about the "sentimental motivations" of those involved with human rights and social and criminal justice causes.
For more on the scientific basis of political and moral disposition, check out social psychologist Jonathan Haidt's May 2012 story for Reason. According to Haidt (and ample research), genetics explains between one-third and one-half of the variability among people in their political attitudes.
* To be clear, that's not precisely what this study says.