"An eye for an eye"? Neural correlates of retribution and forgiveness.

Humans have evolved strong preferences for equity and fairness. Neuroimaging studies suggest that punishing unfairness is associated with the activation of a neural network comprising the anterior cingulate cortex, anterior insula, the ventral striatum, and the dorsolateral prefrontal cortex (DLPFC). Here, we report the neuronal correlates of retribution and "forgiveness" in a scenario, in which individuals first acted as a recipient in an Ultimatum Game, and subsequently assumed the position of a proposer in a Dictator Game played against the same opponents as in the Ultimatum Game. Most subjects responded in a tit-for-tat fashion, which was accompanied by activation of the ventral striatum, corroborating previous findings that punishing unfair behaviour has a rewarding connotation. Subjects distinguished between the human opponent and computer condition by activation of the ventromedial PFC in the human condition, indicative of mentalising. A substantial number of subjects did not retaliate. Neurally, this "forgiveness" behaviour was associated with the activation of the right (and to a lesser degree left) DLPFC, a region that serves as a cognitive control region and thus may be involved in inhibiting emotional responses against unfairness.