Individuals are willing to sacrifice their own resources to promote equality in groups. These costly choices promote equality and are associated with behavior that supports cooperation in humans, but little is known about the brain processes involved. We use functional MRI to study egalitarian preferences based on behavior observed in the “random income game.” In this game, subjects decide whether to pay a cost to alter group members’ randomly allocated incomes. We specifically examine whether egalitarian behavior is associated with neural activity in the ventromedial prefrontal cortex and the insular cortex, two regions that have been shown to be related to social preferences. Consistent with previous studies, we find significant activation in both regions; however, only the insular cortex activations are significantly associated with measures of revealed and expressed egalitarian preferences elicited outside the scanner. These results are consistent with the notion that brain mechanisms involved in experiencing the emotional states of others underlie egalitarian behavior in humans.From their discussion:
...this experiment shows that some parts of the brain are more active during egalitarian outcomes, and these activations are correlated with egalitarian behavior inside the scanner. However, a more crucial result is that the activations are also correlated with behavior outside the scanner, including self-reported preferences for egalitarian outcomes and game behavior that reveals how willing subjects are to use their own resources to obtain egalitarian outcomes within their groups. Taken together, the evidence suggests that the anterior insular cortex plays a critical role in egalitarian behavior in humans. This conclusion is consistent with a broader view of the insular cortex as a neural substrate that processes the relationship of the individual with respect to his or her environment. The predominately left-lateralized activation may point toward the possibility of a positive valence or energy-preserving mode related processing during egalitarian behavior (i.e., individuals may see the group as a greater good that is worth preserving). The fact that the insula is directly involved in physiological, food, and pain-related processing supports the general notion that prosocial behavior, which is important for survival of both the individual and the group/species, is implemented on a fundamental physiological level similar to breathing, heartbeat, hunger, and pain.
Adam Smith contended that moral sentiments like egalitarianism derived from a “fellow-feeling” that would increase with our level of sympathy for others, predicting not merely aversion to inequity, but also our propensity to engage in egalitarian behaviors. The evidence here supports such an interpretation. Although individuals may experience internal rewards when punishing antisocial behavior and may have preferences for social equality, our results suggest that it is the brain mechanisms involved in experiencing the emotional and social states of self and others that appear to be driving egalitarian behaviors.
Our results have important implications for theories of the evolution of prosocial behavior that suggest culturally transmitted “leveling mechanisms”—for example, food sharing and monogamy—stifle within-group competition and create circumstances in which intergroup antagonism generates selective pressure for altruistic behaviors. A concern for equality may have originally evolved because it fostered the conditions necessary for early human groups to maintain a high level of cooperation. Future research should focus on the interconnectivity of regions of the brain involved in egalitarianism and altruism to better understand how these two behaviors may have coevolved.