THE MOST COMPREHENSIVE and up-to-date lay book on the science of behavior is Stanford neuroendocrinologist Robert Sapolsky’s Behave: The Biology of Humans at Our Best and Worst (2017). The book is an encyclopedic compendium of behavioral science, written with folksy, down-home idiom. Here he lays out evidence that the fundamental “default” of human social behavior is trust, and that the brain structure, the amygdala, is involved in applying justice when needed.
The amygdala . . . plays a logical role in social and emotional decision making. In the Ultimatum Game, an economic game involving two players, the first makes an offer as to how to divide a pot of money, which the other player either accepts or rejects. If the latter, neither gets anything. Research shows that rejecting an offer is an emotional decision, triggered by anger at a lousy offer and the desire to punish. The more the amygdala (BLA) activation in the second player after an offer, the more likely the rejection. People with damaged amygdalae are atypically generous in the Ultimatum Game and don’t increase rejection rates if they start receiving unfair offers. . . . [T]hese findings suggest that the amygdala injects implicit distrust and vigilance into social decision making. All thanks to learning. . . . “The generosity in the trust game of our BLA [amygdala]-damaged subjects might be considered pathological altruism, in the sense that inborn altruistic behaviors [my bold] have not, due to BLA damage, been un-learned through negative social experience.” In other words, the default state is to trust, and what the amygdala does is learn vigilance and distrust.Behave, Robert Sapolsky
Sapolsky goes on to explain that, among the inputs to the amygdala is a region in the prefrontal cortex called the insula, which is activated when one tastes or smells something disgusting. This area is also activated by morally objectionable behavior: “Someone does something lousy and selfish to you in a game, and the extent of insular and amygdaloid activation predicts how much outrage you feel and how much revenge you take. This is all about sociality—the insula and amygdala don’t activate if it’s a computer that has stabbed you in the back.” So the amygdala, the brain structure concerned with anxiety, fear and rage, is intricately responsive to moral sensibilities.
Morality has been selected by the deep instincts of our ancestral hominid species for justice, naturally selected over millions of years for permitting the engagement necessary for our principal adaptive advantage—the coordination of divided labor—to take root and blossom.