The evolved capacity for third-party punishment is considered crucial to the emergence and maintenance of elaborate human social organization and is central to the modern provision of fairness and justice within society. Although it is well established that the mental state of the offender and the severity of the harm he caused are the two primary predictors of punishment decisions, the precise cognitive and brain mechanisms by which these distinct components are evaluated and integrated into a punishment decision are poorly understood.

Using a brain-scanning technique known as functional magnetic resonance imaging (fMRI), we implemented a novel experimental design to functionally dissociate the mechanisms underlying evaluation, integration, and decision. This work revealed that multiple parts of the brain – some analytic, some subconscious or emotional – work in a systematic pattern to decide blameworthiness, assess harms, integrate those two decisions, and then ultimately select how a person should be punished. Specifically, harm and mental state evaluations are conducted in two different brain networks and then combined in the medial prefrontal and posterior cingulate areas of the brain, while the amygdala acts as a pivotal hub of the interaction between harm and mental state. This integrated information is then used by the right dorsolateral prefrontal cortex when the brain is making a decision on punishment amount.

These findings provide a blueprint of the brain mechanisms by which neutral third parties make punishment decisions.