The majority of players elected to impose a penalty even when it cost them some of their own money. Doing so, the researchers found, activated a region of the brain known as the dorsal striatum. Previous research has shown that this region is involved in enjoyment or satisfaction.
Brain scans during the experiment also showed a correlation between a person's brain activity and how much punishment they choose to mete out at their own personal cost: Individuals with stronger activations were more willing to incur greater costs in order to punish someone else.
"The nice feature of our study is that the variation in the dorsal striatum predicts these differences in behavior quite well," Fehr said. "Subjects with lower activation in the dorsal striatum punish less."
According to Knutson, the Stanford psychologist, altruistic punishment seems irrational from the standpoint of self-interest. But Knutson wrote that the Swiss study explains this seemingly irrational behavior by showing that "instead of cold calculated reason, it is passion that may plant the seeds of revenge."
Fehr agrees that a person's passion plays a role in altruistic punishment. "But I do not think that our evidence indicates that passion overrides rationality," he said. "In fact, I believe that our evidence shows that people deal quite rationally with their emotions," he said.
The study results suggest that activation of the dorsal striatum reflects some sort of anticipated satisfaction from punishing those who break social norms. The higher the activation, the more people are willing to spend on punishment.
However a second area of the brain, known as the prefrontal cortex, is activated when players need to weigh the satisfaction derived from punishment against the monetary cost of punishing.
The study results show that the higher the cost of the punishment, the lower the actual punishment imposed. "Thus, this all looks pretty rational," Fehr said. "People seem to trade off the expected satisfaction from punishing with the cost of punishing in quite a rational way."
According to de Quervain, the study co-author, understanding the role of the prefrontal cortex in altruistic punishment may also help researchers better understand psychiatric disorders characterized by abnormal social behavior or addiction.
"Deficits in prefrontal cortical functioning may contribute to these psychopathologies by a disturbed ability to weigh beneficial against negative consequences of an action," he said.
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