Brain Study Shows Why Revenge Is Sweet

John Roach
for National Geographic News
August 27, 2004
Revenge is sweet. Many of us have felt that way, and now scientists say they know why.

A new brain-imaging study suggests we feel satisfaction when we punish others for bad behavior. In fact, anticipation of this pleasure drives us to crack the whip, according to scientists behind the new research.

The findings, reported in today's issue of the journal Science, may partly explain a behavior known as altruistic punishment: Why do we reprimand people who have abused our trust or broken other social rules, even when we get no direct practical benefits in return?

"A person who has been cheated is [left] in a bad situation—with bad feelings," said study co-author Ernst Fehr, director of the Institute for Empirical Research in Economics at the University of Zurich in Switzerland. "The person would feel even worse if the cheater does not get her or his just punishment."

Human societies are an anomaly in the animal world. Ours are based on a detailed division of labor and cooperation between genetically unrelated individuals in large groups.

Fehr and his colleagues suggest that the feeling of satisfaction people get from meting out altruistic punishment may be the glue that keeps societies together.

"Theory and experimental evidence shows that cooperation among strangers is greatly enhanced by altruistic punishment," Fehr said. "Cooperation among strangers breaks down in experiments if altruistic punishment is ruled out. Cooperation flourishes if punishment of defectors is possible."

Writing in an accompanying Science commentary, Brian Knutson, a psychologist at Stanford University in Palo Alto, California, described as "elegant" the experiment the Swiss team used to show "this complex emotional dynamic of schadenfreude"—the pleasure felt over someone else's misfortune.

PET Scans

The scientists measured blood flow in the test subjects' brains using a technology called positron emission tomography (PET). The researchers injected water with a unique signature into the bloodstream of each volunteer, then scanned the person's brain when the water, carried via blood, reached the brain.

"Increased cerebral blood flow in a certain brain region means more oxygen consumption and more brain activity in this region," said study co-author Dominique de Quervain, a neuroscientist at the University of Zurich.

The team scanned the brain activity of male volunteers participating in a game of exchanging money back and forth. If one player made a selfish choice instead of a mutually beneficial one, another player could penalize him.

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."

Emotional Balance

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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