"While everyone accepts that physical pain is real, people are tempted to think that social pain is just in their heads," said Matthew D. Lieberman, one of the paper's three authors and an assistant professor of psychology at UCLA. "But physical and social pain may be more similar than we realized."
"In the English language we use physical metaphors to describe social pain like 'a broken heart' and 'hurt feelings,'" said Naomi I. Eisenberger, a UCLA Ph.D. candidate in social psychology and the study's lead author. "Now we see that there is good reason for this."
Eisenberger and Lieberman used functional magnetic resonance imaging (fMRI) to monitor brain activity in 13 UCLA undergraduates while the students played a computer ball-tossing game designed to provoke feelings of social exclusion.
In Cyberball, two computer figures are able to throw a virtual ball to each other and to the game's human player. Although the activities of the figures are entirely computer-generated, the undergraduates were led to believe that they corresponded to other student players elsewhere.
"It's really the most boring game you can imagine, except at one point one of the two computer people stop throwing the ball to the real player," Lieberman said.
In the first of three rounds, experimenters instructed UCLA undergraduates just to watch the two other players because "technical difficulties" prevented them from participating. In the second round, the students were included in the ball-tossing game, but they were excluded from the last three-quarters of the third round by the other players. While the undergraduates later reported feeling excluded in the third round, fMRI scans revealed elevated activity during both the first and third rounds in the anterior cingulate. Located in the center of the brain, the cingulate has been implicated in generating the adverse experience of physical pain.
"Rationally we can say being excluded doesn't matter, but rejection of any form still appears to register automatically in the brain, and the mechanism appears to be similar to the experience of physical pain," Lieberman said.
When the undergraduates were conscious of being snubbed, cingulate activity directly responded to the amount of distress that they later reported feeling at being excluded.
The researchers also detected elevated levels of activity in another portion of the brain -- the right ventral prefrontal cortex -- but only during the game's third round. Located behind the forehead and eyes, the prefrontal cortex is associated with thinking about emotions and with self-control.
"The folks who had the most activity in the prefrontal cortex had the least amount of activity in the cingulate, making us think that one area is inhibiting one or the other," Lieberman said.
The psychologists theorize that the pain of being rejected may have evolved because of the importance of social bonds for the survival of most mammals.
"Going back 50,000 years, social distance from a group could lead to death and it still does for most infant mammals," Lieberman said. "We may have evolved a sensitivity to anything that would indicate that we're being excluded. This automatic alarm may be a signal for us to reestablish social bonds before harm befalls us."
"These findings show how deeply rooted our need is for social connection," Eisenberger said. "There's something about exclusion from others that is perceived as being as harmful to our survival as something that can physically hurt us, and our body automatically knows this."
The explanation is consistent with past research on mammals. Hamster mothers with damaged cingulates no longer take steps to keep their pups near and infant squirrel monkeys similarly affected no longer produce a spontaneous cry when separated from their mothers. In human mothers, fMRIs have shown that infant cries increase activity in the cingulate.
The prefrontal cortex, meanwhile, has been found to be key to thinking in words and controlling behavior, urges, emotions and thought. So researchers theorize that the prefrontal cortex may inhibit the cingulate as opposed to the other way around.
"Verbalizing distress may partially shut down areas of the brain that register distress," Lieberman said. "The regulating abilities of the prefrontal cortex may be why therapy and expressing painful feelings in poems and diaries is therapeutic."
But humans may need a conscious awareness of social exclusion to activate this buffering mechanism, the researchers said. The requirement would explain why the prefrontal cortex did not become activated during the first round of Cyberball, when the students were led to believe that a computer glitch prevented them from being included in the ball toss.
"If we have no reason to consciously believe that we're being excluded," Lieberman said, "we tend not to respond and regulate."
The study's third author is Kipling D. Williams, a psychology professor at Macquarie University in Sydney, Australia. Williams is the architect of Cyberball.
The project received funding from National Institute of Mental Health.