The suggestion of fairness in anything elicits the same reaction as winning money and eating chocolates in the human brain, scientists from the University of California - Los Angeles have found.
They found that being treated fairly turns on the brain's reward circuitry.
"We may be hard-wired to treat fairness as a reward," said study co-author Matthew D. Lieberman, UCLA associate professor of psychology and a founder of social cognitive neuroscience.
"Receiving a fair offer activates the same brain circuitry as when we eat craved food, win money or see a beautiful face," said Golnaz Tabibnia, a postdoctoral scholar at the Semel Institute for Neuroscience and Human Behavior at UCLA and lead author of the study.
The activated brain regions include the ventral striatum and ventromedial prefrontal cortex.
According to Tabibnia, humans share the ventral striatum with rats, mice and monkeys.
"Fairness is activating the same part of the brain that responds to food in rats," she said.
This is consistent with the notion that being treated fairly satisfies a basic need, she added.
In the study, subjects were asked whether they would accept or decline another person's offer to divide money in a particular way. If they declined, neither they nor the person making the offer would receive anything. Some of the offers were fair, such as receiving 5 dollars out of 10 dollars or 12 dollars, while others were unfair, such as receiving 5 dollars out of 23 dollars.
"In both cases, they were being offered the same amount of money, but in one case it's fair and in the other case it's not," Tabibnia said.
Almost half the time, people agreed to accept offers of just 20 to 30 percent of the total money, but when they accepted these unfair offers, most of the brain's reward circuitry was not activated; those brain regions were activated only for the fair offers. Less than 2 percent accepted offers of 10 percent of the total money.
The study group consisted of 12 UCLA students, nine of them female, with an average age of 21. They had their brains scanned at UCLA's Ahmanson-Lovelace Brain Mapping Center. The subjects saw photographs of various people who were said to be making the offers.
"The brain's reward regions were more active when people were given a 5 dollars offer out of 10 dollars than when they received a 5 dollars offer out of 23 dollars," Lieberman said.
"We call this finding the 'sunny side of fairness' because it shows the rewarding experience of being treated fairly," he added.
A region of the brain called the insula, associated with disgust, is more active when people are given insulting offers, Lieberman said.
When people accepted the insulting offers, they tended to turn on a region of the prefrontal cortex that is associated with emotion regulation, while the insula was less active.
"We're showing what happens in the brain when people swallow their pride. The region of the brain most associated with self-control gets activated and the disgust-related region shows less of a response," Tabibnia said.
The study appears in the April issue of the journal Psychological Science.