Economists often use a construct known as "the ultimatum game" in their research: Player one receives an amount of money and must propose a way to split the sum with another player. If player two accepts the offer, both players split the money in the proposed way. If player two rejects the offer, neither player receives any money.
The game essentially pits the desire for personal economic gain against the desire to punish someone else. Although it is financially advantageous for player two to accept any offer—since some money is better than none—previous research indicates that player two will often reject low offers as a way to punish player one for an unfair division of money.
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A collaborative study between Swiss economists and a Harvard University neurologist recently identified a part of the brain that cause us to de-prioritize punishing another player in the ultimatum game.
Previous studies using fMRI indicate that a brain region known as the dorsolateral prefrontal cortex (DLPFC), which is involved in reasoning, is particularly active when a player was presented with an unfair offer.
The authors of the new study, published Oct. 5 in Science, used a technique called repetitive transcranial magnetic stimulation (rTMS) to interfere with DLPFC functioning. The relatively new procedure involves applying a magnetic pulse to the scalp, causing a temporary disruption of activity in the brain region underneath.
Researchers found that subjects with disrupted DLPFC activity were significantly more likely to accept "unfair" offers. Those who received rTMS to the right side of the DLPFC accepted the most unfair offer—$4 out of a possible $20—45 percent of the time. That is compared to a 9 percent acceptance rate among those who received a placebo treatment. (The researchers found that interfering with the function of the left DLPFC had no effect.)
Subjects with disrupted DLPFC activity said they considered low offers unfair, yet frequently chose to accept them anyway.
The neuroeconmic study was the first to show that changing the activity in the DLPFC was sufficient to change people's economic choices.
"There's a big difference between correlation and causality," said Ernst Fehr, an economist at Switzerland's University of Zurich, a researcher on the study. "We established causality."
"We can show that it's DLPFC disruption that leads to the change in behavior," he added.
The work of Fehr and his colleagues challenges previous studies, which suggest that the DLPFC acted to make people rationally take whatever sum they are offered. Because disrupting the region increased player two's acceptance of player one's offers, a normally functioning DLPFC may actually be involved in irrationally punishing player one by rejecting the offer.
"It's a surprising result," said Samuel McClure, a psychologist at Princeton. "People had been thinking the opposite."