Neurological basis for reciprocal fairness in the ultimatum game
Would you give up 3 months of your salary just to spite someone? You might say no. Experiments say yes.
This happens when people play the ultimatum game. In its purest form, one person is asked to split a sum of money with another person in any way the first person wishes, no matter how fairly or unfairly. The second person then decides whether to accept this offer (and the money is split accordingly) or reject the offer (in which case nobody gets any money). While rationally the second person should say yes to any nonzero amount, in practice the second person usually punishes an unfair split by rejecting the offer.
It makes sense in terms of human behavior, financial rationality notwithstanding. Humans punish behavior they see as unsporting or unfair and encourage mutually beneficial decisions. Even if you make $100, the fact that I have $9900 more that I didn’t split with you just offends your sense of fairness. But what part of the brain actually enforces this behavior? Scientists think they have found an answer, or at least part of one.
In this study, titled “Diminishing Reciprocal Fairness by Disrupting the Right Prefrontal Cortex,” researchers have found that the right dorsolateral prefrontal cortex (DLPFC) plays an important role in the implementation of fairness-related behaviors. Although this sense of reciprocal fairness seems to be a strongly innate part of human behavior, and appears to be one of the moral and social constraints that help shape human interaction, disrupting this very specific part of the brain can substantially reduce this behavior.
They found this by using transcranial magnetic stimulation, a noninvasive and temporary procedure where powerful magnetic fields are directed to specific portions of the brain. This invaluable tool allows scientists to temporarily disrupt specific neural structures, allowing them to study phenomena that could only before be observed in people with physical brain damage.
Compared to sham stimulation and left DLPFC stimulation, subjects who experienced right DLPFC stimulation accepted unfair offers at significantly higher rates. For one set of unfair conditions, sham stimulation yielded a 9.3% acceptance rate and left DLPFC stimulation yielded a 14.7% acceptance rate, while right DLPFC stimulation yielded a 44.7% acceptance rate. In all unfair conditions tested, stimulation of the right DLPFC increased acceptance rates by a statistically significant amount.
One of the fascinating points in this study was that all three groups – sham, left DLPFC, and right DLPFC – rated the unfairness levels of the offers the same, within the margin of error. This suggests that the right DLPFC is involved in the implementations of fairness-related behaviors but not the recognition of equity. All of the subjects found the unfair splits equally unfair, but only those with right DLPFC disruption still accepted offers at a rate of almost 50%.
This is intriguing research because it sits at the confluence of behavior science, neuroscience, and information science. The behaviors we study in a networks context are driven by human behavior, which are themselves driven by structures in our brains. Perhaps future research will see if the behavior of the DLPFC extends to other experiments involving reciprocal fairness, such as bargaining networks. It also raises serious ethical questions about using devices repetitive transcranial magnetic stimulation in a social context. Is a diminished sense of reciprocal fairness advantageous in creating a more rational society where people are more able to accurately judge situations without being clouded by emotions? Or does this inhibit the behavior that allows us to be better human beings and have more empathetic societies? Such philosophical questions may seem far removed from day to day life, but the same processes that underlie the ultimatum game are ubiquitous in our society, and ultimately are the same driving processes that underlie most of human behavior.