Abstract:

Interpersonal fairness plays an important role in human life. Punishment is one of the most fundamental factors in the formation of interpersonal fairness. Although many neuroscience studies have used interactive economic games to explore the neural mechanisms that underlie fair norms’ enforcement and compliance, single brain studies cannot make researchers achieve a complete understanding of the intrinsic mechanisms related to dynamic interactions. Hyperscanning techniques can allow researchers to measure the brain activity of two or more persons simultaneously while they complete cognitive tasks under natural conditions. Therefore, we used fNIRS-based hyperscanning to evaluate changes in behavior and interpersonal neural synchronization (INS) during the formation of interpersonal fairness.

The experimental paradigm referred to the adapted ultimatum game (UG). There were two conditions in our study, punishment and voluntary condition. In the punishment condition, the proposer (A) introduced a distribution scheme. When the recipient (B) accepted it, both of them received money according to the distribution scheme. If B thought that this distribution was unfair, he or she could punish A by spending all or part of their money to reduce A’s gains. One RMB invested in punishment led to a reduction in A’s gains by 5 RMB. However, in the voluntary condition, B was obligated to accept the offer no matter how much A allocated to him or her. The task included 50 trials altogether, 25 trials for each condition. The trials of two different conditions were presented in a pseudo-random way. A total of 44 university students participated in this study, including 13 male and 9 female pairs. Before the experiment, pairs of participants drew lots to decide their roles.

Behavioral results showed that compared with the voluntary condition, the average transfer amount was significantly higher in the punishment condition and much closer to the equal split. Moreover, in the face of unfair distribution, more money was transferred to recipients when they punished more the proposers. Consequently, there was no significant difference between proposers’ and recipients’ final gains in the punishment condition. However, in the voluntary condition, proposers gained more. Results from fNIRS revealed that in the punishment condition, the INS of three brain areas was higher than that of the voluntary condition, including the dorsolateral prefrontal cortex (channel 15), inferior parietal lobule (channel 12, 13), and temporo-parietal junction (channel 9). Additionally, the transfer difference (punishment minus control) was positively associated with an increase of INS in the inferior parietal lobule.

These results suggest that punishment can promote the formation of interpersonal fairness, accompanied by enhanced interpersonal neural synchronizations in the brain areas related to strategic decision-making and theory of mind.

Key words: fairness, punishment, interpersonal neural synchronization, fNIRS, hyperscanning