Neural markers of human aggression and cooperation are influenced by zero-sum or non-zero-sum groups.

Cooperation and competition in social interaction are important to social development. There are many studies examining the neural mechanism underlying these behaviors; however, little is known about the potential role of payoff norms in social interactions. This study introduced two distinct payoff norms: zero-sum (ZS) and non-zero-sum (NZS). In the ZS group, only the player with the highest total score would receive a reward; while in the NZS group, rewards were determined by the combined total score of both players, and individual rewards were prorated according to each player's contribution to the total score. Electroencephalography was used to examine individuals' neural responses during the Chicken Game in these different groups. The behavioral findings revealed a higher rate of cooperation in the NZS group compared to the ZS group. Aggressive behaviors from opponents elicited larger feedback-related negativity (FRN) and lower P300 amplitudes in both groups. Notably, larger FRN and smaller P300 amplitudes were elicited by participants' aggressive behaviors compared to their cooperative behaviors in the ZS group, whereas larger P300 amplitudes were elicited by aggressive behaviors compared to cooperative behaviors in the NZS group. Additionally, a greater theta event-related synchronization (ERS) was observed solely when participants exhibited aggression in the NZS group. The type of group moderated the association between posterior theta-ERS and cooperative rates. Specifically, theta-ERS was negatively associated with cooperative rates in the NZS group, whereas a reversed association was found in the ZS group. These findings provide valuable insights into the neurobiological foundations of cooperation under different economic payoff norms.