**2.2.3 Procedure**

First, as an orienting task, the relationship between the participants and six opponent players (two fair, two unfair, and two neutral players) was manipulated through a sequential Prisoner's Dilemma game controlled by a computer program. In each trial, the participants decided whether they cooperate with the opponent or not. If both the participant and the opponent decide to cooperate, they each earn 10 points; if both defect, neither earns any points. If the participant cooperates and the opponent defects, the participants loses 40 points and the opponent earns 40 points, and vice versa. The cooperation rates throughout all trials for the fair opponent, the neutral opponent, and the unfair opponent were set in 80%, 50%, and 20%, respectively.

The opponent's facial expressions were presented on a PC screen after the participants decided their selection. When both the participant and the opponent cooperated, the opponent's expression became happy; when both defected, the opponent's expression became angry. When the participant cooperated and the opponent defected, the opponent's expression became happy, whereas when the participant defected and the opponent cooperated, the opponent's expression became angry. After playing with each opponent, the participants evaluated the opponent on a 7-point scale ranging from "cooperative" to "uncooperative" and "like" to "dislike."

After the orienting task, the participants performed a perspective-taking task with the NIRS device. A block design consisting of 12 blocks was used. The participants observed the facial expressions of the opponents under the two conditions of self-perspective and other's perspective. After a fixation point, the other's facial expression was shown for one second. The participants evaluated the valence of the facial expression on a 7-point scale from "pleasantness" to "unpleasantness" within two seconds.
