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Direct reciprocity plays an essential role in forming cooperative relationships. Direct reciprocity requires individuals to keep track of past interactions and condition their behavior on the previous behavior of their partners. In controlled experimental situations, it is known that children establish direct reciprocity according to the partner's behavior, but this has not been verified in real life. This study aims to identify the establishment of short-term direct reciprocity in response to peers' behaviors among Japanese preschoolers aged 5 and 6. It employs naturalistic observation at a nursery school. In addition, the psychological process for direct reciprocity was examined. The findings demonstrated that after receiving prosocial behavior, the recipient child returned the prosocial behavior more frequently within 7 minutes, compared with control situations; this suggests that 5-to 6-year-olds formed direct reciprocity in the short term when interacting with their peers. Additionally, recipient children tended to display affiliative behavior after receiving prosocial behavior. Positive emotions toward initiating children may have been caused by receiving prosocial behavior, and this psychological change modified short-term direct reciprocity.

Social indirect reciprocity seems to be crucial in enabling large-scale cooperative networks among genetically unrelated individuals in humans. However, there are relatively few studies on social indirect reciprocity in children compared to adults. Investigating whether young children have a behavioral tendency toward social indirect reciprocity will help us understand how and when the fundamental ability to form cooperative relationships among adults is acquired. Using naturalistic observation at a nursery school, this study examined whether 5- to 6-year-olds show a behavioral tendency to engage in social indirect reciprocity in response to their peers' prosocial behavior toward a third party. The results revealed that bystander children tended to display prosocial behavior toward their peers more frequently after observing these peers' prosocial behavior toward third-party peers, compared with control situations; this suggests that 5- to 6-year-olds may have an essential behavioral tendency to establish social indirect reciprocity when interacting with peers in their daily lives. In addition, bystanders tended to display affiliative behavior after observing focal children's prosocial behavior. In other words, observing peers' prosocial behavior toward third-party peers evoked bystanders' positive emotions toward the helpers. Considering both the present results and previous findings, we speculate that in preschoolers, such positive emotions might mediate the increase in the bystander's prosocial behavior toward the helper. In addition, an intuitional emotional process plays an important role in the preschooler's behavioral tendency toward social indirect reciprocity in natural interactions with peers.