“Direct Reciprocity” or “Indirect Reciprocity” ‐ The Mechanisms of Infants’ Sharing Behavior

ABSTRACT Cooperation, such as resource sharing, is a key driving force of the evolutionary success of humans. Researchers have also identified other preferences, such as norms of direct reciprocity, indirect reciprocity, and fairness, which can promote large‐scale cooperation. However, the ontogenetic origins of sharing behaviors, whether they develop independently or in an interconnected manner, are still unknown. If these questions are answered, it would reveal the nature of such preferences and help identify the motivations underlying cooperation and sharing. Through a host of experimental tasks, the development of reciprocity in infants aged 20 to 31 months was examined, and their performance was assessed based on the resource‐allocation choices they made. The results of this study supported three conclusions. First, 20‐month‐old infants did not demonstrate a clear preference for either direct or indirect reciprocity when both options were presented. Second, 25‐month‐old infants showed a significant tendency to share with the direct reciprocator in Experiment 1, but an independent sample of 25‐month‐olds in Experiment 2 showed no clear preference for either reciprocator. This indicated that reciprocity‐based sharing at 25 months is not yet robust across similar tasks. Third, 31‐month‐old infants showed a more pronounced reciprocal pattern, which suggested that they favored direct reciprocators when both direct and indirect reciprocity options were presented. Overall, these findings suggested that reciprocity begins to shape infants’ sharing behaviors during the 20‐ to 31‐month developmental window, but that the pattern for 25‐month‐olds requires replication and careful comparison across highly similar tasks.