‘I see you sharing, thus I share with you’: indirect reciprocity in toddlers but not infants

Human societies are organised around cooperative interactions, the origins and development of which have become a timely topic. In this study, we investigated the development of indirect reciprocity in 18–24-month-old toddlers, and infants aged 6 months, on a two-phase sharing task with non-familiar individuals. In the first phase, we observed whether infants and toddlers differentiated and manifested a preference toward an individual altruistically sharing or acting selfishly. In the second phase, infants and toddlers interacted with the same prosocial and antisocial individuals seen in the first phase, and we observed whether they were willing to share with one of the two. Indirect reciprocity was assessed as the match between the preferences for the prosocial individual in phase one, and the first-person sharing in the second phase. Evidence showed that toddlers, but not infants, indirectly reciprocated the prosocial individual, suggesting that understanding of such a complex behaviour as indirect reciprocity may require prolonged experience in order to emerge.