Return the favour: Preverbal infants represent direct reciprocity

Return the favour: Preverbal infants represent direct reciprocity. Joakim Haugane Zahl, Erik Kjos Fonn, Oda Eidjar, Lotte Thomsen. Human Behavior and Evolution Society 31st annual meeting. Boston 2019. http://tiny.cc/aa1w6y

Abstract> If direct reciprocity sustains selective altruism and cooperation among non-kin (Trivers, 1971), early-developing representations of reciprocity might evolve to facilitate the navigation of such social relations. Here, we show that preverbal infants represent direct reciprocity. We familiarized 32 7-12 month-old infants to a scenario with three novel agents where the benefactor gave one of his two apples to the beneficiary who had none (the third agent simply had one apple). In test trials the former beneficiary now had two apples, while both other agents had none. In Expected trials it  reciprocated by giving its surplus apple to its former benefactor, in Unexpected trials it instead gave it to the third agent. We found that nine-to-twelve month-olds looked longer at unexpected than expected trials (M_unexpected=27,8 seconds; M_expected=21,5; p<.0005, BF10>550), indicating that they expected agents to act reciprocally, but 7-8 month-olds did not. A second study demonstrated that reciprocity is generalized across resources (receiving an apple and returning a banana). Two control studies demonstrated that these effects are specific to resource distributions among self-propelled, intentional agents and not accounted for by low-level mechanisms of mere association.