Human children 18 – 24 months of age and 3 young chimpanzees interacted in 4 cooperative activities with a human adult partner. The human children successfully participated in cooperative problem-solving activities and social games, whereas the chimpanzees were uninterested in the social games. As an experimental manipulation, in each task the adult partner stopped participating at a specific point during the activity. All children produced at least one communicative attempt to reengage him, perhaps suggesting that they were trying to reinstate a shared goal. No chimpanzee ever made any communicative attempt to reengage the partner. These results are interpreted as evidence for a uniquely human form of cooperative activity involving shared intentionality that emerges in the second year of life. From soon after birth, human infants interact with other persons dyadically in coordinated, turn-taking sequences (Trevarthen, 1979). From about 6 to 9 months of age, infants ’ social interactions become more complex, as they often incorporate outside objects and so become triadic (Tomasello, 1995). Some of these triadic interactions are relatively extended and maintain a turn-taking structure, for example, rolling a ball back and forth or taking turns beating a drum (Gustafson, Green, & West, 1979; Ratner & Bruner, 1978). Most of these early triadic interactionsFsometimes called cooperative gamesFseem to rely on adult scaffolding in fairly ritualized situations, because the introduction of novel toys or a peer partner disrupts them almost totally until 18 months of age (Hay, 1979; Ross, 1982). In a series of longitudinal studies, Eckerman and colleagues have investigated the emergence of young children’s skills in cooperative games of a less ritualized

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