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In all human cultures, people gesture iconically. However, the evolutionary basis of iconic gestures is unknown. In this study, chimpanzees and bonobos, and 2- and 3-year-old children, learned how to operate two apparatuses to get rewards. Then, at test, only a human adult had access to the apparatuses, and participants could instruct her about how to obtain the rewards. Children frequently produced appropriate iconic gestures, but with the exception of one human-raised chimpanzee, great apes did not gesture iconically. However, chimpanzees pointed to a reward outside the apparatus in another experimental condition, showing their motivation and ability to communicate with the human to request it. They also manipulated a duplicate apparatus in appropriate ways, though it was unclear if they did this to communicate with the human. Although great apes may have some of the prerequisite skills involved, iconic gestures come naturally to humans in a way that they do not for great apes.

By 3 years of age, children can solve tasks involving physical principles such as locating a ball that rolled down a ramp behind an occluder by the position of a partially visible solid wall (Berthier, DeBlois, Poirer, Novak, & Clifton, 2000; Hood, Carey, & Prasada, 2000). However, the extent to which children use physical information (the properties of the wall) remains unclear because spatial information would suffice (the location of the wall in relation to the ball). We confronted 2- to 6-year-old children with a ball resting on a shelf inside a clear plastic-fronted box. To retrieve the ball, children had to roll it away from a trap or barrier using their fingers. Crucially, a single object acted as a barrier or supporting surface in different conditions, thus requiring a flexible response. Preschoolers solved the task and the critical transfers from 2.5 years of age (Study 1). Interestingly, 2.5-year-olds required to use a tool to displace the ball performed significantly worse than those who could use their fingers (Study 2). In contrast, 2.5- to 4.5-year-olds failed a "covered" trap box that provided only 2-dimensional predictive cues without any visible physical information, and even 6.5-year-olds performed significantly worse on the covered task compared to the uncovered one (Studies 3 and 4). Our results suggest that children from around 2.5 years of age integrate spatial and physical information when solving problems like the trap box task, rather than simply exploit spatial relationships between features.

Human societies are characterized by more cultural diversity than chimpanzee communities. However, it is currently unclear what mechanism might be driving this difference. Because reliance on social information is a pivotal characteristic of culture, we investigated individual and social information reliance in children and chimpanzees. We repeatedly presented subjects with a reward-retrieval task on which they had collected conflicting individual and social information of equal accuracy in counterbalanced order. While both species relied mostly on their individual information, children but not chimpanzees searched for the reward at the socially demonstrated location more than at a random location. Moreover, only children used social information adaptively when individual knowledge on the location of the reward had not yet been obtained. Social information usage determines information transmission and in conjunction with mechanisms that create cultural variants, such as innovation, it facilitates diversity. Our results may help explain why humans are more culturally diversified than chimpanzees., (© 2014 The Author(s) Published by the Royal Society. All rights reserved.)

G. Gergely, H. Bekkering, and I. Király (2002) showed that 14-month-old infants imitate rationally, copying an adult's unusual action more often when it was freely chosen than when it was forced by some constraint. This suggests that infants understand others' intentions as rational choices of action plans. It is important to test whether apes also understand others' intentions in this way. In each of the current 3 studies, a comparison group of 14-month-olds used a tool more often when a demonstrator freely chose to use it than when she had to use it, but apes generally used the tool equally often in both conditions (orangutans were an exception). Only some apes thus show an understanding of others' intentions as rational choices of action plans.