Comprehension of novel communicative signs by apes and human children.

Forty-eight young children (2.5 and 3.0 years old) and 9 great apes (6 chimpanzees and 3 orangutans) participated in a hiding-finding game. An adult human experimenter (the Hider) hid a reward in 1 of 3 opaque containers aligned on a wooden plank. Another adult experimenter (the Communicator) attempted to help the subject find the reward by giving 1 of 3 types of communicative sign: (1) Pointing, for which she placed her hand directly above the correct container with index finger oriented down; (2) Marker, for which she placed a small wooden block on top of the correct container; and (3) Replica, for which she held up a perceptually identical duplicate of the correct container. At both ages, children were above chance in this finding game with all 3 types of communicative sign, with Pointing being easiest (because they knew it prior to the experiment), Marker being next easiest, and Replica being most difficult. In contrast, no ape was above chance for any of the communicative signs that it did not know before the experiment (some had been trained in the use of the marker previously, and one knew pointing), nor was group performance above chance for any of the signs, despite the fact that apes experienced three times as many trials as children on each sign. Our explanation of these results is that young children understand the communicative intentions of other persons--although they may have more difficulty comprehending the exact nature of those intentions in some cases--whereas apes treat the behavioral signs of others as predictive cues only (signals). This may be because apes do not perceive and understand the communicative intentions of others, at least not in a human-like way.