Chimpanzees' (Pan troglodytes) internal arousal remains elevated if they cannot themselves help a conspecific.

Chimpanzees help conspecifics achieve their goals in instrumental situations, but neither their immediate motivation nor the evolutionary basis of their motivation is clear. In the current study, we gave chimpanzees the opportunity to instrumentally help a conspecific to obtain food. Following recent studies with human children, we measured their pupil diameter at various points in the process.

Chimpanzees help others with what they want; children help them with what they need.

Humans, including young children, are strongly motivated to help others, even paying a cost to do so. Humans' nearest primate relatives, great apes, are likewise motivated to help others, raising the question of whether the motivations of humans and apes are the same. Here we compared the underlying motivation to help in human children and chimpanzees.