The behavioural development of 90 chimpanzees (Pan troglodytes) was followed. Of these, 65 had been separated from their mothers to prevent casualties or, at a later age, to increase breeding success. Some showed body rocking and others did not. To obtain insight into the causes of the onset and development of body rocking, chimpanzees raised with peers, with their mother, or in a semi-natural group were compared Rocking was never observed in the semi-natural group. It was occasionally seen when with the mother. Separation from the mother soon after birth induced anxiety and rocking developed after sitting upright had developed. Rocking levels of three per cent of the time were still present at seven to nine years of age.

The most probable causes of the development of rocking are frustrating social circumstances and the inability to cope with these. Merging groups, disturbances and the introduction of a fearful object increased rocking in the individuals that had developed the habit. However, rockers reacted less to those circumstances than non-rockers, showing smaller increase in body contact and less reduction of play. This finding suggests that rocking, instead of being a bizarre reaction to unsurmountable stress, could be a behaviour that helps an individual to cope with difficulties and stress. In terms of animal welfare, rocking in chimpanzees housed in laboratory conditions is an indication of a less optimal reaction pattern to frustrating circumstances. The development of rocking may be prevented if the babies are left with the mother and in their social group. Rocking after (late) separation may be prevented when transfer takes place together with familiar peers.