Abstract
Cognitive development of chimpanzees was directly compared to that of human children in the diagnostic test of “stacking blocks”. Four 4-year-old, one 10-year-old (Ai), and one 27-year-old (Sarah) chimpanzees were served as subjects. The ability of stacking blocks (1 inch cubes) was tested with non-differential reinforcement using social praise in two situations. In the “free stacking” situation, the subjects received a bunch of blocks at once and was asked to use them up to build as many towers as they like. After the subjects received a number of sessions for the “exercise”, they were tested in the “forced stacking” situation, in which the subjects received each block one at a time and was asked to make a high tower. All the chimpanzees constructed the tower of at least 9 blocks. The highest tower made by the chimpanzees had 12 stories. The older two chimpanzees were a little superior to the younger ones on the average. The level of performance matches that of 2-year-old human children. The chimpanzees adjusted the four corners of blocks to make a straight tower and rejected to put one more block “foreseeing” how the tower of blocks turns out. The cognitive aspects found in the stacking behavior were discussed in the framework of the protogrammar of action.
