Object sorting into a two-dimensional array in humans and chimpanzees.

Object-sorting tasks have been used as a means of assessing the cognitive development of humans. In order to investigate cognitive development from a comparative perspective, an object-sorting task was conducted in a longitudinal face-to-face situation involving three juvenile/adolescent chimpanzees (7-9 years old) and 17 children (2-5 years old). The subjects were requested to place nine blocks of different categories (distinguished by three colors and three shapes) into the cells of a box arrayed in a three-by-three pattern. Chimpanzees showed complete or partial categorical sorting in 24-43% of pre-cued trials. The youngest children had difficulty in completing a trial by placing all nine blocks into the box. Humans older than 2 years succeeded in making a one-to-one correspondence by placing a block in each cell, while the end-state pattern remained random. The children gradually increased their rate of categorical sorting, where objects of one category were placed in the same row/column; this tendency peaked at 4 years of age. Above this age, the humans spontaneously shifted their sorting strategy to make a completely even configuration (resulting in a Latin square), which may be more cognitively demanding than categorical sorting. While chimpanzees and older children used both color and shape cues for categorical sorting, younger humans preferred to use shape cues. The results of the present study show fundamental similarities between humans and chimpanzees at the basic level of categorical sorting, which indicates that some autonomous rules are applied during object manipulation.