International Journal of Comparative Psychology

Three of four monkeys were trained successfully on a series of number-related judgment problems ending with "same" and "different" judgments involving pairs of numerousness discriminanda. The discriminanda were black "dots" drawn on cards and constructed using controls to preclude the use of cumulative area or brightness cues and to make specific pattern memorization unlikely. On the final task, all possible same and different pairs of discriminanda representing the numbers 2 through 6 were used, and three monkeys met criterion (two successive sessions of 80% or more correct) in 80, 160, and 200 trials, respectively. Discussion considered possible underlying processes to explain the numerousness judgments as well as the implications of the present work for Piaget's views on conservation of quantity.