Strategies of schedule preference in chimpanzees.

Two chimpanzees were required to choose between a fixed-ratio schedule and a progressive-ratio schedule which increased in response requirement by 20 responses each time it was chosen. Each choice of the fixed ratio reset the progressive ratio to its minimum value. The fixed-ratio requirement was varied from 40 to 1000 responses. The subjects' preferences for the progressive-ratio schedule varied as a function of the magnitude of the fixed-ratio requirement. An analysis of the preference data indicated that the animals tended to minimize reinforcement cost rather than match the progressive-ratio requirement to the fixed-ratio requirement. In a second experiment, selection of the fixed ratio did not reset the progressive-ratio requirement to its minimum value. In this case, the animals matched the progressive-ratio requirement to the fixed-ratio requirement. A model based on reinforcement cost is presented which permits accurate prediction of preferences between fixed and progressively increasing ratio schedules.