Reactions to Intrinsic and Extrinsic Rewards in Protein-Malnourished Monkeys

Abstract

10 rhesus monkeys were tested on a 12-part puzzle manipulation apparatus. 6 of 10 Ss were maintained on a protein-deficient diet containing 3.5% casein by weight, while the remaining 4 Ss received a high-protein diet containing 25% casein by weight. The high-protein animals had an elevated manipulation rate as compared with low-protein fed Ss when the only source of reinforcement was the manipulation of the puzzles (intrinsic reward). However, when food (extrinsic reward) was introduced the low-protein fed Ss manipulated at a level equal to or greater than the high-protein fed animals. Introduction of 100% and partial reinforcement conditions showed manipulation rates to be relatively consistent between the two groups. Extinction conditions, however, showed a significant difference between the high- and low-protein fed animals after partial reinforcement. With the removal of the food reward the low-protein fed animals exhibited a much more rapid reduction of manipulatory activity than the high-protein fed Ss. The more rapid extinction of the manipulation response by low-protein fed monkeys suggests that extrinsic reward is a much more salient variable for these Ss than intrinsic reward.