Rats don't always respond faster for more food: the paradoxical incentive effect

Rats' leverpressing was reinforced on variable-ratio (VR) schedules. As ratio values increased, response rates initially increased with them, then eventually decreased. In Experiment 1, rates were uniformly higher with one-pellet reinforcers than with two-pellet reinforcers- the paradoxical incentive effect. Killeen's (1994) mathematical principles of reinforcement (MPR) described the data quantitatively but failed to predict the advantage for the one-pellet condition. In Experiment 2, rats received one-, two-, and three-pellet reinforcers with counterbalanced preloads of pellets; the continued superiority of the smaller reinforcers ruled out a satiation explanation. Experiment 3 introduced a 20-sec intertrial interval (ITI), and Experiment 4 filled the ITI with an alternate response to test a memorial/overshadowing explanation. In Experiment 5, the rats received one or two standard grain pellets or one sucrose pellet as reinforcers over an extended range of ratios. Once again, rates were higher for one than for two pellets at short to moderate VR values; thereafter, two pellets supported higher response rates. The diminution of the effect in Experiment 3 and its reversal in Experiment 4 and in Experiment 5 at large ratios provided evidence for overshadowing and reconciled the phenomenon with MPR.