Adjusting delay to reinforcement: comparing choice in pigeons and humans.

Indifference functions of amount and delay of reinforcement were compared for pigeon and human subjects by using Mazur's (1987) adjusting delay procedure. A model similar to the matching law (the simple reciprocal model), a hyperbolic model, three modified versions of the hyperbolic model, and a negative exponential model were evaluated. In Experiment 1 the subjects were pigeons, and in Experiments 2 and 3 the subjects were humans. In order to make the nonhuman and human situations more comparable, in Experiments 2 and 3 the reinforcer (points exchangeable for money) was discounted at a constant rate during the delay periods. The rate of this discounting varied between Experiments 2 and 3. The results of all three experiments demonstrated that a power function transformation of the hyperbolic model (in which 1 is added to the delays in the denominator of the simple reciprocal model) provided the best description of both nonhuman and human data.