Differences, not ratios, control choice in an experimental analogue to foraging.

In choice between outcomes with different delays to reinforcement, most theories require that choice be governed by the ratio of the delays, not by the difference between them, a requirement also consistent with Weber's law. Instead, delay-reduction theory and optimal-foraging theory stipulate, under conditions of the present experiments, that the difference between the delays, and not the ratio between them, controls choice. This prediction was assessed using a procedure, widely used in foraging experiments, in which pigeons chose between accepting and rejecting either of two delays when offered. Across conditions, the delays either differed by a constant amount, with the ratio between the delays varying, or differed by changing amounts, with the ratio between the delays constant. In each of six experiments, rate of acceptance of the longer delay depended only on the difference between the two delays and not on the ratio between them, supporting delay-reduction and foraging theory.