Preference between variable-ratio and fixed-ratio schedules: local and extended relations

Although it has repeatedly been demonstrated that pigeons, as well as other species, will often choose a variable schedule of reinforcement over an equivalent (or even richer) fixed schedule, the exact nature of that controlling relation has yet to be fully assessed. In this study pigeons were given repeated choices between concurrently available fixed-ratio and variable-ratio schedules. The fixed-ratio requirement (30 responses) was constant throughout the experiment, whereas the distribution of individual ratios making up the variable-ratio schedule changed across phases: The smallest and largest of these components were varied gradually, with the mean variable-ratio requirement constant at 60 responses. The birds' choices of the variable-ratio schedule tracked the size of the smallest variable-ratio component. A minimum variable-ratio component at or near 1 produced strong preference for the variable-ratio schedule, whereas increases in the minimum variable-ratio component resulted in reduced preference for the variable-ratio schedule. The birds' behavior was qualitatively consistent with Mazur's (1984) hyperbolic model of delayed reinforcement and could be described as approximate maximizing with respect to reinforcement value.