Undermatching, melioration and the discrimination of local reinforcer rates.

Melioration theory suggests that performance in choice situations arises from a process in which subjects allocate more time to alternatives that provide the higher local rate of reinforcers. If this process is unconstrained (as in concurrent aperiodic schedules), melioration predicts a strict equality between molar time-allocation (and, as a result, response-allocation) ratios and molar ratios of overall obtained reinforcers on concurrent variable-interval schedules. The available data, however, suggest that log behavior measures are nearer to indifference (called undermatching that predicted by this theory. The modification of melioration theory suggested here shows that, if animals cannot discriminate local reinforcer-rate differences below a fixed threshold, undermatching is predicted, and the degree of undermatching depends on the absolute size of the threshold. It also predicts the finding that the sensitivity of behavior ratios to changes in reinforcer ratios falls with decreasing overall reinforcer rates.