In the metaphor of behavioral momentum, the rate of a free operant in the presence of a discriminative stimulus is analogous to the velocity of a moving body, and resistance to change measures an aspect of behavior that is analogous to its inertial mass. An extension of the metaphor suggests that preference measures an analog to the gravitational mass of that body. The independent functions relating resistance to change and preference to the conditions of reinforcement may be construed as convergent measures of a single construct, analogous to physical mass, that represents the effects of a history of exposure to the signaled conditions of reinforcement and that unifies the traditionally separate notions of the strength of learning and the value of incentives. Research guided by the momentum metaphor encompasses the effects of reinforcement on response rate, resistance to change, and preference and has implications for clinical interventions, drug addiction, and self-control. In addition, its principles can be seen as a modern, quantitative version of Thorndike's (1911) Law of Effect, providing a new perspective on some of the challenges to his postulation of strengthening by reinforcement.

In several different experiments, pigeons were trained with one schedule or condition of food reinforcement for pecking in the presence of one key color, and a different schedule or condition in the presence of a second key color. After responding in both of these multiple schedule components stabilized, response-independent food was presented during dark-key periods between components, and the rates of pecking in both schedule components decreased. The decrease in responding relative to baseline depended on the frequency, magnitude, delay, or response-rate contingencies of reinforcement prevailing in that component. When reinforcement was terminated, decreases in responding relative to baseline rates were ordered in the same way as with response-independent food. The relations between component response rates were power functions. Internal consistencies in the data, in conjunction with parallel findings in the literature, suggest that the concept of response strength summarizes the effects of diverse procedures, where response strength is identified with relative resistance to change. The exponent of the power function relating response rates may provide the basis for scaling response strength.

Two multiple-schedule experiments with pigeons examined the effect of adding food reinforcement from an alternative source on the resistance of the reinforced response (target response) to the decremental effects of satiation and extinction. In Experiment 1, key pecks were reinforced by food in two components according to variable-interval schedules and, in some conditions, food was delivered according to variable-time schedules in one of the components. The rate of key pecking in a component was negatively related to the proportion of reinforcers from the alternative (variable-time) source. Resistance to satiation and extinction, in contrast, was positively related to the overall rate of reinforcement in the component. Experiment 2 was conceptually similar except that the alternative reinforcers were contingent on a specific concurrent response. Again, the rate of the target response varied as a function of its relative reinforcement, but its resistance to satiation and extinction varied directly with the overall rate of reinforcement in the component stimulus regardless of its relative reinforcement. Together the results of the two experiments suggest that the relative reinforcement of a response (the operant contingency) determines its rate, whereas the stimulus-reinforcement contingency (a Pavlovian contingency) determines its resistance to change.Key words: alternative reinforcement, response rate, resistance to change, concurrent schedules, multiple schedules, satiation, extinction, key peck, pigeon Experimental analysis has distinguished two aspects of operant behavior: the rate of a response and the resistance of that rate to reduction by procedures such as satiation and extinction. These two aspects of behavior are of interest because they vary in orderly ways as functions of rate of reinforcement (Catania & Reynolds, 1968;Nevin, 1974Nevin, , 1979Skinner, 1938Skinner, , 1950 and because of their relation to the theoretical concept of response strength. Although response rate has been taken as equivalent to response strength (Skinner, 1938(Skinner, , 1950 to be (Nevin, 1974(Nevin, , 1979Smith, 1974). Consequently, it is of some importance to examine the variables that influence resistance to change (Fath, Fields, Malott, & Grossett, 1983;Nevin, 1974Nevin, , 1979Nevin, , 1984Nevin, Mandell, & Yarensky, 1981;Nevin, Smith, & Roberts, 1987).The rate of a target response maintained by a given rate of reinforcement decreases when reinforcers are added concurrently from an alternative source. This decrease occurs both when reinforcers are added noncontingently (Rachlin & Baum, 1972) and when they are contingent on a different, concurrent response (Catania, 1963). Adding reinforcers from an alternative source may be viewed as degrading the operant contingency, in that the correlation between the occurrence of the target response and the reinforcer is thereby weakened. Thus, alternative reinforcement might reduce response rate by degrading the operant contingency.If a target response's rate and resistance to chan...