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.

An extension of the generalized matching law incorporating context effects on terminal-link sensitivity is proposed as a quantitative model of behavior under concurrent chains. The contextual choice model makes many of the same qualitative predictions as the delay-reduction hypothesis, and assumes that the crucial contextual variable in concurrent chains is the ratio of average times spent, per reinforcement, in the terminal and initial links; this ratio controls differential effectiveness of terminal-link stimuli as conditioned reinforcers. Ninety-two concurrent-chains data sets from 19 published studies were fitted to the model. Averaged across all studies, the model accounted for 90% of the variance in pigeons' relative initial-link responding. The model therefore demonstrates that a matching law analysis of concurrent chains-the assumption that relative initial-link responding equals relative terminal-link value-remains quantitatively viable. Because the model reduces to the generalized matching law when terminal-link duration is zero, it provides a quantitative integration of concurrent schedules and concurrent chains.

The incidence rates reported here are much higher than those previously found. It is clear that TBIs constitute a major health issue and therefore it is important to have accurate information to enable planning for primary healthcare services and to inform prevention programmes.

Measures of treatment change based on offender self-reports and structured clinical rating systems show convergent and predictive validity, which suggests that effective treatment that targets dynamic risk factors leads to a reduction in sexual recidivism.

Nevin (1979) noted that preference in concurrent chains and resistance to change in multiple schedules were correlated, in that both measures were affected similarly by variations in parameters of reinforcement such as rate, immediacy, and magnitude. To investigate the relationship between preference and resistance to change directly, we used a within-session procedure that arranged concurrent chains in one half of the session and a multiple schedule in the other half. The same variableinterval schedules served as terminal links in concurrent chains and as the components of the multiple schedule, and were signaled by the same stimuli. After performances had stabilized, responding in the multiple schedule was disrupted by delivering response-independent reinforcement during the blackout periods between components. Both preference in concurrent chains and relative resistance to change of multiple-schedule responding were well described as power functions of relative reinforcement rate, as predicted by current quantitative models (Grace, 1994;Nevin, 1992b). In addition, unsystematic variation in preference and resistance to change was positively correlated, which suggests that preference and resistance to change are independent measures of a single construct. That construct could be described as the learning that occurs regarding the prevailing conditions of reinforcement in a distinctive stimulus situation.Key words: resistance to change, multiple schedules, choice, concurrent chains, response strength, key peck, pigeonsBased on an analogy to Newtonian mechanics, behavioral momentum theory (Nevin, 1992b;Nevin, Mandell, & Atak, 1983) identifies two independent dimensions of operant behavior: resistance to change and response rate. Resistance to change is analogous to mass, and response rate is analogous to velocity; their product is behavioral momentum. Although response rate and resistance to change often covary, their independence has been demonstrated in a number of studies employing a variety of multiple schedules (Nevin, 1984(Nevin, , 1992aNevin, Smith, & Roberts, 1987;Nevin, Tota, Torquato, & Shull, 1990). The general conclusion has been that response rate is determined primarily by operant response-reinforcer contingencies, whereas resistance to change is determined by Pavlovian stimulus-reinforcer Portions of these data were presented at the meeting of the Eastern Psychological Association, Boston, March, 1995. We thank Anthony McLean and an anonymous reviewer for helpful comments. Research was supported by NSF Grant IBN-9507584 to the University of New Hampshire (John A. Nevin, Principal Investigator), and by an NSF Predoctoral Fellowship and UNH Dissertation Year Fellowship to Randolph C. Grace.Address correspondence and reprint requests to either author at the University of New Hampshire, Department of Psychology, Durham, New Hampshire 03824 (E-mail: rcg@christa.unh.edu or t.nevin@worldnet.att.net).contingencies. Resistance to change is measured as the reciprocal of the rate of decrease in response rate...