Nevin & Grace's approach is an interesting and useful attempt to find ways to measure “core” effects of a history of exposure to reinforcement. The momentum analogy makes intuitive sense, and the evidence for its utility is increasing. Several questions remain, however, about how the analogy will fare in the case of concurrent rather than sequential activities, about the use of extinction as a method to test resistance to change, and about the generality of some of the effects.

Heyman's view that addiction can be viewed as a natural outcome predictable by melioration and the matching law is provocative. Remaining to be explained more fully, however, are exactly how his view is an improvement on other reinforcement-based accounts. Included in these elaborations should be an account of how different “bookkeeping schemes” are developed and controlled and what new approaches to treatment and prevention of drug addiction are indicated.