PsychologicalReview Copyright 1987 by the American Psychological Assooation, Inc. 1987, Vol.94, No. 4, 469--492 0033-295X/87/$00.75 A Psychomotor Stimulant Theory of Addiction Roy A. Wise and Michael A. Bozarth Concordia University, Montreal, Quebec, Canada The theory is advanced that the common denominator of a wide range of addictive substances is their ability to cause psychomotor activation. This view is related to the theory that all positive reinforcers activate a common biological mechanism associated with approach behaviors and that this mechanism has as one of its components dopaminergic fibers that project up the medial fore- brain bundle from the midbrain to limbic and cortical regions. Evidence is reviewed that links both the reinforcing and locomotor-stimulating effects of both the psychomotor stimulants and the opiates to this brain mechanism. It is suggested that nicotine, caffeine, barbiturates, alcohol, benzodiaz- epines, cannabis, and phencyclidine----each ofwhich also has psychomotor stimulant actions--may activate the docaminergic fibers or their output circuitry. The role of physical dependence in addic- tion is suggested to vary from drug to drug and to be of secondary importance in the understanding of compulsive drug self-administration. Attempts at a general theory of addiction are attempts to isr late--from a variety of irrelevant actionsmthose drug actions that are responsible for habitual, compulsive, nonmedical drug self-administration. The common assumption of addiction the- orists is that general principles of addiction can be learned from the study of one drug and that these principles will have heuris- tic value for the study of other drugs. Thus far, attempts at a general theory of addiction have failed to isolate common ac- tions that can account for addiction across the range of major drug classes. A major stumbling block has been the psychomo- tor stimulants--amphetamine and cocainemwhich do not readily fit models traditionally based on depressant drug classes. The present article offers a new attempt at a general theory of addiction. It differs from earlier theories (e.g., Collier, 1968; Himmelsbach, 1943; Jaffe & Sharpless, 1968; Jellinek, 1960; Kalant, 1977; Lindsmith, 1947; Solomon & Corbit, 1974) in that it is based on the common denominator of the psycho- motor stimulants---amphetamine, cocaine, and related drugs---rather than on the common denominator of the so- called depressant drugs~opiates, barbiturates, alcohol, and others. We take up two topics before presenting the new theory. First, we briefly discuss the heuristic value of a biological approach and suggest that the biologist's distinction between homology and analogy offers a useful insight. Next we discuss the short- comings of earlier theories--variants of dependence theory. Then we outline the new theory and review the relevant evi- dence for its three major assertions: (a) that all addictive drugs have psychomotor stimulant actions, (b) that the stimulant ac- tions of these different drugs have a shared biological mecha- nism, and (c) that the biological mechanism of these stimulant Correspondence concerning this article should be addressed to Roy A. Wise, Concordia University, Sir George Williams Campus, 1455 De Maisonneuve Boulevard West, Montreal, Quebec H3G IMS, Canada. actions is homologous with the biological mechanism of posi- tive reinforcement. Homology and Analogy as Heuristics for Scientific Theory Any attempt to find a common denominator for such diverse conditions as cocaine addiction, alcohol addiction, and heroin addiction is based on the assumption that knowledge of one of these phenomena may provide insights into the others. The assumption is that a common denominator will have heuristic value, that our learning about one addiction will facilitate our learning about another. This assumption need not be valid. In biology there are examples of superficially similar behaviors or organs that have evolved independently, deriving from different ancestors and mechanisms developing out of different embry- onic tissue. Such behaviors or organs are termed analogous, and successful prediction of something about one side of an analogy from knowledge about the other likely reflects the fact that the independent organs or behaviors have evolved under common environmental pressures rather than from a common ancestral origin (Lorenz, 1974). Analogies do not necessarily extend be- yond the superficial similarities that were originally noted (Lo- renz, 1974). The eye of the octopus and the eye of the vertebrate are examples of analogous organs, and the jealousy of the goose and the jealousy of the human are examples of analogous be- haviors; in each case, the analogous details are striking, but there is no commonality of origin and thus no necessary com- monality of mechanism. In other cases, different organs or behaviors can derive from common ancestral origins and common embryonic tissue. Such organs or behaviors are termed homologous: homology is distin- guished from analogy in that homologies result from common biological mechanisms, whereas analogies do not. Since the phyletic or embryonic origins of homologous organs are the same, knowledge of one of a set of homologous organs or behav- iors almost necessarily has some degree of heuristic value for 469