Dr. Duncan developed the self-medication hypothesis while he was with the Texas Research Institute of Mental Sciences. Duncan based his hypothesis on extensive clinical experience with addicts and on principles of learning that had been established by experimental psychology. It was initially published in two papers in 1974. The first was a discussion of reinforcement of drug misuse that appeared in the Clinical Toxicology Bulletin. The second was a response in the American Journal of Psychiatry to Khantzian, Mack, and Schatzberg's (1974) paper on heroin use as a coping mechanism. It was in this second publication that Duncan compared drug misuse to a Bandaid. A further elaboration of the hypothesis appeared the following year in an invited paper for the Journal of Psychedelic Drugs.
The distinction between drug use and drug misuse (or drug abuse as it was termed prior to ICD-10) is critical to the self-medication hypothesis. Dr. Duncan has long asserted that most of the people who take illegal drugs do not meet criteria for substance misuse, let alone for dependence. The great majority exercises control and restraint over their drug use and suffers no harm from using. His model is concerned with describing why a minority (roughly 10-20%) of those who take drugs non-medically do lose control over their use and expose themselves to serious social, interpersonal and medical risks. He has explored the implications of his hypothesis for the primary prevention of drug dependence, the treatment of drug dependence. and the tertiary prevention of the destructive consequences of drug dependence.
Duncan was by no means the first to develop a behavioural model of addiction. Other formulations of drug misuse and addiction as an operant behaviour have dealt with the euphorogenic effects of drugs as positive reinforcement or avoidance of withdrawal sickness as negative reinforcement. Duncan, on the other hand, asserted that drug use is maintained by positive reinforcement (principally from the pleasurable effects of the drug) but that drug dependence is not. It is drug dependence that the self-medication hypothesis attempts to explain.
Drug dependence is the term that has formally replaced addiction in medical terminology. In 1964 the World Health Organizations Expert Committee on Drug Abuse (Eddy, et al., 1965) proposed that the terms addiction and habituation be replaced with the term dependence and distinguished between two types -- psychic dependence and physical dependence. In psychic dependence (which is often referred to as psychological dependence) "there is a feeling of satisfaction and psychic drive that requires periodic or continuous administration of the drug to produce pleasure or to avoid discomfort." Physical dependence, on the other hand, involves the development of tolerance and the occurrence of withdrawal symptoms upon cessation of use of the drug, as a consequence of the body's adaptation to the continued presence of the drug. In Duncan's view, physical dependence is a lesser problem that occurs in addiction to some drugs, such as alcohol and opiates, but not in addiction to cocaine, marijuana, etc., nor in non-drug addictions such as compulsive gambling. It has nothing to do with the problem of relapse that is so much a part of the public health problem of drug dependence.
Duncan argues that drug dependence is just another name for avoidance or escape behaviour when the operant behaviour being negatively reinforced is drug taking. Drug addicts, in his view, have found a drug that provides them with temporary escape from an ongoing state of emotional distress which might be due to a mental disorder, to psychosocial stress, or to an aversive environment. Non-drug addictions, in his opinion, represent similar negatively reinforced behaviour.
Duncan asserts that the characteristics of dependence are all typical of operant behaviours maintained by negative reinforcement. Negative reinforcement maintains high rates of behaviour. Animals that have been negatively reinforced for performing a task such as pressing a bar will often do so to the exclusion of eating, sleeping, sexual activity, etc.. Avoidance behaviours are highly resistant to extinction and even when they appear to have been eliminated, they tend to recur spontaneously. Thus, in Duncan's view, the intensity, compulsiveness and proneness to relapse that characterize drug dependence all result from the fact that the behaviour is maintained by negative reinforcement.
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