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Is Addiction a Heterogeneous Condition? Reflections on Pickard's “The Purpose in Chronic Addiction”

Both the traditional disease model of addiction and Pickard’s “purposive” model (Pickard 2012) attempt to define the motivational influence exerted by substances of abuse on agents that influence the agents’ behavior toward further substance consumption. Such a definition would seem to take the form “For all substances S that cause addiction/dependence, there exists a kind of motivational influence M such that agents affected by M are more inclined to engage in substance-related behaviors B.” In these terms, the argument between Pickard and proponents of the disease model is over the nature of M, and evidence such as the epidemiological data cited by Pickard and neurological changes cited by disease theories (e.g., Everitt and Robbins 2005) are attempts to support differing conceptions of M. Specifically, the disease model argues that M is a change in control from mechanisms underlying voluntary to those underlying compulsive action, whereas Pickard argues that M is a combination of five factors that converge around a putative reason or “purpose” for the agent to consume the substance. This kind of definition assumes a homogeneity in the mechanism by which substances cause dependence. That is, it assumes that M is sufficiently similar across substances such that all substances that can lead to dependence or addiction do so in a sufficiently similar way so that we can classify each of their influences as belonging to M, and all of the substances as belonging to S. However, there is good empirical evidence to suggest that the mechanisms by which substance use results in dependence differ significantly. Pickard’s claim that substance use in nonpsychiatric populations drops off after age 30 assumes that the age of addicted patients does not vary to a significant degree across substances. Yet nicotine dependence does not vary with age (Hughes, Helzer, and Lindberg 2006). Pickard’s conception thus seems ill-suited to explain nicotine dependence: Either her epidemiological claims are unrelated to dependence, or the putative reasons for substance use described by Pickard do not apply to nicotine dependence. The different pattern of use suggests a distinct effect or mechanism underlying nicotine dependence. Similarly, the particular neurological changes thought to underlie the shift to compulsive use (Everitt and Robbins 2005) do not happen after chronic use of all substances that can result in dependence. Specifically, chronic cannabis use