Associate editor: J.L. Katz
Excessive discounting of delayed reinforcers as a trans-disease process contributing to addiction and other disease-related vulnerabilities: Emerging evidence

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Abstract

Delay discounting describes the devaluation of a reinforcer as a function of the delay until its receipt. Although all people discount delayed reinforcers, one consistent finding is that substance-dependent individuals tend to discount delayed reinforcers more rapidly than do healthy controls. Moreover, these higher-than-normal discounting rates have been observed in individuals with other behavioral maladies such as pathological gambling, poor health behavior, and overeating. This suggests that high rates of delay discounting may be a trans-disease process (i.e., a process that occurs across a range of disorders, making findings from one disorder relevant to other disorders). In this paper, we argue that delay discounting is a trans-disease process, undergirded by an imbalance between two competing neurobehavioral decision systems. Implications for our understanding of, and treatment for, this trans-disease process are discussed.

Introduction

Imagine that you cannot wait to engage in an activity, and you do so as often as you can. This behavior, however, causes you and others great sadness and suffering. You want to stop engaging in this behavior, yet you engage in this behavior as often as possible. This scenario is frequently evident among those who are addicted to drugs. Our view is that the choices of the addicted reflect an undervaluation of the longer-term consequences of their behavior, resulting in a limited ability to follow through with plans.

Substantial and growing evidence suggests that this undervaluation may be related to the extensively studied behavioral process known as the discounting of delayed reinforcers. Discounting refers to the observed tendency for the value of reinforcers to decrease as a function of the delay to their delivery (Rachlin & Green, 1972). The systematic measure of this effect, also called “delay discounting,” is often considered a measure of impulsivity (Bickel & Marsch, 2001).

As we will show in this review, drug-dependent individuals discount delayed reinforcers substantially more rapidly than individuals who do not use drugs. These excessive rates of delay discounting are not exclusive to drug dependence, but rather seem to relate to a broader set of disorders. Indeed, as we will address below, excessively high rates of delay discounting are characteristic of individuals with gambling problems, the obese, those who engage in unhealthy actions such as risky sexual behavior, and individuals diagnosed with ADHD or schizophrenia. From our perspective, this evidence of excessive discounting across these differing disorders and disease-related vulnerabilities suggests that excessive discounting should be considered a trans-disease process.

Viewing the excessive discounting of future outcomes as a trans-disease process differs from the traditional and contemporary approach to disease (see Bickel & Mueller, 2009 for a more indepth treatment of the contemporary analysis of disease). Briefly, the contemporary analysis of disease suggests that each disease has its own etiology. This approach is reflected in the variety of scientific organizations focused on particular diseases, and, in the United States, by the various institutes of the National Institutes of Health. The presumption of distinctive etiologies for each disease combines with the need for increasingly specialized research training of scientists to promote a reductionist approach to science, which assumes that a greater understanding of the disease process will be evident in an ever-smaller part of the phenomena under study. Furthermore, to remain fully informed about current developments in a given specialty area, scientists must read and process an ever-increasing number of publications. A consequence is that successful modern disease scientists need to know more and more about less and less. The end result is scientific silos, with failures to recognize characteristics common across different diseases, or across behavioral precursors of various diseases. In other words, researchers working on one disease may be either unaware of relevant observations and experiments of those working on other diseases, or unable to translate relevant observations and methodologies from the study of other diseases into theory and procedures that may advance understanding in their area.

By contrast to the scenario derived from distinct disease etiologies, the notion of trans-disease process suggests that there are processes common across diseases. As a result, the science from one disease can profit from learning about processes in, or precursors of, other diseases. This knowledge of trans-disease processes may drive our understanding of diseases forward — both individually and collectively (see Bickel & Mueller, 2009). Moreover, there may be other trans-disease processes, in addition to excessive discounting, such as those associated with stress (Sapolsky, 2005) or autoimmune response (Rose & Bona, 1993).

In this paper, we will consider humans' excessive discounting of future events a trans-disease process underlying addiction, other disorders, and disease-related behaviors. We will then consider the systematic processes that underlie the discounting of delayed reinforcers, and the dimensional model of addiction that these underlying processes suggest. And lastly, we will consider the implications of this knowledge about excessive discounting for disease therapy.

Section snippets

Research on the discounting of delayed reinforcers

All other things being equal, nearly anyone would prefer to receive a reinforcer (e.g., $1000) now rather than later. This suggests that delayed reinforcers are valued less, or “discounted.” Most individuals, however, do not discount reinforcers at a linearly constant rate. For example, although a reinforcer delivered a year from now may lose half of its subjective value, that same reinforcer would not be worthless (i.e., lose the other half of its subjective value) two years from now. This

The processes behind excessive discounting

If excessive discounting describes a trans-disease process (Bickel & Mueller, 2009) that co- and re-occurs across many patterns of maladaptive responding (e.g., drug addiction, gambling, obesity, etc.), insights into the neurobiological substrates of these patterns of choices may improve our understanding of these various behavioral maladies. Specifically, understanding the neurobiological substrates that underlie excessive delay discounting rates may lead to novel approaches that target these

Implications for treatment

Conceptualizing excessive discounting rates as a trans-disease process may fuel the development of treatments for a range of behavioral maladies. Moreover, the dimensional view that the CNDS view promotes may allow for increased customization of an individual's treatment. Specifically, the particular type of decision systems dysregulation can guide the treatment chosen for a given individual. Consider two cases where there is an imbalance related to addiction. In one case, the impulsive system

Conclusion

In this paper, we suggested that excessive discounting of delayed reinforcers might function as a trans-disease process. To explore that notion, we briefly reviewed important procedural and analytical aspects of discounting and then demonstrated that not only is excessive discounting evident in drug addiction and other disorders such as obesity and ADHD, but also that individuals who excessively discount the future are less likely to engage in health-promoting behaviors. Observing these

Conflict of interest

The authors declare that there are no other potential conflicts of interest.

Acknowledgments

This work was supported by NIDA grants R01DA030241; R01DA024080; R01DA012997; and [NIAAA] R01DA024080-02S1. The authors would like to thank Patsy Marshall for her assistance in manuscript preparation.

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