View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Frontiers - Publisher Connector REVIEW ARTICLE published: 12 March 2014 BEHAVIORAL NEUROSCIENCE doi: 10.3389/fnbeh.2014.00076 Does temporal discounting explain unhealthy behavior? A systematic review and reinforcement learning perspective Giles W. Story 1,2*, Ivo Vlaev 1, Ben Seymour 3,AraDarzi1 and Raymond J. Dolan 2 1 Department of Surgery and Cancer, Centre for Health Policy, Institute of Global Health Innovation, Imperial College London, London, UK 2 Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London, London, UK 3 Center for Information and Neural Networks, National Institute for Information and Communications Technology, Tokyo, Japan Edited by: The tendency to make unhealthy choices is hypothesized to be related to an individual’s Jack Van Honk, Utrecht University, temporal discount rate, the theoretical rate at which they devalue delayed rewards. Netherlands Furthermore, a particular form of temporal discounting, hyperbolic discounting, has Reviewed by: been proposed to explain why unhealthy behavior can occur despite healthy intentions. Barak Morgan, University of Cape Town, South Africa We examine these two hypotheses in turn. We first systematically review studies Matthew Wanat, University of Texas which investigate whether discount rates can predict unhealthy behavior. These studies at San Antonio, USA reveal that high discount rates for money (and in some instances food or drug *Correspondence: rewards) are associated with several unhealthy behaviors and markers of health Giles W. Story, Department of status, establishing discounting as a promising predictive measure. We secondly Surgery and Cancer, Centre for Health Policy, Institute of Global examine whether intention-incongruent unhealthy actions are consistent with hyperbolic Health Innovation, St. Mary’s discounting. We conclude that intention-incongruent actions are often triggered by Hospital, Imperial College London, environmental cues or changes in motivational state, whose effects are not parameterized Praed Street, London, W2 1NY, UK by hyperbolic discounting. We propose a framework for understanding these state-based e-mail: [email protected] effects in terms of the interplay of two distinct reinforcement learning mechanisms: a “model-based” (or goal-directed) system and a “model-free” (or habitual) system. Under this framework, while discounting of delayed health may contribute to the initiation of unhealthy behavior, with repetition, many unhealthy behaviors become habitual; if health goals then change, habitual behavior can still arise in response to environmental cues. We propose that the burgeoning development of computational models of these processes will permit further identification of health decision-making phenotypes. Keywords: discounting, health, addiction, model-based, model-free, habit, preference reversal, hyperbolic INTRODUCTION observing choices between delayed outcomes. Economic theo- Unhealthy behaviors, such as tobacco smoking, excessive alcohol ries of rational behavior posit that goods ought to be discounted intake, physical inactivity and substance misuse, account for sig- exponentially with delay (Samuelson, 1937). Formally, an out- nificant morbidity and mortality worldwide (Smith et al., 2012). come which has utility A if received immediately (t = 0) is worth For example tobacco smoking was estimated to be responsi- A · δt if delayed t periods into the future. The present-time value, ble for a fifth of all deaths in the US in 2005 (Danaei et al., V, of receiving A at time t is thus given by: 2009). In the developed world these behavioral risk factors can be viewed as a direct expression of personal choice, and research has V(A, t) = A · δt (1) therefore been directed toward establishing their psychological determinants (Conner and Norman, 2005). Unhealthy behav- ior frequently has a delayed effect on health, leading researchers Here the discount rate, δ, represents the constant proportional to hypothesize that an individual’s tendency to make unhealthy decrease in value with each added time period of delay. However choices is related to their temporal discount rate, the theoret- both humans and animals violate the exponential assumption of ical rate at which they devalue delayed outcomes (Grossman, a constant proportional discount factor per unit time, appear- 1973; Bickel et al., 2012). In recent years a large number of stud- ing rather to discount rewards occurring in the immediate ies have addressed this hypothesis, forming part of a growing future more steeply than those in the distant future. In other endeavor to identify decision-making phenotypes which correlate words, delaying an immediate reward 1 day into the future with maladaptive behavior (Montague et al., 2012). decreases its implied value proportionally more than does delay- Given a choice, people tend to prefer immediate rewards to ing an already distant reward by 1 day (Strotz, 1957; Chung and those available after a delay, for example preferring to receive $10 Herrnstein, 1967; Ainslie, 1974, 1975; Mazur, 1987; Benzion et al., today over $15 in a month, implying that they discount the value 1989; Green et al., 1994). The discount function estimated from of delayed rewards relative to more immediate ones. A function observed choices is better accounted for by a hyperbolic func- which describes the pattern of discounting can be estimated by tion, written in its simplest form as follows, where k represents Frontiers in Behavioral Neuroscience www.frontiersin.org March 2014 | Volume 8 | Article 76 | 1 Story et al. Temporal discounting and health behavior the hyperbolic discount rate: utility in guiding health behavior change interventions (Conner and Norman, 2005), while understanding the processes involved 1 V(A, t) = A · (2) in unhealthy choice may assist in designing novel interventions. 1 + k · t A proposed alternative is the “quasi-hyperbolic” approximation CAN MEASURED DISCOUNT RATES PREDICT HEALTH to hyperbolic discounting (Phelps and Pollak, 1968; Laibson, BEHAVIOR? 1997; Angeletos et al., 2001; McClure et al., 2004, 2007), which DIFFERENCES BETWEEN DISCOUNTING FOR HEALTH AND MONEY is formalized as exponential discounting, with an additional pref- Measured discount rates are not equal for all commodities, for erence for immediate rewards, expressed as a second discount example, people discount primary reinforcers such as food and β factor, , applied to all time-periods except the first: water more steeply than money (Odum, 2011). This raises an important question: which outcome modality is the best can- Aift= 0 V(A, t) = (3) didate predictor of health behavior? With regard to real-world A · βδt if t > 0 health choices the relevant delayed outcome is a health state. Indeed numerous studies have attempted to measure discount The hyperbolic and quasi-hyperbolic forms qualitatively capture rates for hypothetical health states (Asenso-Boadi et al., 2008). the observation that individuals make far-sighted plans when out- Many such studies require participants to imagine a hypothet- comes are distant, but reverse their choices in favor of short-term ical illness, usually described in generic terms, and to trade-off rewards when the future is reached (see Kalenscher and Pennartz, the severity or duration of the illness against when they would 2008 for a review). Hyperbolic or quasi-hyperbolic discounting prefer the illness to occur (for example Van Der Pol and Cairns, therefore putatively imparts a psychological explanation for why 2001). Individuals who are willing to accept a more severe illness people intend to perform actions they subsequently fail to carry occurring after a delay over a less severe immediate illness are said out.Ineconomicsthisisreferredtoasmyopicpreferencereversal to discount future illness. Alternatively, the health state may be (Kirby and Herrnstein, 1995), and bears considerable relevance to described as an improvement in health from a baseline of illness health choice, where there is a marked discrepancy between health (for example Ganiats et al., 2000), where individuals who prefer intentions and health behavior (Conner and Norman, 2005). immediate over delayed health improvement, and are willing to This article examines the relationship between temporal dis- accept a small improvement in health occurring sooner, over a counting and health behavior at two levels of analysis: empir- larger improvement at a delay are said to discount future health. ical and mechanistic (Rescigno et al., 1987). A distinction is Studies of hypothetical health discounting, for both health often made in psychology and neuroscience between empiri- improvements and illness, demonstrate that some important cal and mechanistic classes of model. Empirical, high-level, or properties of monetary discounting are conserved in the health descriptive models seek to capture structure in observed data, domain. Several studies find that many people indeed dis- allowing predictions to be made, but lack psychological content count future health improvements in the conventional sense, also (Lewandowsky and Farrell, 2011). Thus temporal discounting referred to as positive time preference for health (Lipscomb, 1989; might predict markers of health behavior, without being upheld Olsen, 1993; Bleichrodt and Johannesson, 2001). Also consistent as a psychological process underlying unhealthy choice (see with monetary discounting there is robust