## Abstract

We identify a novel contextual variable that alters the evaluation of delayed rewards in healthy participants and those diagnosed with attention deficit/hyperactivity disorder (ADHD). When intertemporal choices are constructed of monetary outcomes with rounded values (e.g., $25.00), discount rates are greater than when the rewards have nonzero decimal values (e.g.,$25.12). This finding is well explained within a dual system framework for temporal discounting in which preferences are constructed from separate affective and deliberative processes. Specifically, we find that round dollar values produce greater positive affect than do nonzero decimal values. This suggests that relative involvement of affective processes may underlie our observed difference in intertemporal preferences. Furthermore, we demonstrate that intertemporal choices with rounded values recruit greater brain responses in the nucleus accumbens to a degree that correlates with the size of the behavioral effect across participants. Our demonstration that a simple contextual manipulation can alter self-control in ADHD has implications for treatment of individuals with disorders of impulsivity. Overall, the decimal effect highlights mechanisms by which the properties of a reward bias perceived value and consequent preferences.

## INTRODUCTION

Problems with self-control are some of the most detrimental for individuals as well as society, with obesity, excessive debt, and substance abuse representing major health and economic concerns (Madden & Bickel, 2009; Reynolds, Leraas, Collins, & Melanko, 2009; Madden, Petry, Badger, & Bickel, 1997). These issues all have one feature in common: People opt for more immediately rewarding options and undervalue future benefits to their overall detriment. To understand such phenomena, research has posited that future outcomes are evaluated using hyperbolic or quasihyperbolic discount functions, which effectively describe the tendency to overvalue immediate rewards (Frederick, Loewenstein, & O'Dohoghue, 2002). In these functions, value rapidly decreases as rewards are delayed from the present and decreases more slowly as rewards are delayed from future times.

The discount rate expressed in hyperbolic discounting is the critical factor determining relative preferences for immediate rewards. Discount rates depend on a wide variety of contextual and personal variables, such as the nature of the reward, its modality (McClure, Ericson, Laibson, Loewenstein, & Cohen, 2007; Bickel & Marsch, 2001), its magnitude (Green, Myerson, & McFadden, 1997; Thaler, 1981), and even the scent in the experimental room (Li, 2008). Individual factors that predict differences in delay discounting include age (Steinberg, 2010; Sozou & Seymour, 2003; Green, Fry, & Myerson, 1994), health (Chao, Szrek, Pereira, & Pauly, 2009), intelligence (Shamosh et al., 2008), and some psychiatric disorders (Ahn et al., 2011; Heerey, Robinson, McMahon, & Gold, 2007). Peters and Büchel (2011) refer to these dependencies as trait (immutable, e.g., person-related) and state (mutable, framing/context) factors that affect discounting rates. The prototypical disorder associated with greater discounting and poor self-control is attention deficit/hyperactivity disorder (ADHD; Marco et al., 2009; Paloyelis, Asherson, & Kuntsi, 2009; Tripp & Alsop, 1999; Schweitzer & Sulzer-Azaroff, 1988, 1995; Rapport, Tucker, DuPaul, Merlo, & Stoner, 1986).

Process theories of temporal discounting propose a dual system model of decision-making to begin to capture the many influences on relative preferences for immediate reward (van den Bos & McClure, 2013). The first system is posited to be myopic in nature and is linked to positive emotional reactions to rewards. We use the term “affective” to represent this system (Loewenstein, 1996), which is thought to be subserved by brain areas including the nucleus accumbens (NAcc) in the ventral striatum, the ventromedial pFC (vmPFC), and other areas involved in evaluating rewards (Kable & Glimcher, 2007; McClure et al., 2007; McClure, Laibson, Loewenstein, & Cohen, 2004). These brain reward regions have been linked to affective responses (Knutson & Greer, 2008; Panksepp, 2004) and are thought to signal reward value in a stereotyped manner acquired through associative learning (Daw, Niv, & Dayan, 2005; Schultz, Dayan, & Montague, 1997). The second process is hypothesized to be far sighted in nature, slow and rule-based in response, but flexible enough to adaptively control behavior. We refer to this as the “deliberative” system. It is thought to be subserved by the dorsolateral pFC (dlPFC) and posterior parietal cortex (pPC; McClure et al., 2004, 2007).

Here we explore a novel effect on temporal discounting that appears to arise from differences in affective responses to reward prospects. The effect results from changing a seemingly innocuous feature of offered monetary rewards. Specifically, within-subject discount rates differ when choices are constructed from monetary rewards with rounded decimal values (e.g., $25.00) or numbers with nonzero decimal value (e.g.,$25.12). Individuals tend to choose more impulsively when the choice is constituted of monetary rewards that are rounded numbers. We refer to this as the decimal effect. As rounded decimal amounts ($25.00) are more common in daily experience than are nonzero decimal values ($25.12; with .99 a possible exception), we speculate that this effect may result from greater familiarity and hence perceptual fluency with rounded dollar values (cf. Oppenheimer & Frank, 2008; Alter & Oppenheimer, 2006). Our primary aim is to provide a process account of the decimal effect. On the basis of data from several experiments, we will argue that nonzero decimal values in monetary rewards influence affective responses to the rewards and consequently influence how individuals trade off present for delayed rewards.

Our first study, Experiment 1, demonstrates the decimal effect. In Experiment 2, we show behavioral evidence that the decimal effect is related to increased positive affect to rounded monetary rewards. In Experiment 3, we provide fMRI evidence to support our main conclusions. In Experiment 4, we provide an extension of the decimal effect, testing whether rounded values have the ability to increase the value of delayed rewards. Our final study, Experiment 5, examines the decimal effect across a wide developmental period between typically developing controls and participants with ADHD.

## EXPERIMENT 1

Affective processes may signal value in an automatic, stereotyped manner that is slowly acquired through experience. We hypothesized that differential experience with monetary rewards with rounded values relative to nonzero decimal values may bias how the rewards are processed by facilitating automatic responses and consequently influencing intertemporal preferences (Butterworth, 1999). We tested this prediction in our first experiment.

### Methods

#### Participants

We recruited 28 participants; 12 at Stanford University (eight men, mean age = 20.26 years, range = 18–22 years) and 16 from Baylor College of Medicine and the greater Houston area community (10 men, mean age = 26.38 years, range = 20–36 years). (See Table 1 for inclusion/exclusion criteria for all studies and Table 2 for demographic data for Experiments 1–4.) We excluded one participant from each site because they failed to submit choices on all trials. Participants from Baylor College of Medicine completed the task while undergoing fMRI scanning (see Experiment 3).

Table 1.

Inclusion/Exclusion Criteria for All Experiments

GroupExperiments 1–4
Inclusion Criteria
HC Ages 18–50

Exclusion Criteria
HC Clinical history of neurological, major medical or psychiatric disorder
fMRI contraindicationsa

Experiment 5
Inclusion Criteria
IQ over 80 as per WASI
HC t score of 60 or lower on the total DSM total ADHD score
HC 3 or more inattentive and 3 or more hyperactive/impulsive DSM symptoms
ADHD t score of 65 or higher on the total DSM total ADHD score
ADHD 6 or more inattentive and 6 or more hyperactive/impulsive DSM symptoms
ADHD Significant symptoms before age 7 and across at least two domains (e.g., home and school/work)

Exclusion Criteria
HC and ADHD Clinical history of neurological, major medical of psychiatric disorder
HC History of treatment with psychoactive medication
HCa fMRI contraindications
GroupExperiments 1–4
Inclusion Criteria
HC Ages 18–50

Exclusion Criteria
HC Clinical history of neurological, major medical or psychiatric disorder
fMRI contraindicationsa

Experiment 5
Inclusion Criteria
IQ over 80 as per WASI
HC t score of 60 or lower on the total DSM total ADHD score
HC 3 or more inattentive and 3 or more hyperactive/impulsive DSM symptoms
ADHD t score of 65 or higher on the total DSM total ADHD score
ADHD 6 or more inattentive and 6 or more hyperactive/impulsive DSM symptoms
ADHD Significant symptoms before age 7 and across at least two domains (e.g., home and school/work)

Exclusion Criteria
HC and ADHD Clinical history of neurological, major medical of psychiatric disorder
HC History of treatment with psychoactive medication
HCa fMRI contraindications

HC = healthy control.

aExclusion for Experiment 3.

Table 2.

Demographic and Clinical Characteristics for Participants in Experiments 1–4

ExperimentGroupAgeAge RangeGender (male)n
HC 22.4 18–36 17 42
HC 29.5 19–50 19 40
HC 26.1 20–36 16
HC 35.5 19–45 92 183
ExperimentGroupAgeAge RangeGender (male)n
HC 22.4 18–36 17 42
HC 29.5 19–50 19 40
HC 26.1 20–36 16
HC 35.5 19–45 92 183

Data are summarized as mean for the continuous variables.

#### Materials

Each participant was presented with 62 intertemporal choices offering an immediate reward and a larger but delayed reward. For half of the choice trials, rewards had rounded decimal values (e.g., $11.00 today or$21.00 in 6 weeks; rounded condition). The other half had only nonzero decimal values (e.g., $10.87 today or$20.74 in 6 weeks; decimal condition). We omitted decimal values of .25, .50, .75, and .99, as these are common numbers and may have intermediate effects between our rounded and nonzero decimal values. Trials were presented in random order.

The choice trials were derived from the hyperbolic discounting function (Mazur, 1987) that models subjective value as a function of delay according to the function,
where r is the magnitude of the reward, d is the delay until receipt, and V is the discounted value. For each trial, a unique discount rate, keq, implies indifference between the immediate reward and the discounted, delayed reward. Choices were constructed so that each trial in the rounded condition matched a trial in the decimal condition with an equal discount rate (keq) and delay. For the rounded value rewards, magnitudes spanned a range of $2 to$33; nonzero decimal values ranged from $2.14 to$32.90. Delayed rewards were available between 7 and 56 days in the future (in 7-day increments). Reward magnitudes could not be exactly equated; thus, half of the decimal values were slightly larger and the other half slightly smaller than their rounded pairs. As it was not possible to make the average magnitudes exactly the same, decimal values were on average 18¢ (±$1.33) smaller than rounded values. This design ensured that both conditions spanned the same range of intertemporal trade-offs, while controlling for any bias because of differences in reward magnitude (Thaler, 1981). #### Procedure Participants had unlimited time on each trial to make their choice. A 2000 msec blank intertrial interval was used (see Figure 1A). The 62 trials were split into four blocks of either 15 or 16 trials, with one 15 trial and one 16 trial block for both the rounded and decimal conditions. Block order was counterbalanced according to condition, with half of participants beginning with rounded and ending with decimal trials. Trial order within each block was randomly generated. Figure 1. (A) Intertemporal choices for monetary outcomes with nonzero and rounded decimal values elicit different temporal discount rates. (B) Discount rates are consistently higher for rounded dollar values across participants, producing a robust mean decimal effect. Figure 1. (A) Intertemporal choices for monetary outcomes with nonzero and rounded decimal values elicit different temporal discount rates. (B) Discount rates are consistently higher for rounded dollar values across participants, producing a robust mean decimal effect. We used a lottery system in which one of the participant's choices was randomly selected and paid to the participant according to the amount and delay of the selected choice. Participants were instructed to consider each choice seriously as any one could potentially be paid according to their selection. This encouraged participants to remain focused throughout the experiment and to treat all trials as equally determinant of their overall earnings. #### Estimation of Discount Rates For each participant and condition, discount rates were estimated by maximum likelihood. Participants' binary choices between the immediate and delayed rewards were modeled with the exponential version of the Luce choice model (Luce, 2005). If we summarize the subjective value of the two alternatives as V1 and V2 for the immediate and delayed rewards, respectively, then the probability of choosing the immediate outcome for an arbitrary k is given by where VΔ(k) is the difference V1V2 for some value of k. Likewise, the probability of choosing the delayed outcome is equal to 1 − P(ChooseV1). The parameter m captures how consistent choices are with the fitted discount function. The likelihood of any set of choices per participant is the product of the probability for each observed choice. For each condition (c), we form the likelihood function, where J = 1 if the immediate reward is chosen and zero otherwise. We maximized Equation 3 with respect to k and m using a simulated annealing optimization algorithm. This yields condition-specific estimates for k and m. The standard errors of the estimates were obtained by invoking the asymptotic normality of the maximum likelihood estimators. ### Results Choices revealed the decimal effect: Participants made more impulsive decisions in the rounded relative to the decimal condition. We performed analyses on log-transformed discount rates using nonparametric tests because the distributions of log(k) were nonnormal (Kolmogorov–Smirnov tests, p < .001 for both decimal and rounded conditions). The decimal effect held among 22 of our 26 participants (see Figure 1B). Moreover, discount rates in both the decimal and rounded conditions were not significantly different across participants recruited from Stanford University and Baylor College of Medicine (Wilcoxon rank sum test; p > .24 comparing discount rates in rounded and decimal conditions). We therefore analyzed data collectively across these two groups. Comparing the estimated discount rates across conditions within participants, the mean of the differences between the log-discount rates in the rounded versus decimal conditions is positive (0.27) and significantly different from zero (sign test, p < .001). We ruled out two potential confounds associated with the decimal effect. First, we found no difference in RT between the two conditions (mean RT rounded = 3273.04 msec; mean RT decimal = 3088.59; mean rounded − decimal = 184.45 msec, SE 138.59, t(25) = 1.28, p > .20). Second, choice consistency was not influenced by task condition. Comparing m values indicated no significant difference (Wilcoxon signed rank test p = .67). Likewise, fitted k values predicted an average of 90.12% and 88.34% of choices in the decimal and rounded conditions, respectively (Wilcoxon signed rank test, p = .17). Reward magnitude is also known to influence discount rates (e.g., Thaler, 1981). To rule out an influence of magnitude on our results, we split choices (by median) into low- and high-magnitude trials, collapsing across decimal conditions. We then estimated k separately for low- and high-magnitude choices per participant. We performed a sign test on the difference in log(k) values across magnitudes and found no significant difference (p = .33). ### Discussion Consistent with our hypothesis, we found that the nature of the decimal values in monetary rewards influenced intertemporal preferences. We suggested that monetary rewards containing rounded values would be more perceptually fluent and therefore trigger affective valuation processes to a greater degree than would nonzero decimal values. As affective processes are thought to be myopic in nature (Loewenstein, 1996), this would account for our observed differences in discount rates. ## EXPERIMENT 2 Experiment 2 tested the hypothesis that rounded dollar values differ from nonzero decimal values on the basis of affective response. We primed affective processes by asking participants to rate their emotional reaction (Hsee & Rottenstreich, 2004) to the prospect of winning different amounts of money to determine how rounded and nonrounded monetary rewards are evaluated using emotionally based valuation. We manipulated decimal values while holding magnitude comparable. We hypothesized that if valuation of round numbers involves more affective processing, round numbers would generate greater positive affect than comparable nonzero decimal numbers. The alternative hypothesis is that affective processes are unaffected by decimal value, in which case affect ratings between rounded and nonzero decimal values should not differ. ### Methods #### Participants A total of 54 volunteers were recruited (25 men; mean age = 28.8 years) from the Stanford community and gave written informed consent to participate. Because of a technical error in conducting the experiment, 14 participants did not complete all of the ratings and thus were excluded, leaving 40 participants for analyses. #### Materials and Procedure In accordance with the two-dimensional affective circumplex model of emotion (Watson, Wiese, Vaidy, & Tellegen, 1999; Watson & Tellegen, 1985), we separately assessed valence and arousal to measure the subjective emotional impact of rounded versus decimal monetary rewards. Participants received an online questionnaire, asking them to make subjective assessments of 10 monetary rewards, five rounded and five with nonzero decimal values. Each rounded reward was matched to a decimal reward; in each pair, the rounded number had a smaller objective value. Each of the 10 numbers was presented in a random order, and participants were asked the following questions: Imagine you have the chance to win$25.00. How Positive or Negative would you feel? How Activated/Aroused would you feel?

Participants answered the questions using sliding scales numbered from 0 to 100 and anchored to 50 on presentation of the question.

### Results

As valence (v) and arousal (a) ratings were significantly correlated in our data (r2 = .54, p < .0001), we combined these measures on a single dimension of positive arousal as our primary variable of interest (; based on Knutson & Greer, 2008). A two-way, within-subject ANOVA was conducted to compare the main effects of (1) condition (rounded vs. decimal values) and (2) reward magnitude for participants' affect ratings for rewards. We found greater PA for rounded values with a significant main effect of condition, F(1, 38) = 5.48, p = .03. We also found a significant main effect of reward magnitude on PA ratings, F(4, 38) = 29.82, p < .001, with larger values eliciting more positive ratings. These results are shown in Figure 2, where we have plotted normalized ratings (z score corrected within participants across conditions) as a function of reward amount. The interaction between condition and reward magnitude was not significant (p = .18).

Figure 2.

Positive arousal reported for the prospect of earning a rounded dollar amount was larger than that reported for nonzero decimal values or marginally greater objective value. Data have been normalized within participants (z score transformed); error bars are standard errors of the mean.

Figure 2.

Positive arousal reported for the prospect of earning a rounded dollar amount was larger than that reported for nonzero decimal values or marginally greater objective value. Data have been normalized within participants (z score transformed); error bars are standard errors of the mean.

Similar results held when valence or arousal were analyzed using similar ANOVAs. For valence, there was a main effect of amount, F(4, 38) = 30.50, p < .001, and condition, F(1, 38) = 4.98, p = .03, but no significant interaction (p = .38). For arousal, there was a main effect of amount, F(4, 38) = 23.96, p < .001, and a trend for condition, F(1, 38) = 3.43, p = .07, with no significant interaction (p = .23).

Because of the large age range in our participants, we conducted additional ANOVA analyses looking for a main effect of age (split into quartiles) or an Age × Reward magnitude interaction. We found no significant differences on the basis of participants' age (p > .46 for both analyses).

### Discussion

These results suggest that participants feel more positive arousal for monetary rewards with rounded compared with those with nonzero decimal values. Not surprisingly, they also reported feeling more positive arousal for greater magnitudes of monetary rewards. Importantly, this differential affective response overcomes the fact that rounded values were smaller in objective value.

## EXPERIMENT 3

Properties linked to affective and deliberative processes distinguish the functions of the NAcc and dlPFC in intertemporal choice (Peters & Büchel, 2011; McClure et al., 2004). Affective responses to rewards and related NAcc activity predict individual discount rates (Hariri et al., 2006). Cognitive ability correlates with dlPFC activity and lower discount rates (Shamosh & Gray, 2008; Shamosh et al., 2008). Furthermore, manipulating these systems either pharmacologically (Pine, Shiner, Seymour, & Dolan, 2010) or by direct stimulation (Figner et al., 2010) alters discount rates in the expected directions. In this study, we measure correlates of affective and deliberative processing while participants make intertemporal choices containing rounded or nonzero decimal values. Given the results from Experiment 2, we conjectured that rounded values would more effectively recruit the NAcc than would nonzero decimal values. fMRI also allows us to test whether rounded and decimal values differentially recruit deliberative processes by measuring activity in the dlPFC and pPC.

### Methods

#### Participants

Out of 28 participants in Experiment 1, the 16 participants from Baylor College of Medicine performed the task while undergoing fMRI scanning. The two participants excluded from the analysis in Experiment 1 were from this group of 16.

### Discussion

These results replicate our main finding that decimal values influence discount rates—even in those with elevated levels of impulsivity, such as ADHD. The tendency to favor immediately available rewards plays a central role in the delay aversion theory (Sonuga-Barke, Taylor, Sembi, & Smith, 1992) and the steeper and shorter delay-of-gratification gradient theory of ADHD (Sagvolden, Aase, Zeiner, & Berger, 1998). Our replication of the decimal effect in impulsive individuals is particularly significant for populations who display a greater tendency to select immediate rewards, such as adolescents and individuals with substance dependence (Madden & Bickel, 2009). Increased discounting is linked to poor health outcomes and reduced academic achievement and occupational success (Golsteyn, Gronqvist, & Lindahl, 2013). Attempting to improve self-control in individuals with heightened impulsivity by altering reward perception would be a novel approach for reducing the negative outcomes associated with impulsivity. Treatment of ADHD and substance use disorders currently involves contingency management in which rewards are given for appropriate behavior (e.g., Bickel et al., 2010; Barkley, 2006). Although the size and delay of the rewards are typically considered in developing a behavior plan, it has not been considered how to best frame or present rewards in these plans. Our findings suggest that future research should assess how framing effects could enhance the value of delayed rewards to increase self-control across conditions associated with impulsivity.

We also replicate the finding that younger individuals have higher discount rates than do older people, independent of the presence or absence of ADHD (Steinberg et al., 2009). Casey and colleagues (Casey, Duhoux, & Malter Cohen, 2010; Casey, Jones, & Hare, 2008) propose that an increase in risky behavior during adolescence is because of an imbalance between relatively more mature, subcortical brain systems versus less mature functioning in cortical regions linked to cognitive control. Studies suggest impaired modulation of hyperactive reward-related striatal regions by cognitive control regions (i.e., dlPFC) in adolescence (Christakou et al., 2011; Van Leijenhorst et al., 2010; Berns, Moore, & Capra, 2009; Galvan et al., 2006). Brain regions linked to self-control and evaluation of future outcomes (Galvan et al., 2006) mature later in development (e.g., Christakou et al., 2011; Cohen et al., 2010; Olson et al., 2009). Optimal connectivity between dlPFC and other regions (pPC, vmPFC) to support more self-controlled behavior putatively occurs in adulthood (Luna, 2009). Regions such as the NAcc, which have been associated with more impulsive choices in Experiment 3, have also been consistently implicated in ADHD impairments (Hart, Radua, Nakao, Mataix-Cols, & Rubia, 2013; Scheres et al., 2007).

## GENERAL DISCUSSION

Emotional responses have long been hypothesized to underlie the short-sighted behavior evident in choices involving tempting immediate rewards (Loewenstein, 1996; Mischel, 1974). We identify a novel effect on delay discounting consistent with this assertion: subtle features of prospective rewards can change affective responses and impatience.

A large number of effects influence how intertemporal preferences are formed (van den Bos & McClure, 2013). One potential unifying framework for understanding these diverse influences may come from positing independent neurocognitive systems that underlie the evaluation of rewards. We refer to one common dichotomy of such systems herein as affective and deliberative. We have shown that such a framework can explain how a relatively innocuous feature of an intertemporal choice, the numbers following the decimal point, comes to influence discounting. We combined behavioral and neural measures to test how decimal values alter the affective responses that distinguish these two modes of valuation. Overall, we have established a pathway whereby properties of a reward influence consequent discount rates. Although it is possible that the decimal effect is better explained by other effects such as subtle differences in sensory processing or calculation of numerical differences between the rounded and decimal conditions, we believe this is less likely. We found no evidence in to support differences between the rounded and decimal conditions in visual or sensory brain regions nor in decision-related RTs.

It remains to be seen whether the dual system framework will be sufficient to account for the number of factors known to influence intertemporal preferences. For example, people are more patient when the time of reward outcomes is expressed as an exact date as opposed to the duration of time from the present (Read, Frederick, Orsel, & Rahman, 2005). A recent fMRI study has shown that a similar manipulation, switching from delays to dates, modulates dlPFC activity, consistent with dual system theory (Peters & Büchel, 2010). Perhaps as interestingly, the dual system framework suggests novel effects. The idea for the decimal effect arose from considering ways in which we might modulate NAcc activity.

Positing two neurocognitive systems is almost certainly an oversimplification of how intertemporal preferences are actually constructed. The validity of dual system models of discounting is a source of much debate in the neuroscience literature (e.g., Hare et al., 2009; Kable & Glimcher, 2007). Nonetheless, such models have distinct advantages in accounting for numerous phenomena in delay discounting (van den Bos & McClure, 2013). One important future direction will be to relate dual system models to construal level theory (Trope & Liberman, 2003). Recent work by Fujita and colleagues has shown that priming people to think in broader, more abstract terms (high-level construal) increases self-control (Fujita & Han, 2009). It is intriguing to hypothesize that thinking more abstractly depends on the dlPFC and priming this neural system increases that self-control, but this is pure speculation at this point. We also acknowledge that there may be other plausible mechanisms than the dual processing account or the familiarity of rounded numbers that may explain the downstream effect of an increased affective response to the rounded stimuli studied herein. However, our primary goal for this project was to document the outcome of altered affective responses. Future studies will attempt to determine the mechanism underlying the outcome.

The decimal effect also suggests one avenue for interventions aiming to ameliorate the effects of impulsivity. Our approach represents a novel attempt to shift impulsive behavior in populations associated with poor self-control by manipulating the choice context. ADHD is associated with problematic functioning in brain networks implicated in both cognitive (dlPFC/pPC) and affective/reward (vmPFC/NAcc) processes (Fassbender & Schweitzer, 2006). Despite this, attempts to modify self-control in ADHD and adolescents tend to focus on teaching deliberative strategies (Dawson & Guare, 2010). It should be possible to design choice environments in ways that decrease affective responses, reduce NAcc activity, and lead to more far-sighted choices. This suggestion is very similar to Mischel and colleagues' demonstration that, thinking of the abstract, physical qualities of a marshmallow increase ones' ability to delay gratification and ultimately obtain more marshmallows (Mischel & Baker, 1975). The findings here suggest the neurobiological basis by which these framing effects may function. It may also be that differential neural activity relates to distinct symptom profiles in individuals with ADHD. For example, steeper discounting may be because of some combination of heightened sensitivity to immediate rewards, problems with response inhibition, or an ineffectiveness of future outcomes to influence current behavior.

Reprint requests should be sent to Samuel M. McClure, Department of Psychology, Stanford University, 450 Serra Mall, Building 420, Stanford, CA 94305, or via e-mail: smcclure@stanford.edu.

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