Drinking status but not acute alcohol consumption influences delay discounting

Objective The aim of this study was to investigate the following: (a) the effects of acute alcohol on delay discounting; (b) the effects of drinking status on delayed discounting; and (c) whether these effects differ according to reward type (alcohol vs. money). Methods Heavy and light social alcohol users (n = 96) were randomized to receive either an acute dose of alcohol at 0.4 or 0.6 g/kg or placebo in a between‐subjects, double‐blind design. Delay discounting of alcohol and monetary rewards was measured using a hyperbolic model, with higher scores indicative of greater delay discounting. Results ANOVA of discount scores indicated a main effect of reward type, where all participants had higher discount scores for alcohol versus money rewards. A main effect of drinking status was also observed, where heavier drinkers had higher discount scores compared with lighter drinkers. We did not observe a main effect of acute alcohol use on delay discounting or the hypothesized interactions between acute alcohol use and drinking status with reward type. Conclusions Our data suggest that heavier drinkers discount the value of delayed rewards more steeply than lighter drinkers. Delay discounting may therefore be a promising marker of heavy alcohol consumption in social drinkers.

(e.g., alcohol may be less attractive as a long-term reward, compared with money). Secondly, drug rewards are primary reinforcers that have a direct effect on behavior (e.g., intoxication, relaxation), making them valuable as immediate rewards. In contrast, monetary rewards are reinforcers that must be exchanged for other goods in order to impact behavior.
To our knowledge, no studies have examined the effects of acute alcohol use and heaviness of drinking on delay discounting of alcohol versus monetary rewards. Such a study has the potential to inform a growing body of research investigating impulsivity as a potential marker for heavy alcohol use. In this study, we examined the influence of heaviness of drinking, alcohol intoxication, and reward type on delay discounting. We examined the effects of two doses of alcohol on delay discounting of alcohol and monetary rewards in light and heavy social drinkers. Both moderate (0.4 g/kg) and high (0.6 g/kg) doses of alcohol were included to assess the extent to which delay discounting of rewards may be differentially sensitive to different priming doses.
Additionally, light and heavy drinkers were included to examine the influence of heaviness of drinking on delay discounting. Delay discounting was assessed with a hypothetical, question-based measure, given that an experiential paradigm with actual alcohol rewards in this study (e.g., 10 pints of beer or 10 glasses of wine in one single session) would have been unethical (Field et al., 2007). We hypothesized that participants primed with an acute dose of alcohol compared with those given a placebo drink would exhibit increased delay discounting of alcohol-related rewards, relative to monetary rewards.
Additionally, we hypothesized that heavier social drinkers compared to lighter drinkers would exhibit increased delay discounting of alcohol-related rewards, relative to monetary rewards.

| Design
The study employed a double-blind, placebo-controlled design, comprising two between-subjects factors of challenge condition (0.0, 0.4, 0.6 g/kg alcohol) and drinking status (light drinkers, heavy drinkers) and a within-subjects factor of reward type (alcohol, money).

| Participants
Social drinkers were recruited from students and staff at the University of

| Materials
The delay discounting procedure was an experimenter-delivered task adapted from Moore and Cusens (2010). This task provides an ordinal index of participants' discount function using 12-choice items. For each item, participants were asked "Would you prefer £10 now or £a in 3 months time?", where only £a varied and items were arranged so that responses would titrate to a participant's approximate 3-month hyperbolic discount rate (i.e., value = a/[1 + kD], where a is the immediate value of the reward, D is the delay duration, and k is the rate of discount) expressed along an ordinal scale. The main outcome was a calculated discount score. The lowest possible score was "1 for a 3month discount rate of 25% and the highest was 13 for a 3-month discount rate of 99.9%" (Moore & Cusens, 2010, page 2). Higher discount scores are indicative of greater discounting of delayed rewards. All rewards were hypothetical, but participants were instructed to make their choices as if they were going to actually receive the rewards that they selected. Participants were instructed that one unit of alcohol was equal to £1, which was consistent with the price per unit at the time of testing. Alcohol rewards were matched to roughly equivalent monetary rewards; £20 was equivalent to 10 pints of beer or 10 (175 ml) glasses of wine. We fixed the maximum amount of alcohol offered at an amount that could be plausibly consumed in one single drinking session (e.g., 10 pints of beer or 10,175 ml glasses of wine).
Questionnaire measures included self-report measures of drinking behavior (Alcohol Use Disorders Identification Test; Bohn, Babor, & Kranzler, 1995), impulsivity (Barratt Impulsivity Scale; Patton, Stanford, & Barratt, 1995), sensation seeking (Impulsive Sensation Seeking; to consume an alcoholic beverage", "A drink would be very satisfying," "The thought of consuming an alcoholic beverage is appealing," "I need to have a drink," and "I do not want to consume an alcoholic beverage." Each item was rated on an 80-mm scale from "Not at all" to "Extremely."

| Procedure
All participants were tested between noon and 6 pm in a laboratory in the School of Experimental Psychology at the University of Bristol. On the test day, after providing informed consent, all participants completed a screening process to exclude current use of medication and illicit substances, family history of alcoholism, and recent alcohol consumption (i.e., within 12 hr of test session, verified by exhaled breath alcohol). Participants were required to regularly consume wine or beer as these were the alcohol-related rewards offered in the delay discounting tasks and (units per week) were recorded to establish allocation to drinking status group (light drinkers, heavy drinkers). Weight was recorded for drink preparation.

Following the completion of baseline measures (Alcohol Use Disorders
Identification Test, Barratt Impulsivity Scale, Impulsive Sensation Seeking, POMS, and VAS), participants were given 10 min to consume the drink.
Participants were randomly allocated to receive either an alcoholic (0.4, 0.6 g/kg) or a placebo drink. For a 60-kg adult, the 0.4-g/kg dose is approximately equivalent to 2 units of alcohol and the 0.6-g/kg dose to 3 units. The alcohol administration procedure was identical to that described previously (Adams, Ataya, Attwood, & Munafo, 2013). Following drink consumption, participants completed the first awareness check to determine whether they were aware if they had received alcohol or placebo. Awareness of the alcohol content of the challenge condition was determined by asking participants whether they believed that their drink contained alcohol or not.
Next participants were given 15-min absorption time, to ensure all participants had an equivalent period between challenge administration and the delay discounting task. During this period, participants completed postchallenge measures (POMS, VAS). At the end of this time, participants completed the delay discounting task with money and alcohol reward order counterbalanced across participants. The task lasted approximately 5 min. Following the task, participants completed the POMS and VAS measures again and a second awareness check.
On completion of the study procedure, participants were informed of their drink condition (verified by exhaled breath alcohol) and were reimbursed and provided with a full debrief.

| Statistical analysis
All analyses included two between-subject factors of challenge condition (0.0, 0.4, 0.6 g/kg) and drinking status (light drinkers, heavy drinkers). For the delay discounting task, a mixed-model ANOVA of discount scores was conducted including a within-subjects factor of reward type (alcohol, money). Skewness tests for normality indicated that discount data were nonnormal but could be corrected by a log transformation, using log10. For mood and alcohol craving data, mixed-model ANOVAs of POMS and VAS scores were conducted with an additional within-subjects factor of time (baseline, postchallenge, posttasks).
A post hoc sensitivity analysis indicated that the study had 80% statistical power at an alpha level of 5% to detect an effect size of f = 0.16 for the interaction effect of challenge condition (0.0, 0.4, and 0.6 g/kg alcohol) on reward type (alcohol, money). The data that form the basis of the results presented here are available from the data.bris Research Data Repository (http:/data.bris.ac.uk/data/) doi: 10.5523/bris.j2fzlhxc2or234cki8igwtzr.

| Characteristics of participants
Participants (n = 96; 51% male) were, on average, aged 24 years (SD = 4, range 18-39). Table 1 shows characteristics of light and heavy drinking participants allocated to challenge conditions. Allocation of light and heavy drinking participants was equally spilt across 0.0 and 0.6 g/kg conditions, with one participant in the 0.4 g/kg miscategorised as a heavy drinker at data collection, which was corrected during data analysis. Exhaled breath alcohol level (BrAL) was 0.00 μg/L at baseline for all participants. At the end of testing, average BrAL was 0.00 μg/L in the placebo (0.00 g/kg alcohol) condition, 0.15 μg/L in the 0.4 g/kg alcohol condition, and 0.25 μg/L in the 0.6 g/kg alcohol condition. At the end of testing, BrAL did not differ between light and heavy drinkers (p = .32, η 2 = 0.01).

| Correlations
Pearson correlation coefficients of money and alcohol discount scores with alcohol craving at baseline, postchallenge, and posttasks did not indicate any clear evidence of association between delay discounting and alcohol craving (rs < .06, ps > .55).
Note. AUDIT = Alcohol Use Disorders Identification Test; BIS = Barratt Impulsivity Scale; ImpSS = Impulsive Sensation Seeking. There was no evidence for any other main effects or interactions (ps > .11).

| DISCUSSION
Our data suggest that heavier drinkers show greater discounting of delayed rewards relative to lighter drinkers, irrespective of reward type. Additionally, we observed a main effect of reward type, such that all drinkers (in the range examined) showed greater impulsive decisionmaking towards alcohol rewards compared with money rewards. Contrary to our hypotheses, our data did not indicate any interaction effects of acute alcohol consumption or drinking status on delayed discounting of alcohol versus money rewards.
Consistent with previous research (Vuchinich & Simpson, 1998, Petry, 2001a, Field et al., 2007, we observed that heavier social drinkers showed steeper delay discounting than lighter drinkers, indicative of greater impulsive decision-making. However, we did not observe that steeper delay discounting of alcohol versus money rewards was specific to heavier drinking participants, in contrast to previous research (Petry 2001). This discrepancy may reflect differences in the types of drinkers examined, where Petry (2001b) studied alcohol-dependent drinkers. Our delay discounting data show substantial variation in discounting response in heavier social drinkers, suggesting our sample included a wide range of heavy drinkers (e.g., heavy occasional use to problem drinking). Our results add to a growing body of research indicating that heavier drinkers have a general difficulty in delaying gratification, which may reflect an underlying propensity for greater impulsivity. These findings have implications for reducing alcohol intake by increasing sensitivity to longer-term rewards (e.g., improved health, social benefits) and the consequences Our findings also suggest that social drinkers (in the range examined) show greater discounting of alcohol versus money rewards.
This finding is consistent with previous research (Odum & Rainaud, 2003), suggesting that delay discounting is more pronounced when making a decision concerning alcohol versus money rewards. Our results therefore add to a growing body of research demonstrating steeper discounting of alcohol versus money rewards. Steeper discounting of alcohol rewards may reflect a general process associated with consumable rewards (i.e., alcohol is more valued as an immediate reward due to the fact it can be consumed immediately, whereas money must be exchanged in order to obtain a reward).
Additionally, money may be more attractive as a delayed reward, with no expiry.
Consistent with previous research (Richards et al., 1999, Bidwell et al., 2013, Wray et al., 2015, our results suggest that moderatehigh doses of alcohol (in the 0.4 to 0.6 g/kg range) do not influence delay discounting of money rewards. Our study is also the first to examine the effects of acute alcohol on discounting of alcohol versus money rewards, indicating no influence of alcohol intoxication on impulsive decision-making towards alcohol or money rewards. These findings are in contrast to anecdotal reports (Graham, 1980) and previous research (Reynolds et al., 2006), which suggests that acute alcohol increases delay discounting. In addition, our results contradict previous reports of greater discounting of drug versus money rewards during drug intoxication (Giordano et al., 2002, Mitchell, 2004. However, these conclusions are only valid if we accept that delay discounting tasks themselves are sensitive to drug-induced state changes. Reynolds et al. (2006) suggest that experiential delay discounting tasks are more sensitive to the effects of state changes in impulsivity than question-based measures, such as the one used in this study. Nevertheless, a recent study (Wray et al., 2015) using an experiential discounting task failed to show an effect of acute alcohol use on delay discounting. Future work should seek to make a direct comparison of different delay discounting tasks on state changes in impulsive decision-making. Additionally, future work should also examine the reliability and validity of hypothetical measures of delay discounting. Inconsistent findings on the acute effects of alcohol on impulsive behavior reflect the need for replication studies, to establish the role of delay discounting in alcohol-related state changes in impulsive behavior.
Limitations of our study include the use of hypothetical alcoholrelated and money rewards. As we have noted, some researchers have indicated that hypothetical tasks may be less sensitive to acute changes in discounting (Reynolds et al., 2006). However, consistent with previous research, the provision of actual alcohol rewards in this study would have been unethical, given the amount of drinks offered in a single session (Field et al., 2007). Additionally, our between-subjects design did not enable collection of baseline information regarding the socioeconomic status of participants. A baseline assessment of socioeconomic status would have enabled us to control for any group differences in value of money rewards. A further limitation of our between-subjects design was the inability to control for individual differences in impulsive behavior. However, this design was selected to limit participants establishing a stable pattern of responding across time on the delay discounting task (Ortner et al., 2003).

| CONCLUSION
In conclusion, our data indicate that heavier drinkers discount the value of all delayed rewards more steeply than lighter drinkers. This finding suggests that impulsive decision-making is influenced by individual differences in drinking patterns that is irrespective of reward type. Additionally, we observed that all drinkers in the range examined showed greater delay discounting of alcohol rewards, which may reflect the nature of alcohol as a consumable reward. Our data did not suggest any effects of acute alcohol on delay discounting; however, further research is required to establish how sensitive delay discounting tasks are to state changes in alcohol consumption and to determine the reliability and validity of hypothetical measures of delay discounting. This study adds to a body of research suggesting that impulsive delay discounting is a promising marker of heavy alcohol consumption in social drinkers. Additionally, our data indicate a need for research to explore the mechanism underlying the general trend for steeper discounting of drug versus money rewards observed here and in previous studies.