Short communicationDo high rates of cigarette consumption increase delay discounting?: A cross-sectional comparison of adolescent smokers and young-adult smokers and nonsmokers
Introduction
Discounting by delay is the extent to which subjective value for a commodity (e.g., money, food, etc.) is reduced for an organism as a function of its delivery being delayed. Procedures for assessing delay discounting (DD) in humans involve determining indifference values between larger delayed and immediate but smaller reinforcers. An indifference value is the point at which the two choice options (i.e., larger delayed and smaller immediate) are of equal subjective value to the individual. Indifference values obtained across a series of delay lengths can be used to determine rate at which value decreases as a function of increasing delay to delivery. Such indifference values have been best characterized in humans, rats, and pigeons by a hyperbolic function (Mazur, 1987):where ‘Value’ represents the value of the delayed reinforcer, A and D the amount of reinforcer and delay to its delivery, respectively, and k is the free parameter indicating steepness of the discount curve. Higher k-values indicate more discounting, which is often considered an index of impulsivity (e.g., Mazur, 1987; Richards et al., 1999).
Using various types of DD procedures, a robust relation has been found between higher k-values from Eq. (1) and chronic cigarette smoking in adults (e.g., Baker et al., 2003, Bickel et al., 1999, Mitchell, 1999, Reynolds et al., 2004). Most of these studies employed DD procedures involving choices between larger delayed and smaller immediate amounts of hypothetical money (e.g., Bickel et al., 1999). Others used “real-reward” procedures, which are the same as hypothetical procedures except one choice response is randomly selected at the end of the procedure, and the participant receives whatever he or she chose (e.g., Reynolds et al., 2004). Smokers generally discount more than nonsmokers with either version of this procedure. Data presented in the current report are all from a real-reward procedure.
While there does appear to be a robust relation between DD and cigarette smoking status, it is not clear whether individuals with already high rates of DD are more likely to become smokers, or whether smoking itself leads to higher rates of DD. There is cross-sectional evidence to suggest that current cigarette consumption is related to higher rates of DD. Bickel et al. (1999) found that current smokers discounted the value of hypothetical money more by delay than never smokers or ex-smokers who once smoked at the same rate as the current smokers. Based on this finding, it was later suggested that discounting by delay may be reversibly increased by cigarette smoking, thus leading to similar discounting tendencies between never and ex-smokers (Bickel and Marsch, 2001). Other researchers have found that smoking initiation at younger ages is related to higher rates of DD (Kollins, 2003), suggesting that DD may be a predisposing factor to cigarette smoking. However, this research was retrospective and did not consider participant smoking status at the time of data collection. Therefore, current smoking status may have been a confounding factor accounting for the observed relation between age of smoking initiation and rate of discounting if those who initiated earlier smoked more at the time of participation.
In an effort to examine more specifically the causal or consequent relation between DD and cigarette smoking status, DD functions of adolescent smokers and young-adult smokers and nonsmokers from two earlier studies (Reynolds et al., 2003, Reynolds et al., 2004) were cross-sectionally compared. If high rates of DD are a risk factor to becoming a smoker in adulthood, then the adolescent smokers (who reported smoking an average of six cigarettes per day) and young-adult smokers should not differ significantly in rate of DD because daily smoking in adolescence is highly predictive of increased smoking into adulthood (e.g., Chassin et al., 1990, Chassin et al., 1996, Jefferis et al., 2003). On the other hand, if a prolonged high rate of cigarette consumption itself leads to increases in rate of discounting, then young-adult smokers (who reported smoking at least 20 cigarettes per day) should discount more than adolescent smokers. Given this outcome, adolescent smokers are not expected to differ from young-adult nonsmokers in rate of DD. If this latter case were true, it also should be expected that the average number of cigarettes consumed per day and/or length of smoking history would be positively correlated with rate of DD.
Section snippets
Participants
Discounting rates of 19 adolescent male and female smokers, and 54 young-adult male and female smokers (n = 25) and nonsmokers (n = 29) were compared. The average age for the adolescent sample was 15.3 (S.D. = 0.65) years, and 20.04 (S.D. = 0.78) years was the average age for the adult sample. All non-smoking participants reported never having smoked. Adolescent smokers reported consuming an average of 6.77 (S.D. = 3.34) cigarettes per day and had started smoking an average of 2.23 (S.D. = 1.63) years
Results
Fig. 1 is a graph of the median indifference values and overall DD functions of the three groups. R2-values index the variance in data accounted for by Eq. (1). The data of all three groups were well characterized by Eq. (1), with R2-values ranging from .90 to .98. There was a significant main effect of group, F(2, 63) = 9.28, p = .000, two-tailed test. Based on post-hoc analyses, the young-adult smokers discounted significantly more than participants in the other two groups, p < .05. Participants in
Discussion
The results of this cross-sectional comparison lend support to the hypothesis that high rates of cigarette consumption increase rate of DD. First, adolescent smokers discounted less than adult smokers. This difference is opposite from what might be expected due to age differences between the groups, with past research showing younger participants discounting more than older participants (Green et al., 1994). Second, the adolescent smokers of this sample are particularly at risk of becoming
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