Contrary to the study hypothesis and in contrast with previous studies showing drug-dependent groups discount more than control groups, marijuana dependent individuals were not found to discount significantly more than the non-user control group. This lack of effect does not appear to be due to a lack of sufficient power to detect effect sizes found in previous studies with other drug dependent populations. The present study employed a similar number of participants as several published studies showing significant differences between drug dependent individuals and control individuals (e.g., Bickel et al., 1999
; Coffey et al., 2003
; Heyman & Gibb, 2006
; Mitchell et al., 2005
; Petry & Casarella, 1999
; Reynolds, 2006
; Vuchinich & Simpson, 1998
). One possible interpretation is that marijuana dependence, unlike other forms of drug dependence, is not associated with increased discounting. However, this seems unlikely given that there was a trend for currently marijuana-dependent individuals to discount money more than controls. The effect size of this trend indicated a smaller effect than that found in other drug-dependent and control comparisons collected with similar methods in the same laboratory. Consistent with this interpretation, regression results suggest that within the present study, delay discounting was more closely associated with tobacco use than marijuana use. It remains unclear whether a substantially larger study might detect a significant difference between currently marijuana-dependent individuals and matched controls. suggests the possibility that the currently marijuana-dependent group may consist of a flatter distribution (and perhaps a trend toward a bimodal distribution) in money discounting, suggesting that marijuana dependence may be more strongly associated with delay discounting for a subgroup of dependent users. A tentative conclusion at the present time is that marijuana dependence may be associated with delay discounting, but less so than forms of drug dependence previously investigated (e.g., tobacco, cocaine, opioids, alcohol). If this conclusion is correct, then the present data also suggest that, as with tobacco and alcohol (Bickel et al., 1999
; Petry, 2001
), former dependent marijuana users discount less than currently dependent users.
Marijuana dependence shares several commonalities with other forms of drug dependence, including having a neuropharmacological substrate with ties to brain reward systems, a characteristic withdrawal syndrome, and frequent relapse among those in treatment (Budney & Moore, 2002
). Why, then, would it differ from other drugs in the strength of its association with delay discounting? Perhaps an important framework for understanding drug dependence is that different drugs can be rank ordered along multiple dimensions related to their dependence potential. For example, while psychomotor stimulants such as cocaine typically rank high on several measures of laboratory drug reinforcement (e.g., Griffiths, Brady & Bradford, 1979
), they are associated with only relatively mild (i.e., not medically severe) withdrawal symptoms among dependent users (Lago & Kosten, 1994
). In other words, while drug withdrawal is an important clinical phenomenon in drug dependence, it plays a less prominent role for stimulant dependence relative to sedatives and opioid dependence. Similarly, while delay discounting may be an important clinical phenomenon in drug dependence generally, the present results suggest it may play a relatively small role in marijuana dependence relative to other drugs such as tobacco. Indeed, drug dependence may result from multiple underlying processes, and delay discounting may be only one of those processes (Bickel and Johnson, 2003
One possible explanation for a weaker association between delay discounting and marijuana dependence relative to other drugs previously studied may be that marijuana is associated in the public view with less salient future health consequences. Perhaps low discounters may be dissuaded from trying drugs such as tobacco, resulting in a disproportionate number of high discounters initiating these drugs (with some proportion going on to become dependent). To the extent that the public may view marijuana as less associated with future negative consequences, a broader array of individuals along the distribution of discounting in the population may initiate marijuana use, with some proportion of those initiates becoming dependent. Consistent with this hypothesis, data from the National Comorbidity Study indicate a larger proportion of the population has used marijuana compared to other illicit drugs (Anthony, Warner & Kessler, 1994
). Although only a proportion of these become dependent (at a roughly similar rate to other drugs of abuse), the resulting sizeable marijuana-dependent population may include a relatively large distribution of individuals across the delay-discounting continuum. This may seem inconsistent with the strong association between delay discounting and tobacco use, which is even more prevalent than marijuana use. However, it is possible that the very well-known and salient future health consequences of tobacco use make it a special case, that is, a drug especially prone to self-selection of high discounters, as mentioned above. It should be noted that this suggested hypothesis assumes that increased delay discounting causes drug dependence, whereas it may also be possible that drug use increases delay discounting. However, the two possibilities are not mutually exclusive, and it seems likely that both may be true.
Of the 18 personality questionnaire subscales examined in this study, only the Eysenck IVE Impulsivity subscale, which measures the degree to which one behaves without thought of risks or consequences, statistically differentiated the marijuana-dependent group from the control group. There is no clear explanation as to why scores on this specific scale but not others, including the BIS-11, showed differences across groups.
The currently marijuana-dependent group discounted marijuana significantly more than money. The observation that marijuana dependent individuals discount future marijuana more than an equivalent amount of money is consistent with studies demonstrating that tobacco, heroin, and crack/cocaine dependent individuals discount their drug of dependence more so than money (Baker et al., 2003
; Bickel et al., 1999
; Coffey et al., 2003
; Madden et al., 1997
; Petry, 2001
). However, because the discounting of other non-monetary, consumable commodities was not assessed in these studies, one cannot determine if rapid discounting of the drug is indicative of a drug of dependence, or is a more general feature of consumable commodities or primary reinforcers. That is, a number of studies have shown that non-drug consumable reinforcers such as food are discounted more than money (Estle et al., 2007
; Odum & Baumann, 2008; Odum & Rainaud, 2003
; Odum, et al., 2006
). The present results showing that the difference between money and marijuana discounting was no longer significant after controlling for marijuana consumption rate is consistent with the interpretation that marijuana was discounted more than money because it is a consumable, primary reinforcer.
Both discounting measures were positively and significantly correlated with each other. The STPI Present-Fatalistic subscale was positively correlated with money and marijuana discounting, indicating that a greater sense of powerlessness over the future is related to greater delay discounting. However, no significant correlations were found between delay discounting and other questionnaire scales. These results are consistent with the variability observed across studies in previous research examining the correlations between discounting and personality questionnaires (e.g., Coffey et al., 2003
; Madden et al., 1997
; Mitchell, 1999
; Richards et al., 1999
). In the present study, the correlation between money and marijuana discounting was higher than correlations between discounting and personality questionnaires, suggesting that although commodities can be discounted at different rates, individuals nonetheless have some trait-like disposition toward delay discounting that is relatively distinct from what is captured by questionnaire measures.
If future studies yield similar results, they may confirm that the association between discounting and drug dependence may be weaker for marijuana than for previously studied drugs (e.g., tobacco, cocaine, opioids, alcohol). Future research examining discounting in a wider variety of populations (i.e. different drugs of abuse) may help to further examine the contribution of delay discounting to the phenomenon of drug dependence.