Delay and probability discounting by drug-dependent cocaine and marijuana users

Steep discounting of delayed monetary rewards by substance-dependent individuals is well-established. Less is known, however, about discounting other kinds of outcomes, and very little is known about discounting by marijuana-dependent individuals.

To determine how cocaine-dependent individuals and marijuana-dependent individuals discount various delayed and probabilistic outcomes, both positive and negative.

Marijuana-dependent individuals, cocaine-dependent individuals, and controls performed delay and probability discounting tasks with various hypothetical outcomes.

The cocaine-dependent (but not the marijuana-dependent) group discounted delayed liquid rewards and monetary gains, but not delayed losses, more steeply than the control group. In contrast, the marijuana-dependent group (but not the cocaine-dependent group) discounted delayed monetary losses more steeply than controls. There were no group differences in discounting for any of the probabilistic outcomes. Factor analysis revealed a delayed gain factor, a probabilistic gain factor, and a delayed/probabilistic loss factor. The delayed gain factor scores for the cocaine-dependent group, but not the marijuana-dependent group, differed significantly from those of the control group. The groups did not differ in their probabilistic gain factor scores, and the marijuana-dependent group did not differ from the controls with respect to their loss factor scores.

These results are inconsistent with the idea that steep discounting of both gains and losses and both delayed and probabilistic outcomes reflects a general impulsivity trait, as well as with the idea that all drug-dependent individuals are steep discounters. Rather, differences in discounting appear to be related to both the type of outcome and the specific drug on which individuals are dependent.