Single- and cross-commodity discounting among cocaine addicts: the commodity and its temporal location determine discounting rate

Abstract

Rationale: Intertemporal choice has provided important insights into understanding addiction, predicted drug-dependence status, and outcomes of treatment interventions. However, such analyses have largely been based on the choice of a single commodity available either immediately or later (e.g., money now vs. money later). In real life, important choices for those with addiction depend on making decisions across commodities, such as between drug and non-drug reinforcers. To date, no published study has systematically evaluated intertemporal choice using all combinations of a drug and a non-drug commodity.

Objectives: In this study, we examine the interaction between intertemporal choice and commodity type in the decision-making process of cocaine-dependent individuals.

Methods: This study of 47 treatment-seeking cocaine addicts analyzes intertemporal choices of two commodities (equated amounts of cocaine and money), specifically between cocaine now vs. cocaine later (C-C), money now vs. money later (M-M), cocaine now vs. money later (C-M), and money now vs. cocaine later (M-C).

Results: Cocaine addicts discounted significantly more in the C-C condition than in M-M (P = 0.032), consistent with previous reports. Importantly, the two cross-commodity discounting conditions produced different results. Discounting in C-M was intermediate to the C-C and M-M rates, while the greatest degree of discounting occurred in M-C.

Conclusions: These data indicate that the menu of commodities offered alter discounting rates in intertemporal choice and that the greatest rate is obtained when the drug is the later available commodity. Implications for understanding intertemporal choices and addiction are addressed.

Fig. 1

Mean ln(k) (a) and k (b), along with their standard errors are plotted for each discounting condition. Dashed horizontal lines indicate comparable points between the two graphs. M-M is money–money discounting, C-M is cocaine–money discounting, C-C is cocaine–cocaine discounting, and M-C is money–cocaine discounting

Fig. 2

Scatterplots of ln(k) from one condition plotted against the mean of the ln(k)s from the other three conditions. a ln(k) from C-C against mean ln(k) from C-M, M-C, and M-M; b ln (k) from M-M against mean ln (k) from C-C, C-M, and M-C; c ln(k) from C-M against mean ln (k) from C-C, M-C, and M-M; d ln(k) from M-C against mean ln (k) from C-C, C-M, and M-M

Fig. 3

Patterns of apparent discounting predicted by different utility functions or different discount functions for cocaine versus money. a Cocaine and money have the same utility function and the same discount function. b Cocaine and money have the same utility function but cocaine has a faster discount function. When cocaine is the delayed commodity, the apparent discount rate is faster. c Cocaine has a more concave utility function than money but the same discount function as money. When cocaine is the immediate commodity, the apparent discount rate is faster. d Cocaine has a more concave utility function and a faster discount function than money. C-M and M-C are faster than M-M, while C-C is the fastest. e Cocaine has a less concave utility function and a faster discount function than money. M-C is faster than C-C by a greater degree than M-M is faster than C-M