Abstract:

Deficiency in inter-temporal decision making of substance addicts has been consistently confirmed by a vast array of studies using monetary Delay Discounting Task. Substance addicts showed a myopia tendency towards the immediate reward during the process of inter-temporal decision making. This myopia tendency corresponds well with the substance-use behavior in their real life. However, the recently emerging evidence from new variants of the monetary Delay Discounting Task (the addictive substance Delay Discounting Task, the Sexual Delay Discounting Task, and the Cross-Commodity Delay Discounting Task) highlighted some limitations in the single use of this paradigm in the delay discounting study of substance addiction, which need to be considered in terms of external validity.
First of all, the results from monetary Delay Discounting Task may not fully reflect and represent the overall patterns of delay discounting in substance addictive population, the delay discounting level of addictive substance as cigarette, alcohol and heroin, of sex and of other natural rewards were significantly different from the delay discounting level of money. This further confirmed the hypothesis that delay discounting has domain specificity. That is to say, there may not be a general level of delay discounting despite different types of rewards. We need to be cautious when explain the results from monetary Delay Discounting Task in the study of substance addiction, and the additional employing of addictive substance Delay Discounting Task may improve the external validity of studies in this area.
Secondly, by employing Single Delay Discounting task alone may over simplify the intertemporal decision-making process of substance addicts. The intertemporal decision-making process faced by this group in real life may be more complex than the one simulated by Single Delay Discounting Task. In real life, substance addicts often face intertemporal choices between different types of rewards including but not limited to money, for example, choice between immediate drug using VS. delayed financial benefits in future. According to the specific position of reward in intertemporal decision-making (the immediate one or the delayed one), the relative value between different types of rewards may change. It is of great significance for the purpose of description, explanation, prediction and intervention in the study of intertemporal decision-making in substance addicts to employ the Cross-Commodity Delay Discounting Task more frequently in future studies.
Finally, compared with its variants, the monetary Delay Discounting Task was not sensitive to the withdrawal status, addiction severity and clinical characteristics related to risky sexual behaviors of substance addicts, hence the monetary Delay Discounting Task showed poor ecological validity compared with Cross-Commodity Delay Discounting Task and Sexual Delay Discounting Task. Whether the monetary Delay Discounting Task is the most ideal task to capture the dynamic changes of intertemporal decision-making in substance addicts after withdrawal and intervention remains to be determined. And here is reason to suspect the results from monetary Delay Discounting Task in the perspective of external validity.
Considering the external validity of studies on intertemporal decision-making of substance addicts, future research should further enrich and expand the research which employs the Cross-Commodity Delay Discounting Task in the field of substance addiction. Besides, there emerged some evidence which have showed that the authenticity of waiting time between immediate choice and delayed choice would also affect the process of intertemporal decision-making, and the subjective time perception of the waiting duration may also play a nonnegligible role in intertemporal decision-making of substance addicts. So it may be valuable to further elucidate the time factors that may affect intertemporal decision-making. At the same time, researchers also need to consider the question of how to equate non-material rewards with money. The development of effective quantitative methods for non-material rewards such as health, sex and freedom is necessary. In addition, questionnaires and interviews can be introduced to future studies of delay discounting to explore the real motivation of substance addicts to obtain money reward in Delay Discounting Tasks.

Key words: substance addiction, delay discounting, inter-temporal decision making, external validity