Delay discounting as a measure of impulsivity: Task design, reliability, validity and neural mechanisms

Impulsivity is a multi-dimensional construct associated with a range of psychological concerns. Scholars and researchers have investigated impulsivity through different paths, such as personality assessments and behavioral measures. In particular, the delay discounting paradigm as a measure of impulsive decision making has received considerable attention. Delay discounting is used in both animal and human studies. The designs and procedures of delay discounting in different studies are discussed in this chapter. Factors that influence delay discounting parameters include age, state, reward domain, reward magnitude, etc. The delay discounting phenomenon can be described using different models that are based on different assumptions. The current literature indicates that the exponential model describes empirical data well in economic studies when the risk associated with the delay duration is more constant and in human studies when rational strategies are emphasized. On the other hand, the hyperbolic model better describes the decision making processes of humans and animals when irrational components, such as emotions, are involved. A number of neural models have been proposed to explain the underlying mechanisms. Based on the results of some empirical studies, models that allow differentiating different neural process may provide better interpretation for studies on impulsivity, especially when the clinical populations are involved. Based on the studies using hyperbolic model, the reliability of delay discounting rate is high among normal populations but is comparatively lower in clinical populations. The empirical studies support that delay discounting rate reliably differentiates various clinical populations from controls with respect to impulsivity. Positive correlations between delay discounting rate and personality measures of impulsivity are also supported by a number of studies