Moderate Stability among Delay, Probability, and Effort Discounting in Humans

Abstract

The stability of delay discounting across time has been well-established. However, limited research has examined the stability of probability discounting, and no studies of the stability of effort discounting are available. The present study assessed the steady-state characteristics of delay, probability, and effort discounting tasks across time with hypothetical rewards in humans, as well as whether response characteristics suggested a common discounting equation. Participants completed delay, probability, and effort discounting tasks on three occasions. We found moderate relative stability of delay and probability tasks, and similar evidence for absolute stability across time for all tasks. The interclass correlations coefficient showed some correspondence across time points and tasks, and higher levels of between subject variability, especially for the effort discounting task, suggesting trait level variables has a stronger influence on performance than state level variables. Performance on the delay and probability tasks were moderately correlated and similar mathematical functions fit choice patterns on both tasks (hyperbolic), suggesting that delay and probability discounting processes shared some common elements. Lower correlations and different function fits suggested that effort discounting involves more unique features.

Keywords: Delay discounting; Effort discounting; Mathematical functions; Probability discounting; Stability.

Fig. 1

Test–Retest Correlations with 95% Confidence Intervals. Note. Circles represent the AUC_ord values for individuals. The left column shows time points 1 and 2, the middle column shows time points 2 and 3, and the right column shows time points 1 and 3. The top row depicts the test–retest data for delay discounting, the middle depicts the probability discounting time points, and the bottom row depicts the effort discounting data. Each test–retest graph includes its BF₁₀ value and the Pearson correlation coefficient

Fig. 2

AUC_ord for Delay, Probability, and Effort Discounting across Time Points. Note. Box plots of AUC_ord values for the participants who completed the discounting tasks across the three time points. The bottom and top of each box represent the 25^th and 75^th percentiles, the horizontal line within each box represents the group median. The vertical lines extending from the boxes represent the minimum and maximum values that are not outliers. The circle out of the whiskers is an outlier (delay discounting at Time 1). The symbols inside each box represent the individual AUC_ord.

Fig. 3

Model-Fitting to the Median Indifference Points across Time Points for Each Task. Note. Hyperbolic (first column), hyperboloid (middle column), and power function (last column) model fits to the median IPs among tasks and across time points. The top row shows the fits for the delay discounting task, the middle row shows those for the probability discounting task, and the bottom row, the effort discounting task