A second type of magnitude effect: Reinforcer magnitude differentiates delay discounting between substance users and controls

Abstract

Basic research on delay discounting, examining preference for smaller-sooner or larger-later reinforcers, has demonstrated a variety of findings of considerable generality. One of these, the magnitude effect, is the observation that individuals tend to exhibit greater preference for the immediate with smaller magnitude reinforcers. Delay discounting has also proved to be a useful marker of addiction, as demonstrated by the highly replicated finding of greater discounting rates in substance users compared to controls. However, some research on delay discounting rates in substance users, particularly research examining discounting of small-magnitude reinforcers, has not found significant differences compared to controls. Here, we hypothesize that the magnitude effect could produce ceiling effects at small magnitudes, thus obscuring differences in delay discounting between groups. We examined differences in discounting between high-risk substance users and controls over a broad range of magnitudes of monetary amounts ($0.10, $1.00, $10.00, $100.00, and $1000.00) in 116 Amazon Mechanical Turk workers. We found no significant differences in discounting rates between users and controls at the smallest reinforcer magnitudes ($0.10 and $1.00) and further found that differences became more pronounced as magnitudes increased. These results provide an understanding of a second form of the magnitude effect: That is, differences in discounting between populations can become more evident as a function of reinforcer magnitude.

Keywords: crowdsourcing; delay discounting; humans; hypothetical; magnitude effect.

Figure 1

Displayed in panel A are group comparisons of discounting rates (as natural-log-normalized k values) between controls with low AUDIT ( < 8) & FTCD ( < 4) scores and dual-substance users with high AUDIT ( > 10) and FTCD ( > 4) scores, over all five magnitudes of reinforcer examined. Error bars indicate standard error of the mean. Asterisks denote levels of significance of group difference between mean discounting rates. Panels B-F display group data including mean and and 95% confidence intervals as well as individual data points representing individual participant’s natural-log-normalized discounting rates across the $0.10, $1.00, $10.00, $100.00, and $1000.00 conditions, in that order.