Time discounting and time preference in animals: A critical review

Abstract

Animals are an important model for studies of impulsivity and self-control. Many studies have made use of the intertemporal choice task, which pits small rewards available sooner against larger rewards available later (typically several seconds), repeated over many trials. Preference for the sooner reward is often taken to indicate impulsivity and/or a failure of self-control. This review shows that very little evidence supports this assumption; on the contrary, ostensible discounting behavior may reflect a boundedly rational but not necessarily impulsive reward-maximizing strategy. Specifically, animals may discount weakly, or even adopt a long-term rate-maximizing strategy, but fail to fully incorporate postreward delays into their choices. This failure may reflect learning biases. Consequently, tasks that measure animal discounting may greatly overestimate the true discounting and may be confounded by processes unrelated to time preferences. If so, animals may be much more patient than is widely believed; human and animal intertemporal choices may reflect unrelated mental operations; and the shared hyperbolic shape of the human and animal discount curves, which is used to justify cross-species comparisons, may be coincidental. The discussion concludes with a consideration of alternative ways to measure self-control in animals.

Economic Choice as an Untangling of Options into Actions

“…The reward system integrates across dimensions to generate abstract decision variables; that is, ones that are correlated with value (e.g. Barron et al, 2017; Blanchard et al, 2015; Fellows, 2006; Hayden, 2016; Hunt and Hayden, 2017; Raghuraman and Padoa-Schioppa, 2014; Strait et al, 2014; Suzuki et al, 2017). However, unlike the visual system, which has only one major input locus—the eyes—the reward system has many inputs.…”

Impulsivity or Sub-optimal Reward Maximization in Delay Discounting? A Critical Discussion

“…Another problem with the compensating delay discounting design is the assumption that the animals are affected by consequences following reward delivery (Hayden, 2016). In other words, it assumes that the animals are aware of a waiting time between trials.…”

“…A possible explanation for this is an ecological one: In nature, the length of the inter-trial-interval is likely to correlate with the size of the reward, since the animal needs to consume and digest. The inter-trial-interval would not become shorter as LL grows larger (Hayden, 2016). We may use the analogy of hunting prey: An increased delay is analogous to increased hunting or searching time, which may secure a larger prey (LL reward).…”

“…Another approach takes into account perception of time, and describes reward discounting as the actual reward value divided by the sum of pre-and post-reward delays (Blanchard, Pearson, & Hayden, 2013). This latter equation recognizes that an animal's time estimate is affected by learning and attention, and therefore the perceived value of each choice will vary from individual to individual (Hayden, 2016).…”

Cognitive Science: Persistent Apes Are Intelligent Apes

“…The field is characterized not only by the standard empirical debates, but also by elementary definitional debates about what is and is not self-control, and whether it is a single thing or multiple distinct things [2,3]. These issues, difficult enough to approach in human studies, loom even larger in animal studies, where we cannot directly talk to our subjects [4,5]. A new study by Beran and Hopkins, reported in this issue of Current Biology [6], makes a great stride by linking self-control to general intelligence in chimpanzees.…”