Delay discounting in the pigeon: In search of a magnitude effect

Abstract

The magnitude effect, where larger outcomes are discounted proportionally less than smaller outcomes, is a well‐established phenomenon in delay discounting by human participants. To this point in the literature magnitude effects have not been reliably evidenced in nonhuman animals. , however, used a concurrent‐chains arrangement with pigeon and found evidence for a magnitude effect. Grace et al. suggested that in many delay discounting experimental arrangements with nonhuman animals (e.g., adjusting amount, adjusting delay) the organism is not given the opportunity to directly compare outcomes of different sizes. They suggest that because of the lack of direct comparison it is difficult for the organism to determine the relative size of each outcome, which in turn mutes the effect of the amount differences between outcomes. As a test of this “comparison hypothesis,” the present experiment was conducted to assess whether the magnitude effect would be evidenced in pigeon when using an adjusting amount procedure where outcomes of different amounts were presented proximally. In the present arrangement, pigeons were presented two choice panels in an operant chamber where each panel was associated with an independent adjusting amount delay discounting task, but with differing outcome amounts (i.e., a 32‐food pellet panel and an 8‐food pellet panel). In this arrangement the choice panels alternated in their availability within a session from trial block to trial block. The present findings indicate no reliable effect of amount, even when the outcomes were proximal and thus readily comparable. This result suggests that the lack of magnitude effect is not driven by the organism's ability to compare the difference in amount between choice alternatives.

Magnitude effect contributes to the domain specificity in delay discounting

“…The magnitude effect is only observed in humans, but not in other animals, suggestive of requiring higher‐order cognitive alteration of the discount rates (Green & Myerson, ; Green, Myerson, Holt, Slevin, & Estle, ; Holt & Wolf, ; Jimura, Myerson, Hilgard, Braver, & Green, ), and is more prominent in the domain of gains rather than losses (Estle, Green, Myerson, & Holt, ; Jimura et al, ; Mitchell & Wilson, ). The magnitude effect should not be attributed to sloppiness or to lack of deliberation when deciding for small rewards, as the magnitude effect is more pronounced when participants discount small and large amounts in short succession compared with when they evaluate these payoffs separately (Frederick & Read, ).…”

What enables “distraction” to reduce delay discounting for pigeons (Columba livia).

“…Since then, there has been considerable research published on “self-control” with children and other animals. Several factors have been studied that affect the degree to which a more preferred reinforcer available later (larger-later [LL]) is preferred over a less preferred reinforcer available sooner (smaller-sooner [SS]), including the value of each reinforcer (Ballard et al, 2017; Holt & Wolf, 2019), the delay to receive each of the reinforcers (Ainslie, 1974; Miller & Karniol, 1976; Mischel et al, 1989; Rachlin & Green, 1972), and the subjective value of each of the reinforcers (Gailliot, 2013; Ostaszewski et al, 2003; Read & van Leeuwen, 1998).…”