The domain effect in delay discounting: The roles of fungibility and perishability.

There is a growing body of literature demonstrating domain effects where the rate of temporal discounting depends, in part, on the commodity being evaluated. The commodity of money, for example, is typically discounted much less steeply than commodities of entertainment or food. There are several plausible explanations for domain effects: differences in conditioned reinforcer status, degree of fungibility, and differences in metabolic function. While money can be thought of as a conditioned reinforcer exchangeable for a number of different outcomes (highly fungible), comparing money to food (non-fungible) does not separate whether the difference in rates of discounting are due to food having metabolic importance, being perishable, being less fungible, or all of the above. We systematically manipulated the degree of fungibility and perishability of various outcomes and found that while food outcomes tend to be discounted most steeply, the rate of discounting for these outcomes can be moderated by reducing perishability and by increasing fungibility. Important here is that we have identified two independent means of moderating the effect of delay on the value of the outcome.

Discounting of food, sex, and money.

Discounting is a useful framework for understanding choice involving a range of delayed and probabilistic outcomes (e.g., money, food, drugs), but relatively few studies have examined how people discount other commodities (e.g., entertainment, sex). Using a novel discounting task, where the length of a line represented the value of an outcome and was adjusted using a staircase procedure, we replicated previous findings showing that individuals discount delayed and probabilistic outcomes in a manner well described by a hyperbola-like function. In addition, we found strong positive correlations between discounting rates of delayed, but not probabilistic, outcomes. This suggests that discounting of delayed outcomes may be relatively predictable across outcome types but that discounting of probabilistic outcomes may depend more on specific contexts. The generality of delay discounting and potential context dependence of probability discounting may provide important information regarding factors contributing to choice behavior.

Response factors in delay discounting: evidence for Pavlovian influences on delay discounting in pigeons.

Pigeons completed a delay-discounting task where in different conditions the required response was either key pecking or treadle pressing. Because of stimulus-reinforcer relations that are known to form between localized visual cues and the delivery of food (e.g., autoshaping), we predicted that there would be steeper rates of discounting with key pecking than treadle pressing. To account for possible effort differences between key pecking and treadle pressing, pigeons also completed a discounting task where multiple key pecks were required to gain access to the food. The rates of discounting for the key peck and effort-equivalence discounting procedures were similar, and both were steeper than the rate of discounting for the treadle-pressing procedure. While it is tacitly assumed that behavior in choice situations is largely under the control of operant contingencies, the present results suggest that when developing animal analogs to study discounting in a discrete-trial choice procedure, the stimulus-reinforcer relations (Pavlovian conditioning) may need to be taken into consideration.