In previous studies, researchers have found that humans discount delayed rewards orders of magnitude less steeply than do other animals. Humans also discount smaller delayed reward amounts more steeply than larger amounts, whereas animals apparently do not. These differences between humans and animals might reflect differences in the types of rewards studied and/or the fact that animals actually had to wait for their rewards. In the present article, we report the results of three experiments in which people made choices involving liquid rewards delivered and consumed after actual delays, thereby bridging the gap between animal and human studies. Under these circumstances, humans, like animals, discounted the value of rewards delayed by seconds; however, unlike animals, they still showed an effect of reward amount. Human discounting was well described by the same hyperboloid function that has previously been shown to describe animal discounting of delayed food and water rewards, as well as human discounting of real and hypothetical monetary rewards.
Daily life often requires choices between rewards that differ on multiple dimensions. One common situation involves choosing between a larger delayed reward and a smaller reward that is available sooner. In such situations, the subjective value of a reward is typically inversely related to the delay until its receipt-a phenomenon termed delay discounting. Such discounting of delayed rewards has been observed in a wide range of species, including pigeons and rats (Green, Myerson, Holt, Slevin, & Estle, 2004; Richards, Mitchell, de Wit, & Seiden, 1997), monkeys (Stevens, Hallinan, & Hauser, 2005; Woolverton, Myerson, & Green, 2007), and humans (for a review, see Green & Myerson, 2004), suggesting that delay discounting is a fundamental aspect of decision making.
The decrease in the subjective value (V) of a reward as the delay (D) until its receipt increases is well described by a hyperboloid function:
V = A/(1 + kD)^sup s^, (1)
where A represents the amount of delayed reward, k is a constant governing how steeply the reward is discounted, and s is a nonlinear scaling parameter (Green & Myerson, 2004). Although Equation 1 describes discounting by both human and nonhuman animals, two notable differences have emerged. First, animals discount delayed rewards very steeply, so that even rewards that are delayed by as little as several seconds are judged to be of much less value than is an immediate reward. In humans, discounting effects are typically observed on time scales that are orders of magnitude longer (e.g., weeks or months vs. seconds). Second, humans show a magnitude effect, discounting smaller reward amounts more steeply than larger amounts, whereas no such magnitude effect has been observed in previous studies with animals (e.g., Freeman, Green, Myerson, & Woolverton, 2009; Green et al., 2004; Mazur, 2000).
These apparent differences between humans and animals, however, might be attributable to the types of rewards studied and/or to the fact that animals actually experienced having to wait for real rewards. Consistent with this interpretation, humans show steeper discounting when making decisions about directly consumable rewards (e.g., candy, soda, beer) as opposed to monetary rewards, perhaps because money is fungible or because it is not a primary reinforcer (Estle, Green, Myerson, & Holt, 2007; Odum & Rainaud, 2003). For example, Estle et al. found that the subjective value of 40 cans of soda decreased by 50% when receiving them was delayed by 6 months, whereas it took three times as long (18 months) for the subjective value of $40 to show a similar decline. Nevertheless, even with consumable rewards, the discounting function for humans in these studies was still much shallower than that observed in animals, suggesting greater patience. However, the rewards and delays in these previous studies were always hypothetical. …