The evolutionary origins of temporal discounting: an adaptive peak model shows how time and uncertainty impose constraints on selection for optimal decision-making in a temporal framework.

Abstract

AbstractThe propensity of humans and non-human animals to discount future returns for short-term benefits is well established. This contrasts with the ability of organisms to unfold complex developmental sequences over months or years. Research has focused on various descriptive and predictive parameters of ‘temporal discounting’ in behavior, and researchers have proposed models to explain temporal preference in terms of rational outcomes, but the underlying cause of this phenomenon has not been deeply explored. We propose that preference for short-term reward (‘impulsivity’) may not be rational when examined from the perspective of an omniscient observer, but may be the product of the way natural selection acts on events in a temporal framework in the context of future uncertainty. Using a simple Newtonian model for time across a fitness landscape in which movement by organisms is only possible in one direction, we examine several factors that influence the ability of an organism to choose a distant reward over a more temporally proximate reward: including the temporal distance of the far reward, the relative value of the distant reward, and the effect of uncertainty about the value and presence of the distant reward. Our results indicate that an organism may choose a more distant reward, but only if it is not too far into the future, and only if it has a substantially higher-value fitness payoff relative to the short-term reward. Most notably, any uncertainty about the distant reward made it extremely unlikely for an organism to choose the delayed reward strategy compared to choosing a closer reward, even if the distant reward had a much higher payoff because events that are uncertain are only partially visible to natural selection pressures. We argue that these results explain why so many animals have difficulty making 'better' long-term rational strategies for a distant reward over the lower-value short-term reward. Uncertainty is likely to be an especially important ecological factor in promoting and biasing short-term behavioral strategies. These results help illustrate why human and non-human animals have difficulty making the more rational choice when faced with short-term and long-term rewards.