Will Power: Dopamine and Temporal Discounting

Temporal discounting (a.k.a. delay discounting and hyperbolic discounting) is a concept in psychology and behavioral economics that describes the tendency for animals and people to prefer smaller rewards that will happen sooner (SS) over larger rewards that will occur later (LL). This concept has been developed by George Ainslie, who wrote Breakdown of Will, a book I recommend. This approximates the concept of "impulsive choice," which is integral to our understanding of drug addiction. It is also why we may value being healthy and physically fit over eating that piece of chocolate cake, but we still end up eating the cake when it's in front of us.

Temporal discounting is not just associated with pathologies, although it does describe a normal tendency to behave irrationally. For example, most people offered $50 now versus $100 (inflation adjusted) in two years, would choose the $50. On the other hand if they could choose between $50 in five years versus $100 in seven years the vast majority would choose the latter option. How is this possible? In five years, after all, they would be in the identical situation as the first scenario, in which they make the opposite choice.

The rate (shape) of a value curve as a function of delay can explain this discrepancy. Most behavioral data indicate that the change in value as a function of delay follows a hyperbolic function (as opposed to exponential, for example) function.

This figure, from Ross et al. (University of Alabama) shows that when a delay is short, the hyperbolic function allows a SS reward to be more strongly valued than a LL reward, even though the smaller reward is less strongly valued most of the time.Depending on the individual and the reward involved, the value curve can look dramatically different. A smoker, may be certain they are smoking their last cigarette and feel very much in control. Hours later, that same smoker, because of a sharply scooped out hyperbolic value curve, may find himself lighting up once again. This is one way to understand "will power": The sharper the hyperbola, the more impulsive the behavior appears.

In the July 30, 2008 issue of the Journal of Neuroscience Kobayashi & Shultz publish a paper showing that dopamine neurons, which are known to be associated with reward prediction error and incentive salience, also encode temporal discounting in a Pavlovian conditioning task. This suggests that prior to making the choice, our dopamine neurons are likely firing less to the idea of $100 in two years compared than to the idea of $50 now.
If you believe in "free will" (I will do another post on this topic sometime), how do you square away the potential role of dopamine neuronal activity in constraining value? In fairness, it remains to be proven that the activity of these dopamine neurons causes the behavioral phenomenon. Nevertheless, future work will undoubtedly examine how well we can predict subsequent choice based on the early activity of dopamine neurons to differently valued and differently delayed rewards prior to their presentation as a choice. If we are able to do this accurately, it seems to me we will have strong neurobiological evidence for value constraints that precede choice and will.