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When humans are offered the choice between rewards available at different points in time, the relative values of the options are discounted according to their expected delays until delivery. Using functional magnetic resonance imaging, we examined the neural correlates of time discounting while subjects made a series of choices between monetary reward options that varied by delay to delivery. We demonstrate that two separate systems are involved in such decisions. Parts of the limbic system associated with the midbrain dopamine system, including paralimbic cortex, are preferentially activated by decisions involving immediately available rewards. In contrast, regions of the lateral prefrontal cortex and posterior parietal cortex are engaged uniformly by intertemporal choices irrespective of delay. Furthermore, the relative engagement of the two systems is directly associated with subjects' choices, with greater relative fronto-parietal activity when subjects choose longer term options.
In Aesop's classic fable, the ant and the grasshopper are used to illustrate two familiar, but disparate, approaches to human intertemporal decision making. The grasshopper luxuriates during a warm summer day, inattentive to the future. The ant, in contrast, stores food for the upcoming winter. Human decision makers seem to be torn between an impulse to act like the indulgent grasshopper and an awareness that the patient ant often gets ahead in the long run. An active line of research in both psychology and economics has explored this tension. This research is unified by the idea that consumers behave impatiently today but prefer/plan to act patiently in the future (1, 2). For example, someone offered the choice between $10 today and $11 tomorrow might be tempted to choose the immediate option. However, if asked today to choose between $10 in a year and $11 in a year and a day, the same person is likely to prefer the slightly delayed but larger amount.
Economists and psychologists have theorized about the underlying cause of these dynamically inconsistent choices. It is well accepted that rationality entails treating each moment of delay equally, thereby discounting according to an exponential function (1-3). Impulsive preference reversals are believed to be indicative of disproportionate valuation of rewards available in the immediate future (4-6). Some authors have argued that such dynamic inconsistency in preference is driven by a single decision-making system that...