What is it about?

A well established measure of impulsivity is the willingness to wait to receive a reward. This can be expressed as a curve by which the value of a reward decreases as it is delayed, termed delay discounting. The steepness of this curve can be measured for a given person, by offering them choices between immediate and delayed rewards of varying sizes (e.g. £10 today vs £15 in two weeks). When the reward environment is unstable, or volatile, the value of delayed rewards is more variable. For example, if you make an investment in a volatile market, the final return is unpredictable. In fact, unpredictability in the payoff grows with time. Despite this intuition, few previous studies have examined how delay discounting depends on volatility in the value of reward. We show that, in decision making tasks, peoples' willingness to wait for reward adapts to the volatility of the environment, such that people become more impatient in volatile conditions. This sensitivity to volatility correlates with an aversion to risk. Furthermore, people who are more sensitive to volatility are also less willing to wait for rewards in 'real world' choices involving delayed money. Using fMRI, we also show that, when rewards are more volatile, brain regions involved in imagining future scenarios (medial temporal lobe, including hippocampus) show reduced connectivity with prefrontal brain regions involved in representing the value of delayed rewards. These findings show that peoples' impatience to receive reward is partly down to their aversion to risk, coupled with an expectation that delayed rewards are unpredictable, i.e. volatile. Our neuroimaging findings suggest that volatility reduces the extent to which future rewards can be clearly imagined, thus reducing their subjective impact.

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Why is it important?

The study introduces a new model of individual differences in impatient (impulsive) choice, based on an idea that the value of future rewards is less precisely known than that of immediate rewards. Specifically, we show that hyperbolic discounting can be derived if future rewards are valued in inverse proportion to their variance.


This model contributes to an idea that a person's degree of impatience reflects past experience with the cost of waiting for rewards. Past experience in volatile environments is likely to encourage greater impulsivity. Our brain imaging data suggest that volatility also makes it harder to imagine future scenarios, suggesting a possible mechanism for increased impulsivity.

Giles Story
University College London

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This page is a summary of: Discounting future reward in an uncertain world., Decision, June 2023, American Psychological Association (APA),
DOI: 10.1037/dec0000219.
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