Using Nudges in Exercise Commitment Contracts

Featured in print Bulletin on Aging & Health

Sedentary lifestyles are a major contributor to the rising incidence of obesity and obesity-related diseases like hypertension and diabetes in the U.S. Many individuals are aware of the health benefits of exercise and claim to want to be more active, but are unable to translate their good intentions into sustained behavioral change.

Behavioral economics may help to explain this failure. Starting and sustaining an exercise program requires incurring immediate costs in order to obtain future rewards. According to the theory of dynamic inconsistency, decision-makers are more impatient with respect to near-term tradeoffs than to long-term tradeoffs. People believe that exercise is worthwhile when thinking of the long run, but the costs of exercise loom larger when they are deciding whether to exercise today. They may defer exercising to another day, and by making the same decision day after day end up postponing it indefinitely.

Numerous studies of saving, another behavior potentially affected by dynamic inconsistency, suggest that a "nudge" in the form of a default may strongly influence behavior. In Committing to Exercise: Contract Design for Virtuous Habit Formation (NBER Working Paper 16624), researchers Jeremy Goldhaber-Fiebert, Erik Blumenkranz, and Alan Garber conduct a randomized controlled trial to explore the effect of nudges and anchoring on the exercise commitment contracts individuals enter into using a web-based contract creation tool (

In the experiment, users of the tool select a contract length (duration), number of times per week to exercise (frequency), and a financial penalty for failing to live up to the contract (stake). The authors randomly set the default duration seen by users to be either 8, 12, or 16 weeks, though users are free to change the duration with a simple click of the mouse. The authors also randomly assign some users to see a notice informing them that those who agreed to pay a financial penalty for failing to live up to their contract are more successful at fulfilling their commitment.

The authors examine whether these nudges affected the likelihood of users' accepting a contract as well as the chosen duration, frequency, and financial stake. Their experiment included data for 619 adult users, roughly 60 percent of who accepted an exercise contract.

The authors' principal finding is that users who were shown a longer default contract duration chose contracts of longer duration. There was no effect on the chosen exercise frequency, so showing users a longer default contract duration led them to contract for a greater number of total exercise sessions. There was no effect of the default duration on the probability of accepting a contract or on the chosen financial stake.

As the authors acknowledge, one limitation of their study is that they were not able to evaluate contract compliance and so could not determine whether entering into longer contracts helped people to form long-term exercise habits. However, they are already at work on answering this question using a new longitudinal study of users. They conclude, "to the extent that longer contracts increase long-term exercise habit formation, carefully designed default values can lead users towards selecting exercise commitment contracts with lengths optimal for addressing sedentary lifestyles."

The authors gratefully acknowledge funding from the National Institute on Aging (grant P30 AG24957 to the Stanford University Center for Advanced Decision Making in Aging) and by the Department of Veterans Affairs.