Purpose: Regular exercise offers an important solution to the growing burden of obesity-related chronic disease. We evaluated the use of commitment contracts and nudges to promote habitual exercise, focusing on the duration of the contracts and the heterogeneity of individual responses to these behavioral economic devices.
Methods: A randomized controlled trial examined the use of a web-based tool for creating exercise commitment contracts for 3,179 adults (aged 18-77) between September, 2010 and April, 2011. Individuals were randomized to be shown different default contract durations (8 weeks, 12 weeks, or 20 weeks) which they could easily change if they wished. After also choosing the number of exercise sessions per week (frequency) and financial penalty for failing to complete each week, each individual who ultimately signed a contract was followed for the duration of the contract, with weekly reports of their success in meeting exercise goals. For this analysis, follow-up through 13 weeks was available for 1,268 individuals representing 12,574 person-weeks. We analyzed the data using nonlinear multivariable regressions based on a theoretical model of active choice in the context of nudges.
Results: Longer duration nudges increased the mean duration of contracts chosen (13.5 weeks, 14.7 weeks, 18.6 weeks) without altering the likelihood of signing a contract (~70% for all arms), chosen exercise frequency (3.98, 3.93, 3.94 sessions per week), or chosen financial incentives ($6.90, $6.09, $6.81 per week). Based on our active choice model, more than 40% of users were highly susceptible to contract duration nudges, with the greatest effect for individuals interested in contract durations near the nudged defaults. For individuals signing contracts, those nudged to longer contract durations completed statistically more exercise, though this was largely attributable to longer follow-up as success rates did not vary across nudges. Approximately 40% did not complete any exercise sessions (early drop-outs). Early drop-outs were more likely to have accepted the exact nudged duration presented to them.
Conclusions: Individuals can be “nudged” to select contracts with more total exercise. Random use of nudges also causes individuals to reveal two related aspects of their true colors: 1) their activity/passivity of exercise choice; and 2) their likelihood of failing to live up to their exercise commitments. Recognition of such heterogeneity can guide the design of more efficient exercise interventions.