The participants consisted of 213 students attending Tel Aviv University who were selected after a pre-screening process. They were offered the chance to participate in an experiment aimed at increasing their physical activity. Participants received a six-month membership (January-June 2018) to Tel Aviv University sports center. Financial incentives for exercising were provided only for the first two months (January-February 2018), allowing us to observe whether the effects of the incentives persisted during the subsequent four months. We also observed whether participants continued to exercise at two six-month intervals, after the free membership had ended (12 and 18 months after the start of the experiment).
Past research (e.g., Charness and Gneezy, 2009) usually found the most significant effects among participants who did not previously exercise, which guided us in the recruitment. To this end, all university students were invited to answer a short 3-minute lifestyle questionnaire that was distributed online (see the Appendix). To incentivize the students to answer the questionnaire, they were told that 5% of them would be randomly selected to receive 50 NIS (at the time of the experiment, 3.6NIS=$1). The questionnaire included several questions about lifestyle, including filter questions used to determine who was eligible to participate in the experiment.
The students filling out the questionnaire did not know the details of the experiment and its incentives and thus did not have an incentive to lie. We designed the selection criteria to identify individuals who (1) did not exercise at all or exercised only once a week but not in a gym and not swimming, (2) had a commute time of up to 120 minutes from their residence to the university (participants who live closer to the gym can more easily exercise there), and (3) wanted to exercise more. Initially, our criteria were supposed to be stricter, with criterion (1) being not exercising at all, and criterion (2) being a commute time of up to 30 minutes. However, we did not have enough students who answered our stricter criteria. We then sent the selected students invitations to participate. We told them that based on their answers to the lifestyle questionnaire, they were eligible to participate in a research project aimed at encouraging them to exercise in the university gym and that they would be given a free six-month membership (a value of 1,500 NIS).
Participants attended a 90-minute introductory session. The 29 sessions were held between December 17 and December 28, 2017, in the Interactive Decision-Making Lab at Tel Aviv University. Each session was attended by participants from the same treatment in order to minimize the chance of exposure to participants from other treatments. We balanced the treatment assignment according to age, gender, and commute time.
Before being informed about the incentives, participants were asked to sign a consent form to participate in the experiment; 223 agreed to sign the consent form (six participants dropped out at this stage). By signing the form, participants gave consent not only to participate in the study, but also for us to access their future grade transcripts, their Israeli SAT scores, and their matriculation exam scores. After signing the consent form, participants received the experiment’s instructions for the treatment to which they were assigned. The instructions were read aloud to them and fully explained. The instructions included the incentive they would be offered to exercise, how to use the mobile app for the experiment, how rewards would be distributed during the experiment, and so on. Participants also filled out a medical questionnaire, as required by Israeli law, in order to ensure exercising was safe for them.
Participants then answered the following psychological questionnaires: (1) the Propensity to Plan scale (Lynch, Netemeyer, Spiller, and Zammit, 2010), which measures an individual’s tendency to plan, a trait that is typically necessary in order to exercise on a regular basis; (2) CFC—Consideration of Future Consequences (Strathman, Gleicher, Boninger, and Edwards, 1994), which estimates the extent to which participants take into account future consequences, and in this case, those of the decision to exercise; (3) DOSPERT—Domain-Specific Risk-Taking (Blais, and Weber 2006; Weber, Blais, and Betz, 2002), which measures risk tolerance and may be relevant in this context because one of the intermittent incentive schemes in the study involved a certain degree of risk; and (4) a happiness questionnaire, based on a subset of questions from the Oxford Happiness Questionnaire (Hills and Argyle 2002), that measures differences in levels of happiness between people who exercise and people who do not. Participants also answered additional questions about general lifestyle. While participants were answering the questionnaire, they were taken out one by one to a separate room where a nursing student measured their medical indicators (pulse, weight, and body fat percentage) using non-invasive devices—an Omron Body Composition Monitor BF511 for weight and fat percentage, and an Omron M3 device for pulse and blood pressure.
To facilitate comparison, we set the mean reward per visit to the gym at 20 NIS for the three incentivized treatments. Details regarding the treatments can be seen in the treatment section.
As part of the membership, each participant received one personal training session, during which they received an exercise program suited to their needs. The goal of providing an individualized program was to enhance the exercise’s effectiveness (Jeffery, 2012).
The experiment had an associated mobile app (a customized website for smartphones). A direct link to the app was uploaded to all of the participants’ smartphones during the introductory session. The app contained information regarding the participant’s exercise program and membership, including the number of visits, number of visits in the previous week, the rewards that had already been received, the size and timing of their next reward, and so on. Each participant also received a text message on the last day of every week telling them how many times they had exercised in the previous week and whether they had reached their weekly goal (which was set at the recommended three visits). For example, a text might read as follows: “You exercised at the gym twice this week. Good job! You can do even better next week. The recommendation is to exercise at least 3 times a week.”
Visits to the gym were recorded by chip swipes at the entrance to the sports center and to the gym. An employee of the sports center at the entrance verified that the chip used belonged to the individual that swiped it (a picture of the member pops up when the chip is swiped). This system applies to all members of the sports center.
When participants were eligible for a reward for a particular visit, they received a text message approximately 15 minutes after they swiped the chip. We set this delay so participants would still be at the gym if they were actually exercising, in order to prevent participants from swiping the chip to enter, receiving the reward, and leaving immediately. An example of such a message follows: “You are entitled to a reward of 20 shekels for exercising at the gym today. You can pick up the money at the gym reception desk. In order to receive the payment, please enter the app or the link in order to confirm receiving the money.”
After receiving this message, participants had until the end of the day to pick up their reward from the gym reception desk. If a participant forgot to collect the payment on the same day, he or she could pick it up from the experiment’s administrator.
During the four months following the incentivized period (March–June 2018), participants could still access the gym and use the app, and they continued to receive weekly text messages, but they did not receive any rewards. Of the 213 participants who started the experiment, eight canceled their participation at some point during the six months: three from Control, one from Per-visit, two from Increasing, and two from Unexpected. Thus, 205 participants remained in the study until its completion.
During the first two weeks of June 2018, all the participants were invited to a concluding lab session similar to the introductory one held in December, and received 100 NIS for attending. During the session, participants answered the same questions as in the original questionnaire, along with some additional ones (see Appendix). One hundred seventy-one participants attended the concluding session (roughly 85% of the remaining participants).
Participants were sent two short follow-up questionnaires in which they were asked whether they had continued to exercise and where. The first online questionnaire was sent in January 2019, 12 months after the beginning of the incentivized period, and the second was sent in June 2019, 18 months after the beginning of the incentivized period and prior to the university’s exam period.