Can you pay someone into a gym habit? Two 2009 experiments, one question at a time

Yes, for some people. In two experiments reported by Gary Charness and Uri Gneezy in Econometrica in 2009, students paid to visit a campus gym eight times in four weeks kept visiting after the money stopped, with weekly visits up 107 and 181 percent in the two studies, and nearly all of the gain came from students who had not been regular gym-goers. Here is how they got there, one question at a time.

Who were the subjects and what did they get?

Study 1 recruited 120 University of Chicago students from an experiment mailing list, with no mention of exercise in the invitation. Every student already had gym access through fees. Forty served as controls in an unrelated experiment. The other eighty were paid $25 to visit the gym once in the following week. Half of those were then promised another $100 for at least eight more visits over the next four weeks. The gym’s swipe-card records gave the researchers each student’s attendance before, during and for seven weeks after.

Study 2, at UC San Diego, recruited 168 first- and second-year students. To keep money out of the comparison, everyone was paid the same $175 for turning up to three body-measurement appointments. Only the gym requirement differed: none, one visit, or eight visits in the following month. Twelve students who missed appointments were dropped from the analysis.

What happened to attendance?

In Study 1, the control group went from 0.59 visits a week to 0.56. The paid-once group moved from 0.70 to 0.76. The eight-visit group went from 0.60 to 1.24, a 107 percent increase averaged over the seven weeks after payment ended; the authors report no decline across that period.

Study 2 looks different because every group trended up; the authors speculate that first-year students were still settling in. Controls rose from 0.81 to 1.10 visits a week. The one-visit group rose from 0.62 to 0.87. The eight-visit group went from 0.52 to 1.46, an increase of 181 percent.

Did it work for everyone?

No, and that is the most useful part of the paper. The researchers split students into regulars (at least one visit a week before the study) and everyone else. For regulars, the eight-visit payment did nothing: their attendance drifted down by 0.07 visits a week in Study 1 and 0.20 in Study 2. For students who had not been going, the average rise was 0.98 visits a week in Study 1 and 1.20 in Study 2. In Study 1, 12 of the 27 non-regulars in the eight-visit group, 44 percent, became regulars. In Study 2 it was 26 of 49, or 53 percent.

So the payment bought almost nothing from people who were already going and quite a lot from people who were not.

Wasn’t the payment supposed to ruin the motivation?

That was one of two hypotheses tested. Psychologists have long argued that paying people for something they like can leave them less interested once the pay stops. Charness and Gneezy set it against a rival idea from Becker and Murphy’s 1988 model of habit, in which past consumption makes present consumption more rewarding. The data came down on the second side. Attendance after payment was higher than before it, not lower. The authors add that the two forces are not mutually exclusive, so some students may have been crowded out while more were pulled in.

Was the money doing the work, or the visits?

Study 2 was built to answer this. Because every group was paid the same $175, the control group’s extra income and free time cannot explain the eight-visit group’s higher attendance. In attendance, the one-visit group, which received the same pay with one required visit, did no better than controls. Eight visits in a month appears to be a different dose from one.

Did they get healthier?

A little. Between the first measurement and one taken 20 weeks later, the eight-visit group’s average weight fell 0.34 kg, while controls gained 0.57 kg and the one-visit group 0.59 kg, a gap of 0.91 kg against controls. Body fat fell by 0.78 percentage points in that group and rose by 1.41 in controls. Waist and pulse moved the same way relative to controls, and blood pressure did not. The authors flag that some of the gap comes from controls getting worse, and mention the “freshman 15.”

What would a skeptic say?

Several things, and the paper raises some of them itself. These are students at two campuses who volunteered for experiments, and the gym was free to them. A $100 payment is not trivial on a student budget. The follow-up lasted seven weeks in Study 1 and 13 in Study 2, so a habit measured in weeks is not a habit measured in years, and the often-quoted 66 days to form a habit comes from self-reported automaticity, a different yardstick. Self-reported exercise logs did not line up with swipe data, and the authors call the discrepancy puzzling, suspecting self-reports. And the questionnaire data offered almost no way to predict who would respond.

One detail stands out. Before the study, 84 percent of participants said they wished to exercise more, and 82 percent said being paid would help, 79 percent of those expecting the effect to last. In other words, most participants expected payment to work, yet their questionnaire answers did little to predict who actually changed.

What does this say about getting yourself up on time?

Nothing directly, since no one in these experiments was waking up for anything. Gyms sell the opposite arrangement, and their own pricing model assumes you will stop coming. What the experiments offer instead is a structure: a reward large enough to matter, tied to a count of repetitions rather than a single act, over a period short enough to finish.