Habit Formation and Naiveté in Gym Attendance: Evidence from a Field Experiment∗

We extend the gym-attendance study of Charness and Gneezy (2009) by incentivizing subjects to attend the gym for a month, observing their preand post-treatment attendance relative to a control group, and eliciting subjects’ preand post-treatment predictions of their post-treatment attendance. We find a habit formation effect similar to that of Charness and Gneezy in the shortrun, but with substantial decay caused by winter vacation. We additionally find that subjects seriously over-predict future attendance, which we interpret as evidence of partial naivete with respect to self-control problems. Subjects also appear to have biased beliefs about their future cost of gym attendance. Our design allows us to estimate the monetary value of habit formation—equivalent on average to a $0.40 per visit subsidy—as well as the welfare cost of present bias and naivete.