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A “nudge” is a policy intervention that “alters people’s behavior in a predictable way without forbidding any options or significantly changing their economic incentives,” according to Richard Thaler, the University of Chicago economist who last year won a Nobel Prize in part for his work on the subject.

Nudges are typically used to get people to do things that are good for them or society as a whole, but which they may be otherwise disinclined to do. Famous nudges include painted flies on urinals, reducing “spillage” by giving men something to aim at; automatic 401(k) enrollment; and getting people to use less electricity by showing them how much their neighbors are using.

One type of nudge that's shown a great deal of promise is known as a planning prompt, which asks people to lay out the concrete steps they will take to achieve a certain goal. Research has shown these prompts are effective at getting people to do things such as vote, get their flu shots and go to the dentist.

What about going to the gym?

That's what the team of researchers behind a new working paper set out to discover when they ran a randomized field experiment among 877 members of a private gym in the Midwest. In the realm of exercise, in particular, there's a notoriously large “gap between intentions and actions,” as the researchers describe it. Most Americans know they should be exercising more, but less than a quarter of them are getting federally recommended amounts of physical activity each week. A 2015 experiment conducted among workers at a Fortune 500 company found that “workers’ targeted levels of exercise are 43 percent higher than their actual levels of exercise,” according to the authors of the new working paper.

The researchers recruited subjects from among the gym's active members and divided them into two groups. The treatment group were asked to fill out a schedule indicating the days and times they planned to attend the gym in the following two weeks. A control group filled out no exercise plan, instead simply listing the amount of times they had exercised in the previous two weeks. The central question: Would the people who filled out an exercise schedule go to the gym more than the people who didn't?

To keep respondents honest, the researchers used the gym's records of member check-ins to track actual exercise frequency. What they found was slightly dismaying, at least for anyone hoping to nudge themselves into exercising more: “The treatment group made an average of 2.3 visits over the two-week period, compared to an average of 2.6 for the control group,” the authors write. Statistically speaking, the difference between the two groups was zero.

The authors tried to suss out why this was. The subjects certainly believed that planning out their visits could help them exercise more: Before the study period, 60 percent of all subjects agreed with the statement “I don't go to the gym as much as I would like because I don’t set aside time for it in my schedule; then my schedule fills up and I no longer have time to go to the gym.”

The treatment subjects also appeared to try to stick to their plans — “subjects are more than twice as likely to attend the gym on planned days than on unplanned days,” the study found.

But the researchers found that not all plans were fulfilled. There remained a considerable gap between a stated intent to exercise on a given day and actually showing up to the gym that day. In fact, one of the biggest predictors of overall gym attendance during the study period was not whether people made plans to visit the gym but rather how often they visited the gym before the study period.

In other words, people already inclined to go to the gym continued to go to the gym, regardless of whether they made concrete plans to do so or not. This creates a discouraging circularity for anyone hoping to change their exercise routine: If you want to start going to the gym, it's best to already be going to the gym.

Why does the nudge fail in this case? The researchers suspect that planning nudges may be more useful for one-time events, like doctor's appointments. Activities that are repeated, such as going to the gym, are easier to put off. “Repeated behaviors like exercise . . . are very unlikely to produce a feeling of urgency, since many individuals likely have the mind-set that they can always exercise 'later,'" the authors explain.

Unfortunately for many of the would-be exercisers among us, that “later” never comes.