The 91% Exercise Study Behind Atomic Habits Meets an 877-Person Gym Experiment

James Clear’s article on implementation intentions, the idea behind the “I will [behavior] at [time] in [location]” template, rests on a single experiment. In 2002, Sarah Milne, Sheina Orbell and Paschal Sheeran put 248 people into three groups for two weeks. One group tracked its exercise. A second tracked it and read about the benefits of exercise. A third read the same material and then finished the sentence “During the next week, I will partake in at least 20 minutes of vigorous exercise on [day] at [time] in [place].” By Clear’s account, 35 to 38 percent of the first two groups exercised weekly, against 91 percent of the third. Motivation, he quotes the authors, “had no significant effects on exercise behavior.”

I confirmed the design and the verdict from the paper’s abstract in the British Journal of Health Psychology: the motivational material raised intentions but not exercise, and the plan “had a dramatic effect.” The three percentages come from Clear’s page and secondary write-ups, because I could not reach the paper’s results section.

What the study did not include

Prestwich, Lawton and Conner (2003, Psychology & Health), writing about exercise planning, note that the study had no group that made a plan without the motivational intervention. So the 91% shows that motivation plus a plan beat motivation alone, which is a more modest sentence than the one usually repeated.

Two other limits are structural. The measure was whether someone exercised at least once in the week, which says little about someone aiming for five sessions. And the Carrera team, reading the same paper, points out that the plan focused people on acting once. Exercising once next week is a one-time action. Building a habit is a repeated one.

The gym test of the repeated version

In 2018, Mariana Carrera, Heather Royer, Mark Stehr, Justin Sydnor and Dmitry Taubinsky published the repeated-action test in the Journal of Health Economics (I read the NBER working-paper version). They recruited 877 members of a private gym. Half were asked to pick the days and hours they planned to go over the next two weeks. The other half were asked to recall when they had gone in the previous two weeks. The outcome came from the gym’s check-in records, not from self-report.

Everything that should have made the plan work did. About 90 percent of the planning group chose at least one visit. Their plans predicted behavior: a planned day carried a 21-percentage-point higher chance of attendance, roughly doubling the baseline. Yet the planning group averaged 2.3 visits against 2.6 for the control group, and the confidence interval ruled out a gain of more than 2 percent. The planners had also been three times too ambitious, planning 7.04 visits and making 2.33.

Why one-time and repeated behaviors split

The paper’s frame is useful. Milkman and colleagues (2011, PNAS) found that a prompt to write down a vaccination date and time raised flu-shot uptake by 4.2 percentage points over a 33.1% control rate. According to the Carrera paper, the effect appeared where employees had a single clinic day, not where they had several. The Carrera authors lean on a line from that paper: plan prompts work best when the opportunity for action is fleeting. A gym is always open, so you can go later, and “later” is where a plan gets lost.

The first habit of the day has no earlier habit

This is where the “make it obvious” advice meets waking up. A plan works by tying an action to a cue you will recognize. Most of the day’s cues sit inside a day you are already living in: after coffee, after parking, after brushing your teeth. The first action of the day has nothing before it. Its cue is an alarm, and it goes off at someone who is the least able to act on a plan they wrote at night. In a six-person laboratory study (Burke et al., 2015), attention and inhibition stayed impaired for hours after waking.

Waking does have one feature the evidence favors: a single, unrepeatable window, since the moment is gone in minutes. The flu-shot pattern says plans do best there. What the waking window lacks is something that closes it. DontSnooze, a free alarm app, adds that: once the alarm goes off you have 30 seconds to record a video proving you are up, or a random camera-roll photo goes to the friends you chose.

Plans that nothing stands behind are the subject of a piece on keeping promises to yourself, which sorts them by what enforces them.

That is my inference from two sets of studies, not a trial of alarms. Wood’s work on place and habit and Lally’s range of habit timelines both point to habits that take weeks and a stable setting to form, whatever the first plan looks like. What the gym null leaves open is the one thing a morning plan can still try: say in advance not just when, but what you will do about the excuse that arrives.