#675 2011 · Katherine Milkman, John Beshears, James Choi, David Laibson & Brigitte Madrian (field experiment) · Workplace health / behavioral economics
A free on-site flu clinic still left a third of employees unvaccinated, until researchers added one blank line asking when they'd go
the problem
Employees who already intend to get their free flu shot still skip it
background
Seasonal flu causes over 200,000 US hospitalizations a year, and the vaccine that prevents most of them is cheap and, at many large employers, free and delivered on-site during work hours — every practical objection an employer can remove (cost, travel, scheduling) had already been removed. Yet compliance with CDC recommendations stayed stubbornly incomplete, and the standard response was more of the same: another reminder mailing, another poster, occasionally a small incentive, on the assumption that people who weren't vaccinating either didn't know the clinic existed or needed a nudge on price.
Five researchers ran a three-arm randomized trial at a large Midwestern utility company to test a different theory: psychology research on 'implementation intentions' had shown, mostly in small lab studies, that asking someone to specify WHEN and WHERE they'll act closes a gap that reminders and information don't touch — the gap between deciding to do something and actually doing it. The 3,272 employees eligible for the study, all 50 or older or with a chronic condition raising their flu risk, were split into a control group that received a standard clinic-times mailer and two treatment groups whose otherwise identical mailer added one blank line.
what everyone would do
The standard move against low vaccination uptake is to remove friction — free clinics, on-site delivery, more reminders, sometimes a cash incentive — treating the holdouts as people blocked by cost or access. That was already true here and a third of eligible employees still skipped it, so doing more of the same (another mailer, a bigger discount) would have kept hitting a ceiling the researchers correctly guessed wasn't about access at all.
what they saw
The remaining gap wasn't information or incentive, it was that a general intention to 'get around to it sometime during the clinic window' never turns into a concrete plan on its own — and the psychological research on implementation intentions said that simply asking someone to name a specific moment is enough to forge the mental link between the goal and a trigger, without needing to check whether they kept the plan.
the move
The control mailer listed the on-site clinic's dates and times, nothing more. The 'date plan' mailer added a line asking the employee to write down the date they planned to get vaccinated. The 'time plan' mailer asked for both a date and a specific time. No group received extra information, a discount, or any contact from the employer about what, if anything, they wrote down — the plan was never collected or checked.
why it works
Writing down a date and time makes an abstract goal into a cued action: the written moment becomes a trigger that fires when it arrives, rather than a vague obligation competing with everything else on the employee's calendar. The date-only prompt barely moved the needle because a day alone still leaves the actual moment unscheduled against a multi-day clinic; specifying a time removed that last degree of freedom, which is why only the time-plan condition reached significance, and why the effect was larger at sites with only a single clinic day, where there was less room for a vague plan to survive contact with a busy day.
the payoff
The control group's vaccination rate was 33.1%. The date-only prompt raised it to 35.6%, a 2.4-point gain that was not statistically significant. The date-and-time prompt raised it to 37.1%, a 4.0-point gain (regression-adjusted 4.2 points, p=.03) that was both significant and, for a single line of ink costing nothing to print, of meaningful magnitude — comparable to gains employers spend real money chasing through incentives or extended clinic hours.
where it breaks
The prompt only works on people who already want to act but haven't scheduled it — it does nothing for someone who hasn't decided to get vaccinated at all, and the paper's own date-only condition shows that an under-specified prompt can fail to move behavior even when a specific one does. It also depends on there being an actual bounded window and location to commit to; a prompt to 'plan to eat healthier' with no fixed moment to anchor on doesn't carry the same mechanism, and repeated or generic planning prompts risk becoming as ignorable as the reminders they replaced.
what came after
The paper became one of the most-cited field demonstrations that implementation-intentions prompts work outside the psychology lab, and the same team later scaled the logic into a 2021 PNAS megastudy testing text-based planning nudges for vaccination appointments across a million patients. Planning-prompt lines now appear routinely in public-health reminder mailers and workplace benefits communications, usually without the sender knowing this is the study that established the effect held up under a real administrative outcome, not a self-report.
references
- [1]Using Implementation Intentions Prompts to Enhance Influenza Vaccination Rates (NBER Working Paper 17183)National Bureau of Economic Research, 2011nber.org
- [2]Using implementation intentions prompts to enhance influenza vaccination ratesProceedings of the National Academy of Sciences, 2011pnas.org