Across a series of studies, researchers at HBS and IESE found that a simple nudge—making people feel as if they are completing a set—can push them to donate more aid items to charity, spend more time on tasks, incur more risk in a gambling game and buy more or fewer beers. This has implications for real-world product packaging and digital presentation of any activities in which organizations want consumers to engage.

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It’s no secret that people like to finish things; there’s something deeply and inexplicably satisfying about crossing the last item off a to-do list or acquiring the final piece of a collectible set. But just how far are people willing to go to achieve “completeness”? Recent research I conducted with Leslie John, Elizabeth Keenan, and Michael Norton of Harvard Business School investigated whether it’s possible to harness this desire to motivate people in specific ways.

In a series of studies, we used visual cues and verbal descriptions to artificially reframe individual items, from donations to tasks to gambles, as cohesive but otherwise arbitrary groups. We then measured the effect of this pseudo-set framing on people’s effort levels and completion rates, and found that behavior changed in significant and meaningful ways.

Our first test was in the field; we teamed up with the Canadian Red Cross (CRC) to conduct an experiment during their 2016 Holiday Campaign. The CRC randomly directed more than 7,000 donors to one of three nearly-identical websites. One version displayed the business-as-usual website, a platform offering donors the option to give money and/or up to six aid items (e.g., hot meals, blankets). A second version encouraged donors to give the six aid items—the more, the better – and “rewarded” them with a badge for each one added to the cart. A third version also encouraged donors to give the six aid items, but this time described them as component parts of a “Global Survival Kit,” presented a graphic that filled in as items were added, and showed text marking progress toward “100%” completion, indicating all six goods were in the cart.

Once the final donations were tallied, we found that the Global Survival Kit framing led four to seven times as many people to donate all six items (as compared to the business-as-usual site and the badge prompts.) By merely tweaking the framing—and without changing anything about the choices themselves—we were able to systematically shift what donors chose to give.

Next, we wondered: Just how arbitrary could these sets be and still elicit the same behavior? We tested this in several follow-up laboratory studies, which depicted pseudo-sets in different ways: a five-slice pie chart that “filled in” as tasks were completed, an image of a subdivided coin that “filled in” as gambles were won, and the description of a “batch” of greeting cards that could be written. In all of these cases, the framing made people significantly more likely to reach completion—spending more time, or in the case of the gambles, incurring more risk—relative to control conditions, even though there were no rewards for doing so, and even when the total arbitrariness of the grouping was made explicit.

So what exactly makes pseudo-set framing work? To explore the “why,” we turned to a common real-world scenario: beer purchases. In an online experiment, we showed one group of study subjects images of one, two, or three loose beers with no product packaging, and then asked how many additional bottles they’d want to buy. Most said they’d purchase either nothing more or the number needed to add up to six, representing a traditional six-pack. However, when we presented a second group of subjects with a four-pack container—pre-filled with one, two, or three bottles—they overwhelmingly said they would purchase only the extras needed to fill all four slots, no more and no less. They did so, we discovered, because they were uncomfortable leaving the case incomplete. Our conclusion is that organizations can fairly easily shift consumers’ go-to quantity for purchases with a simple tweak in product packaging.

And there are many other possibilities for implementing pseudo-set framing in the real world. People frequently encounter tasks with no obvious stopping point, prompting the question: “How much is enough?” How many items should we buy? How many friends should we refer? How many times should we donate? Although firms and fundraisers would always prefer more engagement, they might instead consider finding a sweet spot for engagement and setting that as a point of “completion”—via text, graphics or other nudges. We’d bet that many people won’t be able to resist their inherent desire to finish.