Gotta Get ’em All? Why Incomplete Sets Drive Consumer Behaviors

August 16, 2022 • By Keith Giles

Consumer behaviors are difficult to predict, but psychologists and researchers have uncovered some fascinating details about why certain approaches succeed where others fail. Professor Eric Spangenberg of The UCI Paul Merage School of Business has studied consumer behavior for a long time now. In a recent article, he and his fellow researchers discovered how what they have labeled “The Incompleteness Effect” drives consumer choice, and why it’s very difficult for consumers to resist.

Understanding consumer behaviors inspired the research article that Spangenberg recently published with co-researchers Christoph Bauer of Simon-Kucher & Partners, Hamburg, Germany, Andreas Herrmann of the Institute for Mobility, University of St. Gallen, Switzerland, and his daughter, Katie Spangenberg of The Albers School of Business and Economics, Seattle University, Seattle, Washington. The results were published in the Journal of the Academy of Marketing Science (December, 2021) under the title, “Collect Them All! Increasing Product Category Cross-Selling Using the Incompleteness Effect.”

Unconscious Choices

Ironically, Eric Spangenberg can’t help himself when it comes to human behavior. He admits that he’s relentlessly curious about why humans act in certain ways—especially when deciding what and how much to purchase. “The inspiration for anything I work on is typically centered around consumer decision-making,” he says. “I am a consumer psychologist by training, so I’m always interested in why people make decisions and how we can influence them.”

Spangenberg identifies himself as somewhat of an “anti-marketing marketer” preferring to inspire positive buying choices rather than using manipulative methods to merely increase consumer purchases. “Rather than just find new ways to sell people more rolls of paper towels, I’d prefer to find ways to encourage consumers to make smarter, healthier, and more societally conscious choices,” says Spangenberg.

Inspired Curiosity

The inspiration for the study came from our co-author Christoph Bauer,” Spangenberg says. “It was taken directly from his doctoral dissertation completed at the University of St. Gallen where I served on his doctoral committee.”

One of the study’s underlying premises is that people often have an unconscious sense of unease about incomplete tasks. “Sometimes that unease can reach the conscious level if it becomes strong enough,” Spangenberg says. “But either way, that feeling of unease drives people to complete a set or finish a task, and that drive  is what we were most interested in exploring in our research.”

Ancient Wisdom

An experimental social psychologist by training, Spangenberg was fascinated with testing theories and hypotheses that are nearly a hundred years old. “Lewin’s Field Theory, for example, was first published in Germany in the 1920s,” he says. “Outside of our work, no one had looked at Lewin’s Field Theory in a cross-buying context, and to our knowledge, this is the first research to demonstrate how online product configurators can be used to trigger this impulse for consumers to complete a set. This effect is based on the state of tension people feel whenever there are unmet needs or desires, which increases the likelihood of cross-buying products.”

Complete Success

Their study showed that framing products and services as sets was effective, regardless of different presentations. “Using the incomplete set framework increased cross-selling of products, whether from a single purchase instance or over multiple interactions over a longer period of time,” Spangenberg says.

Their study also examined the underlying drivers of the incompleteness effect and how online stores can visually group products. “When you arrange products as part of an interrelated set, you increase perceptions of incompleteness,” he says, “increasing the likelihood that consumers will buy more, or all products within the set created.”

While the benefits of this research for retailers appear quite obvious, there are benefits for consumers as well. “People may enjoy a more satisfying consumer experience, and certainly enjoy a more complete experience when given these options,” he says. “The whole premise of our work is based on the fact that people like complete sets better than incomplete sets.”

Total Experience

To illustrate this, Spangenberg suggests a scenario where someone has an upcoming dinner party, and they go online to purchase a nice wine for the occasion. “Under normal circumstances, that person would probably just buy one red and one white wine and be done,” he says. “But if you offered a complete dinner party experience, including a non-alcoholic cider, sparkling wine to serve with appetizers, red wine, white wine, and after dinner grappa, the incompleteness effect greatly increases the likelihood that consumers will choose to purchase the complete set.”

Accentuate The Positive

The perception that the experience would be incomplete without the entire collection of beverages is one that most consumers would not consciously (or subconsciously) prefer. The effect also comes in to play when you offer a customer an entire  set and ask which options they’d like to eliminate. “A previous study we worked on with Audi found that customers who were shown a fully loaded vehicle and asked which features they wanted to eliminate ended up driving away with more features than those asked to build up a vehicle from a base model with few features,” he says. “The perception that the fully loaded vehicle would somehow be incomplete by removing features led people to not to eliminate those options.”

Multiple Choice

Of course, this model isn’t effective 100 percent of the time—many factors play into its success. “We have to ask if the cost of completing a set is feasible or rational. In cases where the consumer feels that it’s both feasible and rational, we found that they are more likely to choose to complete the set that’s offered to them.”

Personal Enjoyment

On a personal note, Spangenberg was thrilled to work on this project with his daughter Katie Spangenberg. “This was the most fun I’ve had working on a paper because it meant we could work on it together,” he says. “A lot of the managerial and theoretical implications of the study came from her in addition to much of the statistical analyses. She’s a professor at Seattle University now, and not only did she have the most current methodological expertise, but as a younger scholar she was current in the literature.”

Future Insights

While their research doesn’t explain why hot dogs come in packs of ten and hot dog buns in eight, it offers guidance for how companies can improve customer experience and increase product cross-selling by incorporating these principles.

What we found suggests that this methodology could be used across multiple settings,” Spangenberg says. “Whether you’re selling all the ingredients in a recipe, a complete line of cosmetics, home furnishings or spa services, it really doesn’t matter. No matter what you’re selling, if you frame things as a set, you’re more likely to find retail success and consumers are more likely to feel fulfilled as buyers.”


Eric Spangenberg, PhD, is Professor of Marketing and Psychological Science, and The Director of the Center for Global Leadership, Paul Merage School of Business at the University of California Irvine. As a a highly cited and widely recognized international scholar, Spangenberg has authored or co-authored more than 50 journal articles and book chapters, as well as dozens of other works across several research areas in marketing and consumer psychology.