Caffeine increases impulse purchases: Experimental design practice and ethics discussion

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I think of the Intro Psych course as an owner’s manual for being human. Throughout the course, we explore the multitude of ways we are influenced to think, feel, or behave a certain way that happens without our conscious awareness. Here’s one such example we can use to give our students some experimental design practice. It’s suitable for the methods chapter or, if you cover drugs, in that chapter after discussing caffeine.

Caffeine, as a stimulant, increases arousal. It’s plausible that consumers who are physiologically aroused engage in more impulsive shopping and, thus, spend more money than their uncaffeinated counterparts.

Give students this hypothesis: If shoppers consume caffeine immediately before shopping, then they will spend more money.

Ask students to take a couple minutes thinking about how they would design this study, and then invite students to share their ideas in pairs or small groups. Ask the groups to identify their independent variable (including experimental and control conditions) and their dependent variable. If you cover operational definitions, ask for those, too. Invite groups to share their designs with the class.

Emphasize that there is no one right way to conduct a study. Each design will have its flaws, so using different designs to test the same hypothesis will give us greater confidence in the hypothesis.

Share with students the first two of five experiments reported in the Journal of Marketing (Biswas et al., 2022). In study 1, researchers set up a free espresso station just inside the front door of a store. As shoppers entered, they were offered a cup of espresso. The experiment was conducted at different times of day over several days. At certain times, shoppers were offered a caffeinated espresso. At other times, they were offered a decaffeinated espresso. As the espresso drinkers left the store after having completed their shopping, researchers asked if they could see their receipts. Everyone said yes. Researchers recorded the number of items purchased and the total purchase amount. (Ask students to identify the independent and dependent variables.) As hypothesized, the caffeinated shoppers purchased more items (2.16 vs. 1.45) and spent more money (€27.48 vs. €14.82) than the decaffeinated shoppers. Note that participants knew whether they were consuming a caffeinated or decaffeinated beverage, but did not know when they accepted that they were participating in a study.

There are a few ethical questions about study 1 worth exploring with your students. First, this study lacked informed consent. Participants were not aware that they were participating in a study when they accepted the free espresso. As participants were leaving, it became clear to them that they were participating in a study. Given the norm of reciprocity, did participants see not handing over their receipts as a viable option? Lastly, the researchers expected that caffeine would increase consumer spending. In fact, it nearly doubled it. Was it ethical for the researchers to put unwitting shoppers in a position to spend more money than they had intended?

In study 2, students from a marketing research class “in exchange for course credit” were asked to recruit family or friends to participate. The volunteers, who were told that this was a study about their shopping experience, were randomly assigned to an espresso or water condition which were consumed in a cafeteria next to a department store. After consuming their beverages, the volunteers were escorted to the department store and were asked to spend two hours in the store “shopping or looking around.” As in study 1, caffeinated shoppers spent nearly twice as much money (€69.91 vs. €39.63).

Again, we have the ethical question of putting unwitting shoppers in the position to spend more money than they would have. We also have the ethical question of students recruiting friends and family to participate as course requirement. And then from a design perspective, how certain can we be that the students didn’t share the hypothesis with their family and friends? Is it possible that some of the students thought that if the study’s results didn’t support the hypothesis, their grade would be affected?

As a final ethics question, what should we do with the knowledge that we are likely to spend (much) more money when shopping when we are caffeinated? As a shopper, it’s easy. I’m not going stop on the coffee shop on my way to the store. For a store manager whose job it is to maximize, it’s also easy. Give away cups of coffee as shoppers enter the store. The amount of money it costs to staff a station and serve coffee will more than pay for itself in shopper spending. Here’s the bigger problem. Is it okay to manipulate shoppers in this way for financial gain? Advertising and other persuasive strategies do this all the time. Is free caffeine any different? Or should coffee cups carry warning labels?

To close this discussion, ask students in what other places or situations can impulsive behavior encouraged by being caffeinated be problematic. Casinos come readily to my mind. Are caffeinated people likely to bet more? Would that study be ethical to conduct?



Biswas, D., Hartmann, P., Eisend, M., Szocs, C., Jochims, B., Apaolaza, V., Hermann, E., López, C. M., & Borges, A. (2022). Caffeine’s Effects on Consumer Spending. Journal of Marketing, 002224292211092.


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About the Author
Sue Frantz has taught psychology since 1992. She has served on several APA boards and committees, and was proud to serve the members of the Society for the Teaching of Psychology as their 2018 president. In 2013, she was the inaugural recipient of the APA award for Excellence in the Scholarship of Teaching and Learning at a Two-Year College or Campus. She received in 2016 the highest award for the teaching of psychology--the Charles L. Brewer Distinguished Teaching of Psychology Award. She presents nationally and internationally on the topics of educational technology and the pedagogy of psychology. She is co-author with Doug Bernstein and Steve Chew of Teaching Psychology: A Step-by-Step Guide, 3rd ed. and is co-author with Charles Stangor on Introduction to Psychology, 4.0.