“When we compare it to other in-store mediums … in-store product demonstration has the highest [sales] lift,” says Giovanni DeMeo of the product-demonstration company Interactions, a department of which handles Costco’s samples. That department is Club Demonstration Services, and it—not Costco—staffs the sample tables.

While DeMeo insists that the short-term spike in sales isn’t the only effect of product sampling that matters—it's great for making customers loyal to stores and brands over longer periods of time—the figures are impressive. In the past year, Interactions’ beer samples at many national retailers on average boosted sales by 71 percent, and its samples of frozen pizza increased sales by 600 percent. (These figures are in line with the few others that are publicly available.)

Average Percentage Increase in Sales After Product Samples in the Past Year, by Product Type

Data courtesy of Interactions

It’s true that free samples help consumers learn more about products, and that they make retail environments more appealing. But samples are operating on a more subconscious level as well. “Reciprocity is a very, very strong instinct,” says Dan Ariely, a behavioral economist at Duke University. “If somebody does something for you”—such as giving you a quarter of a ravioli on a piece of wax paper—“you really feel a rather surprisingly strong obligation to do something back for them.”

Ariely adds that free samples can make forgotten cravings become more salient. “What samples do is they give you a particular desire for something,” he says. “If I gave you a tiny bit of chocolate, all of a sudden it would remind you about the exact taste of chocolate and would increase your craving.”

Percentage of Shoppers Who Purchased Items Being Sampled, by Product

Heilman et al.

Plenty of marketing research has been done on the fruits of retail strategies like couponing and loyalty cards, but the literature on free samples is relatively sparse. One narrowly applicable study from 1978, for example, found that samples were more likely to cause obese customers to purchase something than customers of normal weight.

A 2011 study in the British Food Journal sought to illuminate the lucrative but uncharted (at least in the academic literature) practice of putting out free samples. The researchers surveyed shoppers at a grocery store on six different weekends, and their findings color in a detailed picture of the mechanisms that underlie free samples.

For starters, about three-quarters of people surveyed took a free sample when offered one. And those who did take a sample were more likely to have taken other samples than those who didn’t—which suggests that people are driven to samples more by their dispositions than by their perceptions of a product’s relevance to them. Interestingly, people who took samples were less likely than non-samplers to have graduated from college.