New York University marketing professor Priya Raghubir’s lightbulb moment about consumer behavior came in the late 1990s, when she was a new mother struggling with a fussy baby who refused to eat.

One particularly exasperating day, she switched the bowl she normally used to feed her son and was amazed that he gobbled up everything.

Raghubir immediately called her friend and colleague, Aradhna Krishna, who was having the same trouble feeding her young son, and advised her to try switching bowls. It worked like a charm.

“Aradhna and I, chatting and laughing, said that if we are able to trick our own children, who of course have to be the smartest people on Earth, into eating by changing the shape of the bowl, there must be a paper in this,” Raghubir recalled. “And that is how we began exploring volume perception.”

More than two decades after publishing that paper with Krishna in 1999, Raghubir is sharing her latest co-authored research on how the shape of packaging influences purchase decisions. The paper, titled “The Stability Heuristic for Weight Judgments,” examines how perceptions about the weight and volume of containers affect consumer preferences and their willingness to pay. The co-authors are Dengfeng Yan, marketing professor at NYU Shanghai, and Lu Yang, marketing professor at Nanjing Agricultural University.

Through a series of online and lab experiments, the researchers found that consumers judge stable shapes to be heavier than unstable shapes. This judgment leads them to conclude, sometimes inaccurately, that stable-looking containers hold more weight, greater volume and higher calories. For example, consumers believe that flatter, wider yogurt containers hold more product than taller, cylindrical yogurt containers — even if both containers are the same volume. Offering a more exaggerated example, the researchers point out that the Leaning Tower of Pisa appears to be unstable although it is very heavy, while a hat looks stable although it is quite light.

This new “stability heuristic” proposed by the researchers extends to logos as well; fast-food chains such as KFC and Burger King have switched to skinnier, less stable fonts such as italics to convey lower calories.

Marketers can use the study results to craft packaging and logo designs that appeal to exactly the right audience. According to the research, a shorter, wider yogurt container indicates a full-fat, rich-tasting product is waiting to be enjoyed, while a skinnier, cone-shaped cylinder promises a low-fat, healthier choice.

“One of the key takeaways is that if you are in a product category and you are trying to signal low calories or low weight, you should not use a very stable-looking container,” Raghubir said. “If you’re trying to signal a greater weight or that you’re getting more for your money in this pack, then a stable base should lead to perception of greater weight.”

The Elongation Effect

In addition to positing the stability heuristic, the paper also expands what is known about the elongation effect, a theory explored in Raghubir’s 1999 paper.

Like the stability heuristic, the elongation effect is a perception bias. Consumers perceive taller containers as more voluminous than shorter ones, even when the volume is held constant. Think about drinking glasses or shampoo bottles or beer cans. An elongated 12-ounce glass, bottle or can appear to hold more liquid than a squat glass, bottle or can of the same volume.

Both the elongation effect and the stability heuristic are functions of human reasoning, Raghubir explained. People make these simplified judgments about weight and volume because they lack more precise information, so the brain fills in the blanks.

The researchers found the stability heuristic [stable=heavy, heavy=stable] works consistently when study participants were asked to estimate the weight of containers first. But when they were asked to estimate the volume of containers first, before estimating weight, the elongation effect takes over and they perceive the taller container to have more.

“The 1999 paper has been around for a while and the elongation effect has been shown to be fairly robust, so the fact that we were able to reverse it was theoretically interesting,” Raghubir said.

Another aspect of the study was how the researchers structured their experiments. In each one, participants were given successively more information. At first, they were asked to make judgments about shapes they saw only in pictures, such as a tortilla chip that was pictured upright or tilted. In another experiment, they viewed a container up close without touching it. In yet another, blindfolded participants were allowed to lift and hold a container.

The researchers found that the stability heuristic was mitigated with more information. In other words, participants didn’t rely solely on the stable=heavy assumption when they were armed with more facts.

What It Means for Marketers

Raghubir acknowledged that the stability heuristic seems intuitive. Yet the study offers a more nuanced understanding of how package shape can affect purchase choices, which has implications for marketers trying to appeal to consumer aesthetics.

“It’s not a surprise to anybody that heavier packages are more stable. That we know. That’s the law of gravity,” she said. “But we are saying it leads to this reverse causality that more stable packages are perceived to be heavier, and that may not necessarily be the case.”

The research also ties into other observations about size, shape and color, including the notion that packages are perceived to be heavier when darker colors are used at the bottom.

In the paper, the co-authors note that weight judgment is an area of marketing research that hasn’t received enough attention. But for Raghubir, the subject continues to pique her curiosity after all these years.

“I fall prey to this effect, even though I study it,” she said, sharing a story about pouring water from one glass into a tumbler that she thought was much larger, only to find the volume was the same. “I said to myself ‘wow,’ because even I was fooled.”