CHAMPAIGN, Ill. - Your eyes play tricks. And your brain makes it worse.
Both teenagers and adults misjudge how much they pour into glasses. They will pour more into short wide glasses than into tall slender glasses, but perceive the opposite to be true. The delusion of shape even influences experienced bartenders, though to a lesser degree, a researcher at the University of Illinois at Urbana-Champaign has found.
How shape can alter a person's notion of size has been widely investigated. For instance, triangles are generally perceived to be larger than squares, and horizontal shapes are seen as smaller than vertical objects of identical volume.
Yet research examining the effects of shape on how people determine how much they consume is limited, said Brian Wansink, a professor of marketing and nutritional science at Illinois. To understand the process better, Wansink examined how shape influences teenagers, adults and bartenders who pour beverages into empty glasses.
The results of his study will be published in the December issue of the Journal of Consumer Research.
Wansink, director of the Food & Brand Lab at Illinois, conducted three tests. In the first, he looked at how much juice 97 teenagers poured for themselves during breakfast at a summer camp in New Hampshire. The male and female campers, 12 to 17, with an average age of 15, had come to the camp to learn about nutrition and lose weight. They were taught about dieting and portion control in daily lectures and demonstrations.
Upon entering the cafeteria line for breakfast on the ninth day, the campers were randomly given a tall and short glass of identical capacity in which to pour their orange juice. The tall glass was slightly less than twice the height of the small glass.
The teenagers poured 76.4 percent more orange juice in the short, wide glasses than in the tall glasses (9.7 ounces versus 5.5 ounces). Although the girls poured less juice in their glasses than the boys, both groups equally overpoured in the short, wide glasses.
When questioned by Wansink's team, however, the teenagers believed that they had poured less (7 ounces) into the short, wide glasses, and more (7.5 ounces) in the tall, slender glasses. This mistaken impression translated into drinking more juice when placed in the short glass, with 97 percent of all campers finishing the juice they had poured.
The psychologist Jean Piaget (1896-1980) believed that young children tended to be caught and fixed by the vertical dimension of a visual field. Piaget thought that as they grew up humans developed strategies to isolate and better compare vertical and horizontal dimensions.
But the tendency to overestimate the vertical dimension persisted in a second experiment conducted by Wansink. He used the same basic procedure of the teenager study to measure how much juice was poured by 89 adults eating breakfast at a camp in western Massachusetts. The group ranged from 16 to 82, with an average age of 37.
The adults poured and consumed 19.2 percent more juice in the short wide glass than in the tall slender glass (6.8 ounces versus 5.7 ounces). "These results were consistent with Piaget's notion that older people are less likely to focus their attention merely on the vertical dimension and are better able to account for the other dimensions as well. Still age did not eliminate the elongation effect," Wansink wrote.
The adults, like the teenagers, mistakenly perceived that they had poured less into the wide glasses than into tall, slender glasses. Seventy-nine percent of the adults given the wide, short glasses underestimated how much they poured, as compared with 17 percent of those given tall glasses.
When informed of the overpouring, most of the adults expressed surprise. "We heard remarks like 'You're kidding' and 'Can you weigh it and show me?' which is consistent with the general lack of awareness by participants of how much they actually poured," Wansink said in an interview.
In a final study, Wansink examined how accurately bartenders could estimate drink volumes. He asked 45 bartenders in Philadelphia to pour 1.5 ounces of liquor into drink glasses. Half the bartenders were given slender highball glasses, and the others had short tumbler glasses. Each glass held 12 ounces. The bartenders were asked to pour rum for a rum and Coke, whiskey for a whiskey on the rocks and vodka for a vodka tonic.
On average, the bartenders poured 31.3 percent more into the tumbler glass than into the highball glass (2.1 ounces versus 1.6 ounces).
Less experienced bartenders tended to overpour more (2.2 ounces in tumblers versus 1.6 ounces in highball glasses), but even bartenders with an average of nine years of experience poured 1.8 ounces in the short glass compared with 1.7 ounces in the tall glass.
There are various policy implications in these findings, according to Wansink. The tricks of the eye and brain could play havoc with dieters seeking to monitor and better control food and beverage consumption.
"Because people believe there is greater capacity in a tall, slender glass, they will pour less into it, but thinking the opposite with a short, wide glass, will keep pouring," the researcher said. Aside from overconsumption of alcohol, inadvertent overpouring of medications and over-the-counter drugs could pose a potential health risk.
Wansink's paper is titled, "Bottoms Up! The Influence of Elongation on Pouring and Consumption Volumes."