Cornell Professor Brian Wansink's study shows that people overpour into short, squat glasses by twenty to thirty percent compared with tall, thin glasses, probably because of the vertical-horizontal optical illusion that people consistently perceive vertical lines as longer than horizontal ones of the same length.

"Yet, people who pour into short, wide glasses consistently believe that they pour less than those who pour into tall, narrow glasses," said Professor Wansink.

The reason for the difference, Wansink speculates, is the classic vertical-horizontal optical illusion: People perceive vertical lines as longer than horizontal ones.

"People generally estimate tall glasses as holding more liquid than wide ones of the same volume," Wansink said. "They also focus their pouring attention on the height of the liquid they are pouring and insufficiently compensate for its width."

The study, by Wansink and Koert van Ittersum, assistant professor of marketing at Georgia Institute of Technology, is published in the newest issue of the British Medical Journal.

In separate studies, the researchers asked 198 college students of legal drinking age and 86 professional bartenders to pour a shot of spirits into either short, wide tumblers or tall, thin highball glasses.

The college students consistently poured thirty percent more into the short glasses than into the tall, and the bartenders poured twenty percent more.

When the researchers asked one group of students to practice ten times before the actual pour, they still poured 26 percent more into the short glasses. When the researchers asked one group of bartenders to "please take your time," the bartenders took twice as long to pour the drink, but still poured ten percent more into the short glasses.

Because people generally consume most of what they serve themselves, the issue of pouring accuracy is relevant to policy-makers, health professionals, consumers, officials, and counselors, write the authors. For example, they note, the hospitality industry wants to control serving sizes and thus costs, those in public policy want to minimize waste, and health professionals want to discourage overconsumption.

Advice from Wansink for bars and restaurants and for those who don't want to drink too much: Use tall glasses. For parents: Use tall, thin glasses when pouring sugary drinks but short, wide glasses for healthful drinks.

Wansink studies the psychology behind what people eat and how often they eat it.