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Glass Shape Illusions

Short, Wide Glasses Induce Us to Over-Pour Despite Serving Experience

Short, Wide Glasses Induce Us to Over-Pour Despite Serving Experience

People underestimate the amount of alcohol they pour into short-wide glasses, even bartenders

To accurately assess how much alcohol you pour, use taller, slimmer glasses

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Wansink, Brian, and Koert van Ittersum (2005). Amount of Alcohol Poured: Comparative Study of Effect of Practice and ConcentrationBritish Medical Journal, 331(7531),1512–1514. doi: http://dx.doi.org/10.1136/bmj.331.7531.1512

If you ask someone how many drinks they have had, they are likely to give you a confident answer especially when they poured their own drinks. Despite this perceived awareness, people may not be as aware of how much they are pouring, largely because of the shape of their glass. Research by Dr. Brian Wansink of Cornell University and Dr. Koert van Ittersum of University of Groningen, The Netherlands revealed that when people pour into short-wide glasses, they generally pour about 30% more than in tall-slender glasses. They also found that, on average, people consume 92% of what they serve themselves. 

To examine how the shape of glass affects pouring behaviors, researchers asked 198 students of legal drinking age to practice pouring wine into two types of glasses—one tall and thin and one short and wide.

Although the tall-slender and short-wide glasses had the same volume (355ml), people perceived that the tall-slender glass contained more liquid than short-wide glass. They also believed that they actually poured more into the tall-slender glasses. However, the measurements indicated that participants poured an average of 29.75% more alcohol into the short-wide glass than the tall-slender glass. The more times the participants practiced pouring into slim-tall glasses, the more accurate they became, but accuracy did not improve with practice when pouring into the short-wide glass.

Afterwards, 86 professional bartenders participated in the same experiment. Despite their average 6.3 years of bartending experience, they also showed inconsistencies when pouring into different shaped glasses. Although bartenders were highly confident about their estimation, they still on average poured 20.5% more in short-wide glasses.

Based on their findings, the researchers concluded that people generally make pouring decisions based on the height of the liquid in the glass. Short wide glasses give the illusion of containing less liquid causing servers to over-pour despite serving experience.  So, in order to be more accurate when pouring for yourself or others, try using taller slimmer glasses or glasses that are marked to show the level of one serving. 

Summary by Vince Wen