Even professional bartenders pour more into short, wide "tumblers" than into "highball" glasses, suggesting that experience of pouring alcohol has little effect.
The study involved 198 college students and 86 bartenders from a large city in the United States.
After several practice pours, half the students were given tall, slender 355 ml glasses and half were given short, wide 355 ml glasses. They were then asked to pour a standard "shot" of alcohol (1.5 ounces, 44.3 ml) for four mixed drinks (vodka tonic, rum and Coke, whiskey on the rocks, and gin and tonic).
Each bartender was also asked to pour the same four drinks, either with no instructions or after being told to take their time.
Both students and bartenders poured more into short, wide glasses than into tall, slender glasses. Among students, practice reduced the tendency to overpour into tall glasses, but not into short, wide glasses. Most students also believed that the tall glasses held more.
Despite an average of six years of experience, bartenders poured 20.5% more into short, wide glasses than tall, slender ones. Paying careful attention reduced but did not eliminate the effect.
These findings suggest that alcohol consumption studies should include questions about the shape of the glass, say the authors.
To avoid overpouring, they suggest using tall, narrow glasses or ones on which the alcohol level is marked. And to realise that when alcoholic drinks are served in a short, wide glass, two drinks are actually equal to two and a half.