As you prepare to celebrate New Year’s Eve with your favorite organic wine, champagne or cocktail, you’ll need to pay special attention to the glasses you—or your party hosts—use.

People pour 20% to 30% more alcohol into short, wide glasses than into tall, narrow glasses of the same volume, but they wrongly believe tall glasses hold more, according to researchers. Even professional bartenders pour more into short, wide “tumblers” than into “highball” glasses, suggesting that experience in pouring alcohol has little effect.

Dr. Brian Wansink of Cornell University and Dr. Koert van Ittersum of the Georgia Institute of Technology studied 198 college students and 86 bartenders. 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 oz, 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 his time.

Both students and bartenders poured more alcohol 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 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.

“People focus their attention on the height of the liquid they are pouring and insufficiently compensate for its width,” says Dr. van Ittersum. “If short tumblers lead people—even bartenders—to pour more alcohol than [into] highball glasses, then there are two easy solutions: Either use tall glasses or ones with alcohol-level marks etched on them, as is done in some European countries.”

The researchers, whose findings were published in this week’s British Medical Journal, believe future studies of alcohol consumption should include questions about glass shape.

For some nonalcoholic New Year’s Eve beverages, check out our recipes for Organic Wassail and Organic Cranberry Party Punch.