Professor Brian Wansink studied the phenomenon with Koert van Ittersum of Georgia Institute of Technology. Reporting in the new issue of the British Medical Journal, the researchers said their study showed one way to prevent overpouring is to use tall, thin glasses.

They conducted a study in which subjects were asked to put a 1.5-ounce shot of liquor into short, wide tumblers or tall, thin highball glasses. The participants routinely put 30 percent more alcohol in the short glasses than in the taller ones.

The outcome apparently resulted from an optical illusion: People generally believe vertical lines are longer than horizontal ones of the same length, so even experienced bartenders pour more liquor into short, wide glasses than tall, thin ones with the same volume.

"Education, practice, concentration and experience don't correct the overpouring," said Wansink.

The lesson can even be applied by parents serving non-alcoholic drinks to children -- use tall, thin glasses for sodas but short, wide ones for milk and other healthy drinks, said Wansink.