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Variations in pouring and drinking behavior mean that the amount of alcohol poured into a mixed drink can vary widely. Although correction efforts have been suggested, an important unaccounted source of bias in self–reported consumption of spirits may have to do with the shape of the glass into which a drink is poured. This is the result of two perceptual biases: that people generally believe tall glasses hold more liquid that wide ones of the same volume and that they focus their pouring attention on the height the liquid reaches, insufficiently compensating for the width of the glass.
We had 198 students of legal drinking age practice pouring alcohol into different glasses; those in a low pouring education conducted one practice pour into a 1.5 ounce shot glass and those in a high pouring condition conducted the same practice pour 10 times. Afterwards, all participants were supplied with alcohol bottles and asked to pour the amount of alcohol in 4 standard drinks and estimate how much they thought they had poured. Half of the participants were asked to pour into tall, slender glasses and half were asked to pour into short, wide glasses. Both glasses were equal in volume (355 ml.) Although the students believed the taller glasses held more liquid, they poured 30% more liquid into the shorter glasses. Although practice reduced the tendency to over pour into tall glasses, it did not do so for the shorter glasses. In a follow up study, 86 bartenders were asked to do the same pouring tasks. Half of the participants were instructed to pay close attention to how much they poured to create a high attention versus low attention condition. Despite their professional experience, bartenders still poured 20.5% more liquid into the shorter glasses; paying careful attention reduced but did not eliminate this effect. We conclude that to avoid over pouring, consumers and bartenders should use tall, narrow glasses or shorter ones on which the alcohol level is pre–marked. Since previous studies have found that the entire amount of alcohol poured in a drink is usually drunk, consumers should be aware of these perceptual biases when drinking alcoholic beverages.
Wansink, Brian, and Koert van Ittersum (2005). Amount of Alcohol Poured: Comparative Study of Effect of Practice and Concentration. British Medical Journal, 331(7531),1512–1514. doi: http://dx.doi.org/10.1136/bmj.331.7531.1512