The participants were almost twice as slow when drinking alcohol from the straight-sided glass compared to the curved glass. There was no difference in drinking rates from the glasses when the drink was non-alcoholic.
The researchers suggest that the reason for this may be because it is more difficult to accurately judge the halfway point of shaped glasses. As a result, drinkers are less able to gauge how much they have consumed.
In order to test this, participants attended another session in which they completed a computer task that presented numerous pictures of the two glasses containing varying volumes of liquid. By asking participants to judge whether the glass was more or less than half full, the researchers were able to show that there was greater error in accurately judging the halfway point of the curved glass.