The Shape Of A Glass Can Influence How Much We Drink

By Matthew Warren

Recent years have seen the government take measures to try and limit people’s consumption of sugary drinks and other unhealthy foods. Take the so-called “sugar tax” placed on soft drinks, for instance, or the proposal to ban adverts for junk food before the 9pm watershed.

Some psychologists hope that small changes in design can also help “nudge” people into healthier behaviours. For example, a study from last year found that the order in which drinks are presented on the McDonald’s menu could encourage people to choose the sugar-free options more often.

Now a new paper in Scientific Reports suggests that the shape of a glass could also subtly influence people’s drinking behaviours.

Past work had found that people were slower to drink both beer and soft drinks from glasses with straight sides compared to those with outward-sloping sides. But it was unclear whether there was any effect on the total amount consumed or exactly why the shape of a glass should have an effect.

To investigate these questions, Tess Langfield from the University of Cambridge and colleagues conducted a series of experiments using different shaped glasses. First, the team gave 198 participants a fizzy apple drink in either a straight-sided tumbler, or one with sides that sloped outwards. The participants were allowed to drink at their leisure while watching a nature documentary.

Langfield et al (2020)
The glasses used in the first study (left; each glass contains 330ml of apple drink) and second and third studies (right; each glass contains 165ml of passion fruit drink). Via Langfield et al (2020)

The team found no difference in the total time participants spent drinking from each kind of glass. But those who drank from the sloping glass did show a more “decelerated” pattern of drinking: they tended to drink more of the drink in the first half of the session than the second half.

Participants were then asked to fill the glass until it was half-full. Those with the sloping glass tended to underestimate where the midpoint was compared to those with the straight-sided glass. That makes sense: if a glass slopes outwards, there is more volume at the top than at the bottom, so the midpoint is higher that it might initially seem.

These findings suggest glass shape could influence people’s pace of drinking, and potentially their perceptions of how much they’ve had to drink. But they don’t reveal anything about the actual amount consumed. So in a second study, the team asked participants to perform a (fake) taste-test, rating four passion fruit drinks, half served in straight-sided champagne flutes and half in sloped martini glasses.

Participants did indeed drink significantly less from the champagne flutes than the martini glasses. They also took smaller sips from the champagne flutes. This time, however, glass shape didn’t influence people’s accuracy at finding the midpoint of the glass, suggesting that this kind of perceptual bias couldn’t explain why people drank less from the straight-sided glasses.

Instead, the researchers wondered whether the results could be related to the physical shape of the lips when sipping from each kind of glass. So in a final experiment, they had participants drink from the champagne and martini glasses while their face was rigged up with electrodes to measure muscle activity. The team found that people’s lips were more pursed when sipping from the straight-sided flute than the sloped martini glass, and there was some evidence that pursing the lips more was related to taking smaller sips.

Overall, then, the study suggests that sloping glasses can cause us to drink more of a soft drink — but this seems to be due to the way the shape affects the actual mechanics of how we sip from a glass, rather than the result of perceptual biases.

So should health-conscious bars and restaurants start chucking out their sloping glasses? Well, it seems pretty clear that more work needs to be done in real world settings. Drinking in a bogus taste-test in the lab — particularly when hooked up to electrodes — is a far cry from how people might consume soft drinks in daily life. And it remains unclear whether the results hold for alcohol as well as soft drinks.

The last two studies also used glasses which normally have quite specific purposes: they’re used for serving sparkling wine and cocktails. Could participants’ associations with those drinks have influenced their behaviour? Maybe they’re used to sipping slowly on champagne, for instance, while more readily knocking back martinis. Unlikely, perhaps, but it seems worth also exploring the effects of glasses with novel designs that are free from these kinds of existing associations.

Still, the study shows that the design of our drinkware is worth examining in the context of public health. And even small “nudges” could theoretically have a large impact at a population level, particularly alongside other measures. “In combination with other behaviour change strategies, adopting straight-sided glasses may prove to be one intervention contributing to the many needed to reduce consumption of drinks that harm health,” the team concludes.

Glass shape influences drinking behaviours in three laboratory experiments

Matthew Warren (@MattBWarren) is Editor of BPS Research Digest

One thought on “The Shape Of A Glass Can Influence How Much We Drink”

  1. The researchers assume that the champagne glass v. martini glass was testing the sloping v. straight sides. However, given the pursing of the lips it is at least as likely that the critical factor is the diameter of the rim, and that the pursing is done to conform the lips to the smaller aperture.

    Like

Comments are closed.