Theresa Marteau & Natasha Clarke
You and your friend have different favourite restaurants. You always order the same bottle of wine in each. In Theresa’s Bistro, the bottle usually lasts the duration of the meal. At Tash’s Pace, you somehow always end up ordering a second bottle. Might this be due to the restaurants’ wine glasses?
The size of wine glasses varies across bars and restaurants – from the modest “Paris Goblet” holding 190ml, to the glasses for red wine capable of taking half a litre, often favoured in restaurants. The amount wine glasses can hold has also changed over time. In the only study to examine this, we found an almost seven-fold increase in their size from 1700 to 2017 – from 66ml to 450ml. The most marked change was over the last 25 years, in which time glasses have almost doubled in size, from 230ml to 450ml. Pricing, automated technologies for making larger, more robust glasses, and the role of glasses in the drinking experience are some likely explanations for this. But what impact does wine glass size have on how much we drink?
In 2015, we worked together with an establishment in Cambridge, with separate bar and restaurant areas, to run the first study to examine this. They were serving wine in a 300ml glass. We purchased glasses of the same design but that were smaller (250ml), or larger (370ml) than their usual glasses. We changed the glass size used every two weeks, for a period of 16 weeks in total. This meant that for two weeks they served the wine just one glass size only– while keeping the amount of wine they served in each glass the same. We then compared sales for wine when served in each of these three glass sizes. When the larger glasses were used, sales increased by almost 10%. This was larger in the bar area – where sales increased by 14% – than in the restaurant – where sales increased by 8%. We repeated this study a number of times in other bars and restaurants in Cambridge between 2015 and 2018. We found similar effects in one other bar but not in another. We then found an effect of larger wine glasses on sales in another restaurant but this finding was not consistently replicated. So, larger wine glasses sometimes increased wine sales but not always, with the overall pattern being hard to interpret.
To provide a clearer picture and a robust estimate of the impact of wine glass size on sales we did a ‘mega-analysis’, bringing together the results of all our previously published studies carried out at bars and restaurants. We used 300ml glasses as the comparison size. The overall pattern of results showed that in restaurants, when glass size was increased to 370ml, wine sales increased by 7.3%. There was also a trend to lower sales when the glass size was 250ml. These effects were not found in bars.
Why might glass size have an effect in restaurants but not bars?
One reason is that in restaurants maybe more wine is sold in bottles or carafes, rather than by the glass, which require free-pouring by customers or staff. For example, if you are out for a meal, you might get a bottle of wine for the table, which you or restaurant staff would then pour into glasses. In a bar, you might order a fixed portion – like a 175 ml glass of wine. Pouring wine from a bottle or a carafe allows people to pour more than a standard serving size – e.g. over 175ml – an effect that may increase with the size of the glass. People may still think of this as ‘one glass of wine’, and drink the same number of glasses as they usually would with a smaller glass and, consequently, drink more wine with larger glasses.
It is also possible that wine glass size does affect sales in bars but the effect may be smaller than the study was able to detect.
Does it matter?
If you run a restaurant you might now want to increase the size of your wine glasses to sell more wine. This is exactly what happened after the results of our first study were published. The manager of that establishment told the Wall Street Journal that they removed smaller glasses and used only 370ml glasses.
If you are responsible for public health, you will take a different view. Globally, excess alcohol consumption is the 7th largest contributor to premature death. Our results suggest that reducing the size of glasses – perhaps returning to the classic Paris Goblet with its 190ml capacity- might make a useful addition to other well-established approaches for reducing consumption, including increasing the price of alcohol, reducing its availability and curbing its marketing. It’s also a potentially feasible intervention, if we consider that in the UK serving sizes are currently regulated by law.
So how can we explain why a bottle of wine lasts the evening in one of your favourite restaurants but not the other? Many factors will be at play but the results of our new study put the size of the wine glasses squarely in the frame.
Professor Theresa Marteau is Director of the Behaviour and Health Research Unit at the University of Cambridge and Director of Studies for Psychological and Behavioural Sciences at Christ’s College, Cambridge. The research in her group is focused on identifying cues in our environments that can be changed in order to reduce consumption of food, alcohol and tobacco to improve health across populations. This includes the first studies worldwide of the size of wine glasses – how the size has increased over the centuries and how this may be affecting how much we drink.
Her research is funded by Wellcome, National Institute of Health Research and Medical Research Council UK.
Dr Natasha Clarke is a Research Associate in the Behaviour and Health Research Unit at the University of Cambridge and works full time on the Behaviour Change by Design Project. Natasha obtained a PhD from the University of Liverpool in 2017 which focused on developing interventions to reduce alcohol-related harm by altering cues in the drinking environment, such as glass shape and labelling. Natasha’s current work includes how such interventions can be applied in naturalistic settings.
Her research is funded by Wellcome.