Consistent with evidence from the recent Cochrane systematic review that tableware size influences consumption of food and non-alcoholic beverages [5], the larger glass size had an effect on overall wine sales (compared to standard glasses). However, results were inconclusive for the smaller glass size. Separating out the results in the different areas of the establishment, the effect for larger glasses was significant in the bar area, but not in the restaurant area.

Almost all sales in the bar were by the glass. As glasses containing pre-served portions may change perceptions of portion size [20, 21], the results for the larger glasses in the bar area are consistent with larger glasses being perceived to contain less, given the same actual portion [21]. As a result, portions may be drunk faster [20], which may lead people to drink more. Alternatively, there may be decreased satisfaction with perceived-to-be smaller portions or a perception that the portion did not comprise a full glass, leading to additional purchases. However, these mechanisms might be expected to operate across glass sizes (i.e., we should observe similar patterns when comparing the smaller and standard glasses), which was not evident in this study. This requires further investigation as to which glass comparisons yield similar results. Additional field studies are warranted alongside laboratory studies to test the two hypotheses for the observed effect outlined above. These findings can be considered within the broader literature of the many sensory and behavioural cues that influence the consumption of alcohol, of which size is just one [27, 28]. Future research might ultimately attempt to elucidate the combination of cues that reduced alcohol consumption the most.

The effect size of the larger wine glasses in the restaurant area, where sales by bottles and carafes represented 1/3 of the sales but 2/3 of the volume, while they represented 1/15 of the sales and 1/5 of the volume in the bar, did not reach statistical significance. This may reflect a smaller effect size of wine glass size in this context that the study was insufficiently powered to detect. Replication of the current study is needed in other settings where wine is served primarily using bottles or carafes in studies powered to detect smaller but potentially important effect sizes.

Strengths and limitations

This study is the first, to our knowledge, to explore the impact of glass size on wine sales for on-site consumption in a real-world setting (although it is likely that industry research exists). Examining how wine sales were affected in the bar area, where sales were predominantly by the glass, suggests several possible mechanisms that might underlie the influence of glass size, as described above.

However, several limitations should be noted. First, the multiple treatment reversal design has a higher risk of bias than an experimental design. Second, the predominance of sales by the glass in both the bar and restaurant areas meant that we were unable to robustly examine the effects of sales by the bottle. Third, the study took place in only one establishment in a relatively affluent English city. Finally, our outcome measure was sales for on-site consumption rather than consumption itself, although this still represents an objective measure of behaviour.

Implications for research and policy

Replication of the current intervention is needed. Further field studies could reduce further the risk of bias through experimental designs involving more observations, conducted in settings that include less affluent areas. Further investigations need to establish the contexts in which the strongest effects are likely to occur, including the extent to which these results might extend to in-home alcohol consumption. While further research is needed to establish the reliability of these findings – and in particular, explore the use of different glass sizes – the results offer initial evidence that reducing the use of larger glasses may reduce consumption of alcohol. If further work does suggest glass size might be an effective target for intervention, the next step would be to explore how this might be implemented [29]; for example, ensuring that all glasses were below a certain size could be one criterion amongst alcohol licensing requirements.