Plastics pollutions is a major global issue, and single-use beverage cups are one of top-ten items littered around beaches. To increase the use of reusable cups, and thereby increase circularity, we in this study explore nudging as a method to shift behaviour from buying coffee and other beverages in single-use cups to reusable cups. In collaboration with coffee shops, cafeterias and gas station convenience stores in Denmark and Sweden, we test the effect of several nudging techniques. By nudging customers, we promote the sustainable behaviour without imposing restrictions or significantly altering incentives. Reusable cups are available to customers at many coffee shops, but they are not as salient as the single-use alternative. Single-use cups are also the default choice. To overcome some of the barriers facing an increased uptake of reusable cups, we implemented several nudging techniques and combination of techniques at different coffee shops. We compared sales with similar locations without any interventions to assess the effect of the different nudges. Our findings reveal that prompts are the most impactful type of nudge among those tested. However, successful prompts also require a normal implementation with a dynamic approach to ensure that instructions are followed in situations where staff already follow several instructions, may forget, and may perceive additional questions to customers as an inconvenience. We further find that increasing saliency may be necessary, but not sufficient to meaningfully impact behaviour in isolation. Installing posters had only a limited or no impact on reusable cups sales, and tests with prompts and posters combined revealed that the positive effects disappeared once prompts were removed. Leveraging social norms by providing on-going information on the behaviours of others (showing how many reusable cups had been sold) was found to have a positive impact. However, this positive effect was only in situations where customers were more likely to identify with the other customers, such as when the information was based on local behaviour rather than general, and when beverages are sold in closed settings. Closed settings refer to coffee shops in office spaces or at universities, where many customers are repeat customers, as opposed to more openly available coffee shops. Lottery based incentives were also found to have a significant impact, but mainly when the lottery was provided in the purchase moment rather than the return moment.
Nudging provides a promising method to reach impactful sustainable behaviour only if successfully implemented at scale. For coffee shops with the ambition of increasing the uptake of reusable cups, we recommend using prompts and increasing the saliency of the reusable cups. Even though saliency alone was not found to have a meaningful impact, we believe it creates awareness and facilitates a successful implementation of prompts. We advocate scaling solutions carefully, adopting a dynamic approach to ensure that nudges are implemented with as little noise as possible. Nudges should ideally be tested at scale by both using the same nudge type at many locations (open and closed settings etc), and by using different nudge types at similar locations. Even though prompts are found to be the most impactful intervention, we do not endorse mandating coffee shops to use prompts, as they can also be perceived as an inconvenience both for customers and staff. Instead, we recommend encouraging voluntary employment of prompts and saliency. We also recommend investigating possible effects of monetary incentives, both for customers and coffee shops, as well as awareness campaigns in collaboration with coffee shops.