This is an open access article distributed under the terms of the Creative Commons Attribution License (Food production is one of the largest drivers of environmental damage caused by human activity (WWF, 2020), with global climate targets in the Paris Agreement unable to be met if current trends within our food systems continue (Clark et al., 2020). While systemic change is needed to sustainably transform our food system, shifting western consumers’ diets can significantly contribute to reducing the environmental impact of food production and help achieve climate commitments (Behavioural Insights Team, 2021; Clark et al., 2020). Owing to the environmental damage caused by animal agriculture (Poore & Nemecek, 2018), and the overconsumption of animal products in the global north (Ritchie & Roser, 2023), consumers’ diets need to become more plant-based (Scarborough et al., 2023).
Insights from behavioural sciences can be applied to help promote plant-based diets. One framework is nudge theory (or ‘nudging’), which aims to change people’s behaviour by altering the environmental context in which individuals make decisions while maintaining freedom of choice (Thaler & Sunstein, 2008). Nudging is based on the recognition that, when faced with decisions, individuals often use cognitive shortcuts instead of making well-reasoned choices (Kahneman, 2011). This can bias people’s behaviour, resulting in them making less-optimal choices for themselves, society and the planet (Thaler & Sunstein, 2008). Having received much attention among policymakers, nudging has become widely embedded in western policy in recent decades (Halpern, 2015). Nudging has also become increasingly common in the environmental sector. Termed ‘green nudging’, it can address environmental issues by promoting pro-environmental behaviours and there has been growing support to integrate green nudges into sustainability-related policy, including governmental and higher education policy (Carlsson et al., 2021; UNEP et al., 2020).
Despite this potential, the impact of nudging on changing people’s behaviour has been heavily debated, with concerns that impact may be limited (Chater & Loewenstein, 2023; DellaVigna & Linos, 2022; Maier et al., 2022). Research suggests that the influence of nudges depends on the type of nudge and area of focus, with nudges being particularly effective at altering individuals’ food choices (Mertens et al., 2022). However, research in this area is still developing (Reisch & Sunstein, 2021). For example, carbon labels promoting plant-based diets do not work effectively in real-world settings (Kaljonen et al., 2020), despite positive results in online experiments (Betz et al., 2022), and the impact of positive food descriptions (e.g., dish of the day’) are mixed (Saulais et al., 2019; Zhou et al., 2019). Furthermore, nudges based on social norms can have unintended adverse effects by reducing or reversing the desired behaviour (the ‘boomerang effect’; Osman et al., 2020; see Griesoph et al., 2021, and Sparkman et al., 2020).
One nudge which seems particularly effective at increasing plant-based food consumption is the ‘default’ nudge, which makes sustainable food the default option (similar to using an ‘opt-out’ strategy; Meier et al., 2022). Although studies testing the influence of default nudges on real-world food choices are still emerging, they seem effective at encouraging plant-based food intake, even when meat-based options were available upon request. For example, Hansen et al. (2021) found that conference attendees whose catering choice defaulted to a vegetarian option ate significantly more vegetarian meals compared to attendees who received a meat option default (86%–89% versus 2%–12.5%). Similarly, Danish students ate approximately twice as many meat-free meals when allocated a default vegetarian meal choice (Randers & Thøgersen, 2023); see also Campbell-Arvai et al. (2014) and Taufik et al. (2022) for similar findings. Although only a handful of studies have explored default nudges and sustainable food consumption, their positive impact on decreasing meat intake has encouraged public support for policy to incorporate default nudges to increase sustainable diets (Reisch & Sunstein, 2021).
Default nudges may be effective because of various cognitive biases driving inertia, including:
Status quo bias, where people have a tendency to continue with the current situation instead of changing (Samuelson & Zeckhauser, 1988), which is even stronger for individuals paying less attention (Thaler & Sunstein, 2008).
Loss aversion, where people’s perception of the negative impact of losing something (i.e., the default) is greater than the potential benefit of gaining it (Kahneman, 2011).
Perceptions of endorsement, where people believe that the default has been selected to benefit them and is the socially acceptable action to take (Meier et al., 2022).
Although people’s predisposition to take no action makes default nudges effective, the extent of their impact depends on how the nudge is implemented (Sunstein, 2017). Based on a systematic review of studies applying default interventions to decrease meat reduction, Meier et al. (2022) identified four factors which increased effectiveness of this nudge. These were:
The alternative requiring more effort.
Awareness of the alternative being low.
The appearance of the default being more attractive than the alternative.
People paying low attention.
These factors emphasise the myriad of influences that affect the success of default nudges and highlight the complexities of delivering such nudges in real-world settings.
Further research is needed to determine the impact of default nudges on encouraging sustainable food choices beyond meat reduction and whether these replicate in real-world settings. Existing research has focused on reducing meat consumption, but no studies have tested whether default nudges can reduce dairy milk intake and increase plant-based milk consumption (Reisch et al., 2021), even though plant-based milk has a substantially lower environmental impact than dairy (Ritchie, 2022). The environmental damage caused by dairy production (Poore & Nemecek, 2018) adds urgency to research examining the role of default nudges in shifting consumers’ diets away from this food group (Grundy et al., 2022). Even more important is the need for research which tests the effectiveness of default nudges in real-world settings since this is currently very limited (Meier et al., 2022). While default nudges are known to be effective (Meier et al., 2022), conceptual replications of default nudges in different contexts will help determine the robustness of this intervention in enhancing sustainable food consumption, as well as the potential effect sizes of such interventions. For instance, although default nudges encouraging meat-free meal choice have shown to be effective in restaurant settings (Campbell-Arvai et al., 2014; Hansen et al., 2021; Randers & Thøgersen, 2023; Taufik et al., 2022), it is not clear the extent to which these results generalise to drink choices in café settings; it is perhaps plausible that factors influencing drink purchasing and the context in which this behaviour occurs differ. For example, café customers may have stronger location-bound preferences than restaurant customers because they are likely to buy the same drink from the same location repeatedly, meaning they may be less influenced by default nudges (Venema & Jensen, 2024).
The present study contributes to the growing literature on the effectiveness of nudge interventions on sustainable food consumption by testing the effect of a default nudge on sustainable drink behaviour (plant-based milk consumption). Specifically, this study aims to answer the research question: Does changing menu defaults increase plant-based milk consumption in a UK university café? To do this, we test the hypothesis that making plant-based milk the default option in drinks will increase plant-based milk consumption by conducting an experimental study which changes the default milk option in one university café but not in a second café at the same university.
Method and Materials
Intervention
An intervention where plant-based milk is the default milk option for drinks sold at a university café was tested during the study (for the theoretical framework behind this intervention, see Supplementary Section S1 in Major-Smith, 2024b). Specifically, customers purchasing drinks from Plymouth Marjon University’s (PMU) main café (known as Barjon Café) were automatically given oat milk in their drink unless they requested otherwise. Oat milk was chosen because it is the most requested plant-based milk among the café’s customers and has low environmental impact; using significantly less land and water, and emitting fewer greenhouse gas emissions, than dairy milk (Ritchie, 2022). While delivering the intervention, three signs were displayed informing customers that oat milk would be automatically applied to drinks for environmental reasons and that alternative milks were available upon request (Supplementary Section S2; Major-Smith, 2024b). Two signs were located on the café’s counter, one of which was next to the till where customers ordered and paid for their drinks. A third sign was displayed on the wall before entering the café. The other milk options available upon request were almond, coconut, dairy and soy; the baristas asked customers if they had any allergies before serving them. The study received ethical approval from PMU’s Research Ethics Panel, which did not require informed consent from café customers.
Setting
The experiment took place at PMU’s Barjon Café. Managed by an external catering company, this café is located on the main campus and is predominantly used by staff and students. The café is much busier during weekdays than weekends. A second café, the Grandstand Café, was used as a comparison site during which data on drink sales was collected but no intervention applied. Managed by the same catering team as Barjon Café, the Grandstand Café is situated in the university’s sports centre, located just off the main campus. During weekdays, this café is used by staff, students and members of the public visiting the sports centre, while during weekends it is mainly used by the public.
Study Design
To test the intervention, an ABAB experimental design was implemented from 30th January–31st March 2023. As Barjon Café’s drink sales are highest during term time, the study was implemented during a semester. An ABAB design contains four phases: A1 (baseline), B1 (first intervention), A2 (second baseline) and B2 (second intervention). This design was selected because it effectively tests the impact of an intervention by using baseline data to describe any changes to behaviour prompted by the intervention (by comparing B1 to A1), predicts behaviour if the intervention was not implemented (A1), tests if the intervention replicates (by comparing B2 to A2), and indicates potential carryover effects (by comparing A2 to A1; Kazdin, 2019; Nock et al., 2007). Furthermore, given the restrictions of conducting research in a real-world setting, applying this type of experimental design is more feasible than other research designs (e.g., conducting a randomised-controlled trial because of the challenges of randomly allocating customers to conditions). The Grandstand Café was used as a control comparison where no intervention was conducted to provide stronger support that any observed changes in plant-based milk consumption at Barjon Café could be attributed to the intervention (Rare & Behavioural Insight Team, 2019).
To determine the sample size of drink sales needed for each condition given various effect sizes, and thus the length of each study phase, power analyses were conducted. This was based on the mean number of milk-based beverages sold in Barjon Café during a semester weekday in 2022, and the number which contained plant-based milk. An average of 256 drinks were sold daily, with 21 (8.2%) containing plant-based milk. Power analyses explored the sample size needed within each study phase to detect a 2.5, 5, 10 and 20%-point difference in plant-based milk consumption between conditions. For a proportion test with an alpha level of .05 and power of .8, to detect a small effect of a 2.5%-point increase in plant-based milk consumption the sample size for each study phase would need to be 2148 drink sales (approximately eight days). Taking this into consideration, as well as the 2022–2023 academic timetable, it was decided that two weeks for each study phase would be sufficient to detect even a small change in plant-based milk consumption of 2.5%-points. While this study may have been overpowered, especially considering the larger effect sizes from previous default nudges on meat reduction, this also allowed us to examine the longer-term effect of the default nudge intervention.
During the first phase (A1) baseline data were collected on customers’ drink purchasing and milk preferences in Barjon Café. During this time, drinks continued to be served in the same way as normal (i.e., dairy milk default) and data were collected for three weeks from 30th January–19th February. This phase incorporated a half term week from 13th–19th February and even though data were collected during this week, these data were removed during analysis because a higher proportion of the public visited the Grandstand Café, and fewer students visited Barjon Café, which could bias results (for more information on this, and robustness checks which show that the inclusion of this week does not alter the conclusions of the results reported below, see Supplementary Section S3 in Major-Smith, 2024b). The first intervention phase (B1) was then implemented, which ran for two weeks from 20th February–5th March. This was followed by the second baseline phase (A2) during which the intervention ceased and dairy milk was again the default. This A2 phase ran for two weeks from 6th–19th March. The final phase (B2) repeated the intervention and ran for two weeks from 20th–31st March. During all phases, baristas collected data on the number of drinks sold daily (from 7.30am to 5pm) and which milk was consumed, with data hand-recorded on paper-based recording sheets. The same information was collected from the Grandstand Café, except for 17th March when no data were collected because of staff changes. The two Barjon Café baristas received training prior to the study starting, and both baristas were present for the duration of the study except for a few hours on 20th and 21st March when two different baristas managed the café, both of whom received brief training. Due to differences in customers of both cafés during weekends (e.g., a greater proportion of the public visiting the Grandstand Café and substantially less customers visiting Barjon Café) and the challenges of training the weekend baristas because of frequent staff changes, weekend data were not collected.
Pre-Registered Analysis Plan
This study was pre-registered on the Open Science Framework (see Major-Smith, 2024a) on 27th January 2023. The primary analyses using aggregate-level data were conducted as described in the pre-registration document, but with the addition of non-preregistered equivalence tests for the Grandstand Café (Control Café (Grandstand) Section). However, we were unable to conduct mixed-effects models using individual-level data (see Supplementary Section S4 in Major-Smith, 2024b).
Data Analysis
To test whether the intervention impacted plant-based milk consumption, a Poisson regression model was conducted to estimate the risk ratio effect of each phase (i.e., B1, A2 and B2) relative to the A1 baseline phase. This analysis: 1) examined whether the B1 intervention phase affected plant-based milk consumption (comparing B1 to A1); 2) examined differences between the two baseline phases, indicating potential carryover effects from the first intervention phase (comparing A2 to A1); and 3) examined the effect that the B2 intervention phase had on plant-based milk consumption compared to the first baseline phase (comparing B2 to A1). To estimate unbiased standard errors when using Poisson regression with a binary outcome, we applied robust standard errors (Zou, 2004). For this analysis, data were disaggregated into one row per drinks purchase, with the outcome a binary variable of whether the drink contained plant-based milk or dairy milk.
As the third component uses the first baseline phase (A1) as a comparison for the effect of the second intervention phase (B2), this may not accurately reflect the true effect of the second intervention phase if differences occurred between the two baseline phases. For example, if plant-based milk consumption was higher during the second baseline phase (A2) than the first (A1), then the intervention’s effect would be different when comparing it to the first baseline phase instead of the second. Therefore, to more accurately assess whether the intervention replicated, using the Poisson model above a post-estimation hypothesis test was conducted to compare whether the effect size of the B1 intervention phase differed from the B2 intervention phase, adjusting for any potential A2 differences from the A1 baseline. If a difference was found, then a further Poisson model comparing B2 to the baseline A2 phase was conducted to estimate this risk ratio.
To examine whether there was a causal effect between the intervention and plant-based milk consumption, analyses were replicated in the comparison Grandstand Café site, in addition to non-preregistered equivalence tests to formally test for the absence of an effect (Lakens et al., 2018). For all analyses, data on consumption of the default plant-based milk (oat milk) were collated with consumption data of other plant-based milks (e.g., if customers requested a different plant-based milk instead of the default oat milk). This is because separate analysis of plant-based milk consumption was not necessary for answering the research question. Analyses were conducted in R 4.2.3 using the packages ggplot2 (Wickham, 2016), marginaleffects (Arel-Bundock, 2023), readxl (Wickham & Bryan, 2023), sandwich (Zeileis et al., 2020) and Tidyverse (Wickham et al., 2019).
We also conducted a carbon footprint analysis to calculate any changes in greenhouse gas emissions due to the intervention. The carbon footprint of each drink and milk option was quantified, after which the mean carbon footprint of milk per drink consumed in each condition was calculated (to account for different sample sizes between the phases), which combined the carbon footprints of dairy milk, oat milk and other plant-based milk options (see Supplementary Section S5 in Major-Smith, 2024b).
Results
Intervention Café (Barjon)
The mean number of drinks sold daily in Barjon Café during the study period was 206 (Standard Deviation = 54, range = 103–404). The percentage of plant-based milk consumed during the A1 baseline phase was 16.6%, which increased to 51.9% during the B1 intervention phase. This decreased to 23.0% during the A2 baseline phase and then increased again to 46.0% during the B2 intervention phase. The amount of plant-based milk consumed was 5.9%-points lower in the B2 intervention phase compared to the B1 intervention phase, showing that rates of plant-based milk consumption reduced as the default nudge progressed (Table 1 and Figure 1; rates of plant-based milk consumption also appeared to decrease over time within the B1 and B2 intervention phases). Plant-based milk consumption increased by 6.4%-points during the A2 baseline phase compared to the A1 baseline phase.
Table 1
Mean Percentage of Plant-Based Milk Consumed, and Total Number of Drinks Sold, in Barjon Café and Grandstand Café
| Café | A1 | B1 | A2 | B2 |
|---|---|---|---|---|
| Barjon Café | 16.6% (15.1, 18.2) | 51.9% (49.7, 54.0) | 23.0% (21.2, 24.9) | 46.0% (43.7, 48.4) |
| Total drinks sold | 2315 | 2076 | 2021 | 1829 |
| Grandstand Café | 15.0% (12.7, 17.4) | 12.1% (9.78, 14.6) | 12.2% (10.1, 14.6) | 14.4% (11.9, 17.1) |
| Total drinks sold | 909 | 730 | 868 | 730 |
Note. A1 = Baseline phase, B1 = Intervention phase, A2 = Baseline phase and B2 = Intervention phase. Binomial confidence intervals for each percentage are shown in brackets.
Figure 1
Percentage of Plant-Based Milk Consumed During Weekdays in Barjon Café and Grandstand Café
Note. A1 = Baseline phase, B1 = Intervention phase, A2 = Baseline phase and B2 = Intervention phase. Each point represents a weekday and error bars for each percentage represent binomial confidence intervals. Grandstand Café data for the 17th March 2023 during the A2 phase is missing as data were not recorded due to staff changes.
The Poisson regression model showed that, compared to the A1 baseline phase, customers were three times more likely to consume plant-based milk during the B1 intervention phase (risk ratio (RR) = 3.13, 95% confidence interval (CI) [2.83, 3.46], p < .001) and just under three times more likely in the B2 intervention phase (RR = 2.78, 95% CI [2.50, 3.08], p < .001). Customers were also 40% more likely to consume plant-based milk during the A2 baseline phase compared to the A1 baseline phase (RR = 1.39, 95% CI [1.23, 1.57], p < .001), suggesting that carryover effects occurred from the B1 intervention phase to the A2 baseline phase.
Next, a post-estimation hypothesis test was conducted to assess whether effect size of the B1 intervention differed from the B2 intervention while adjusting for any differences between the A1 and A2 baseline phases. This test showed that the size of the B2 intervention was approximately one-third smaller than the B1 intervention (RR = 0.64, 95% CI [0.56, 0.73], p < .001). Finally, a comparison of the A2 baseline phase and B2 intervention phase showed that customers were twice as likely to consume plant-based milk in the B2 intervention phase compared to the A2 baseline phase (RR = 2.00, 95% CI [1.82, 2.20], p < .001), confirming that the B2 intervention phase had a weaker effect than the B1 intervention phase.
Control Café (Grandstand)
The mean number of daily drinks sold in the Grandstand Café was 83 (Standard Deviation = 22, range = 35–173). The percentage of plant-based milk consumed was approximately equal through all study phases (A1 baseline = 15.0%, B1 intervention = 12.1%, A2 baseline = 12.2%, B2 intervention = 14.4%; see Table 1 and Figure 1).
A Poisson regression model confirmed this interpretation, finding little difference in plant-based milk consumption between the four phases. Plant-based milk consumption did decrease slightly during the B1 intervention phase (RR = 0.81, 95% CI [0.63, 1.03], p = .114) and A2 baseline phase (RR = 0.82, 95% CI [0.64, 1.03], p = .117), compared to the A1 baseline phase, although no difference was found for the B2 intervention phase (RR = 0.96, 95% CI [0.76, 1.22], p = .762). Non-pre-registered equivalence tests suggested that these Grandstand Café results were consistent with effect sizes +/- 2.5%-points or smaller, but likely inconsistent with effect sizes larger than +/- 5%-points (see Supplementary Section S6 in Major-Smith, 2024b for full details).
Carbon Footprint of Drinks
The mean carbon footprint of milk per drink was lower in the B1 and B2 intervention phases when oat milk was the default milk compared to the A1 and A2 baseline phases (Table 2). While estimated carbon footprints vary between Poore and Nemecek (2018) and Singh-Povel et al. (2022), differences found between each phase for both sources are similar. Compared to the A1 baseline phase, there was a 30%–34% decrease in milk-related greenhouse gas emissions during the B1 intervention phase, and a 25%–28% reduction in the B2 intervention phase. When comparing the B2 intervention phase against the A2 baseline phase, carbon footprint reduction is slightly lower at 21%–22%.
Table 2
Mean Carbon Footprint of Milk per Drink (kgCO2eq) Consumed in Barjon Café
| Phase | Using Poore and Nemecek (2018) data | Using Singh-Povel et al. (2022) data |
|---|---|---|
| A1 | 0.79 (0.77–0.81) | 0.39 (0.38–0.40) |
| B1 | 0.56 (0.54–0.58) | 0.26 (0.25–0.27) |
| A2 | 0.75 (0.73–0.76) | 0.36 (0.35–0.37) |
| B2 | 0.59 (0.5–0.61) | 0.28 (0.27–0.29) |
Note. A1 = Baseline phase, B1 = Intervention phase, A2 = Baseline phase and B2 = Intervention phase. This incorporates all milk types consumed during each study phase (i.e., oat milk, dairy milk and other types of plant-based milk). 95% confidence intervals for these mean values are displayed in brackets.
Discussion
These results demonstrate that changing a menu’s milk option default from dairy to oat milk can increase customers’ plant-based milk consumption in a UK university café. Customers were approximately three times more likely to consume plant-based milk when oat milk was the default option instead of dairy milk, with the mean milk-based carbon footprint per drink reducing by 25%–34%. Comparable data in a second café at the university found no changes in plant-based milk consumption during the study period, suggesting that the observed differences in intake were due to the intervention. These findings suggest that default nudges can significantly enhance sustainable food consumption beyond meat reduction and help shift consumers’ diets, providing implications for the wider adoption of default nudges in the food sector. To our knowledge, this is the first study examining the impact of default nudges on dairy reduction and plant-based milk consumption, and it is intended that these findings will contribute to the growing literature on, and support for, the usefulness of default nudges in encouraging sustainable food consumption.
Utilising default nudges to reduce dairy milk intake among consumers is also likely to benefit organisations. Food businesses commonly employ sustainability strategies to reduce their environmental impact, with targets linked to reducing companies’ carbon footprints and rate of unsustainable procurement (Scott, 2018). Utilising default nudges to increase plant-based milk intake would help achieve such targets (Ritchie, 2022). The university sector, in which this study took place, is committed to reducing their emissions and mitigating climate change impacts. Globally, over 1100 educational institutions have committed to becoming net zero by 2050 (UNFCCC, 2025), while in the UK the development of the Climate Commission for Further and Higher Education (Climate Commission for UK Further and Higher Education, 2021a) and the Department for Education’s sustainability strategy (Department for Education, 2022), aim to help educational institutions achieve this. The current study has high ecological validity because it tested the intervention in a university café, thus increasing the generalisability of findings to other university contexts (Schmuckler, 2001). The use of default nudges encouraging plant-based milk consumption at universities, in combination with other efforts, could help the sector achieve net zero.
Furthermore, integrating default nudges into real-world food environments is relatively straightforward to employ and seems to have low impact on customer satisfaction. While the prospects of increasing consumer loyalty and attracting new customers are important drivers for businesses implementing sustainable standards (Sharma, 2019), companies are reluctant to take actions which could negatively impact customer satisfaction (Chkanikova & Mont, 2015). Encouragingly, the present study received little negative customer feedback; only one formal complaint was made and four drinks (out of 3905; 0.1%) were returned by customers who disliked the taste of oat milk. Although formal customer feedback was not obtained during the study, anecdotal evidence from customers and baristas suggest that customers were receptive to the change. Importantly, no negative feedback was received by the baristas (who are first point of call for informal customer feedback). However, this finding may be context-dependent. Attending university offers students an opportunity to break previously practiced behaviours and acquire new habits (Verplanken et al., 2008), with attendance found to increase environmental action, including sustainable transport use (Haggar et al., 2019). Delivering interventions during this life stage could be more effective than at other times. Furthermore, environmental concern is stronger among younger and more highly-education individuals (Li et al., 2019; Panzone et al., 2016), suggesting that university students and staff may be more receptive of changes made for environmental purposes. Additionally, climate change has become a prominent theme among university students, with UK students demanding stronger sustainable action from universities (Climate Commission for UK Further and Higher Education, 2021b). Thus, customers in non-university settings may react differently to milk default changes, meaning that generalisability of the study findings may be limited beyond the university sector.
While the present findings provide promising implications for the employment of default nudges in aiding plant-based milk consumption, the study raises an important consideration on their long-term effectiveness. Research examining default nudges (and nudges more generally) often focused on short-term impact (Hansen et al., 2021; Randers & Thøgersen, 2023; Taufik et al., 2022). While the present study took the same short-term perspective, the decrease in plant-based milk consumption during the second intervention phase compared to the first, as well as within both intervention phases, illustrates the potential for the effectiveness of default nudges to diminish over time. A possible explanation for this could be the ease customers experienced requesting dairy milk (Meier et al., 2022), with the baristas initially directing customers to read the information sign outlining the change in milk default and option to request an alternative milk, despite being asked not to during their training. Although this action only occurred for a few days, it may have increased customers’ awareness of alternative milks being available and reduced efforts of requesting it. Customers’ engagement with the information signs was not measured, so the number of customers aware of the plant-based milk default or the impact that the signs had on the type of milk customers consumed is not known. These issues highlight the challenges of conducting experiments in real-world settings and mirror the unexpected problems reported in previous nudge-based research on sustainable food consumption (Kaljonen et al., 2020; Sparkman et al., 2020). Further research exploring the long-term impacts of default nudges and the role that accompanying information signage has on the intervention’s effectiveness, both on plant-based milk intake and sustainable food consumption more generally, is needed.
An additional limitation of this study is not being able to examine individual-level data and assess repeat purchases among customers, which meant that each drink purchase was treated as an independent datapoint in analyses. While this unlikely impacted the study’s overall conclusion given the large effect size, using repeated individual-level data may have provided a more detailed assessment of the association between the intervention and purchasing behaviour, highlighting, for example, if the intervention effect differed between repeat customers and one-time customers. This is important because previous research suggests that nudges may not work as effectively for individuals who have developed location-bound preferences through repeated interaction with the choice architecture in which the nudge was implemented. For example, in a hospital canteen, a nudge influenced new customers/visitors more than repeat customers/hospital staff (Venema & Jensen, 2024). While the effect sizes of the default nudge found in the present study are large (i.e., ~35%-point increase in plant-based milk consumption), they are slightly lower than the effect sizes of default nudges on meat reduction in restaurants (e.g., ~40%-point to ~80%-point increase; Campbell-Arvai et al., 2014; Hansen et al., 2021; Randers & Thøgersen, 2023; Taufik et al., 2022). While requiring replication, this suggests that location-bound preferences may have impacted the effectiveness of the default nudge. Furthermore, differences may have occurred in the present study because customers who repeatedly purchased drinks could have learnt to state their milk preference when ordering drinks while one-time customers may not have. Alternatively, repeat customers may be more inclined to follow the default nudge because they perceived consuming oat milk as being socially acceptable. Therefore, the intervention may have been either more or less impactful on repeat purchasers compared to those who purchased a drink once. If true, treating drink purchases as independent datapoints may have caused an underestimation or overestimation of the true effect size. Furthermore, individual-level data would have allowed the exploration of carryover effects (Sparkman et al., 2020).
Despite these considerations, this study shows the positive impact of default nudges on encouraging sustainable food consumption and the value that implementing them in policy would have on increasing plant-based diets (Reisch & Sunstein, 2021). To further maximise the effect of default nudges, consideration needs to be made on how to support the continuation of sustainable food consumption after exposure to the default nudge has ended, since their promotion of long-term behaviour change can be restricted (Sunstein, 2017). Discussions on the continuing impact of nudges are occurring in the context of behavioural public policy (Sanders et al., 2018) but further research is needed to establish best methods of ensuring they support repeated performance of sustainable behaviour in the future. By taking this long-term perspective, the utility of implementing default nudges widely throughout the food sector will increase, helping encourage the long-term adoption of sustainable diets in western society.
Conclusion
This study examined the impact of a default nudge intervention on sustainable plant-based milk consumption in a UK university café. We found that customers were approximately three times more likely to consume plant-based milk when oat milk was the default milk option instead of dairy milk and that the mean milk-based carbon footprint per drink reduced by 25%–34%. These findings suggest that default nudges can help encourage plant-based diets among consumers by reducing dairy consumption and enhancing sustainable plant-based milk consumption. However, the effect of this intervention decreased slightly as the study progressed, emphasising the need for future research to examine the long-term impact of default nudges on sustainable food consumption. These findings highlight the potential impact of default nudges on reducing the environmental impact of animal agriculture by encouraging plant-based diets among consumers and provide implications for the adoption of plant-based milk default nudges in university cafés and more widely across the food sector.