The research, led by the University of Bristol, found that providing a traffic light rating of eco-friendliness next to dishes on the menu significantly increased the likelihood of diners choosing more sustainable options.
Lead author Katie De-loyde, Research Associate in Psychological Science, said: "Adding a traffic light eco-label to menus increased the selection of more sustainable food items. Furthermore, and somewhat surprisingly, participants were positive about the eco-label, with a huge 90% of participants supporting the idea.
"Pending replication in real-world settings, our results suggest future policy could include mandatory eco-labelling, just like the health traffic light system, on food products as a way to promote more sustainable diets."
The researchers asked participants whether they would order a burrito with a beef, chicken, or vegetarian filling. Each burrito was accompanied with a traffic light-style ranking of sustainability, with the vegetarian option being green indicating it was the most sustainable.
Findings showed five per cent more of the 1,399 adult participants went veggie when the eco-labels were included, while 17 per cent more went for vegetarian or chicken, the second most sustainable option.
Katie added: "The eco-label was particularly effective among those people who reported already being motivated to act sustainably. This suggests these kinds of labels help people make dietary decisions which are in line with their personal values."
Eating meat and dairy products can have various adverse effects on the environment, largely owing to the huge amounts of methane, a harmful greenhouse gas, cows, pigs, and other farm animals release into the atmosphere.
More trees are cut down to convert land for crop growing, as around a third of all grain produced in the world is used to feed animals raised for human consumption. Overall, studies have shown that going vegetarian can reduce your carbon emissions from food by half, and going vegan can reduce this further.
The researchers wanted to discover if increasing an awareness of the impacts of different dishes would influence consumers to choose a more sustainable option, supporting the social ideal.
Three mock-ups of food delivery app menus were created, each showing the three burrito options with different accompanying information.
All menus contained a photo of each item as well as the calorie content, a Fairtrade logo, a spice indicator and the price, which was the same for all options.
But one mock-up also featured a 'social nudge' – an indicator encouraging people to act according to the most sustainable option. This resembled a gold star, including the words 'Most Popular' placed alongside the vegetarian burrito.
In another mock-up, each burrito was given the eco-label, with the beef option scoring '5' in red, highlighting it is unsustainable. The chicken option had a yellow '3', indicating it was neither sustainable nor unsustainable, and the vegetarian option got a green '1' for sustainable.
Participants were randomly shown one of the three menu mock-ups and asked to pick a burrito option, as if they were normally ordering food. They were also asked follow-up questions designed to measure their level of motivation to act sustainably.
The results, published in the journal Behavioural Public Policy, found a third of the participants who were given the 'control' menu – without a social nudge or eco-label – went for the beef burrito. However, this dropped to 29 per cent for those who had the social nudge menu, and to 16 per cent for those who had the eco-labelled menu.
Findings showed only nine per cent would order a vegetarian burrito if they were given the control menu, but this increased to 13 per cent with the social nudge and 14 per cent with the eco-label.
The University of Bristol was the first UK university to declare a climate emergency and the first to work towards Climate Action Plans (CAP) for all its schools.
This academic year, catered halls for students introduced eco-labels on menus, empowering students to make more informed and sustainable decisions on what they eat.
Students are also helping with a trial by the university's School of Psychological Science to monitor which sandwiches, each featuring details of its carbon footprint, sell best.
Co-author Dr Olivia Maynard, from the University of Bristol, said: "In 2020 the UK's Committee on Climate Change advocated that beef consumption must be considerably reduced if the UK is to reach its net-zero greenhouse-gas emission target by 2050. Although further research on eco-labelling is essential, future policy may wish to consider a mandatory eco-label to help meet global climate change targets."