Changing the position of items on the menu of fast food restaurants can "trick" consumers into choosing healthier options, a trial in McDonald's has found.
Customers using touch-screen order kiosks were persuaded to choose less sugary options when researchers changed the order of the list of soft drinks.
The icon for Coke Zero, which contains no sugar, was moved to the first spot on the top left of the touch screens in 622 stores for 12 weeks in 2016.
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Coca-Cola was moved down the list to the lowest location on the screens.
Researchers from universities in Manchester and the University of Warwick found that sales of Coke Zero increased and sales of Coca-Cola decreased - both by more than 300 per store.
Purchases of Coca-Cola fell by 34 per store (9%) on average in the week following the change, while sales of Coke Zero increased by 19 (21%).
Sales of Coca-Cola fell by 345 per store (7%) on average in the 12 weeks after the change, while sales of Coke Zero increased by 317 (30%).
Customers expecting Coca-Cola to be in a certain place were prompted to consider the sugar-free alternative when the two were swapped.
The authors believe that, by disrupting the expectation of the consumer, they were given a chance to reconsider "their otherwise habitual menu choice".
The swap was effective regardless of how deprived the area the restaurant was in, the study, published in the Psychology & Marketing journal, found.
Dr Ivo Vlaev, a behavioural scientist from Warwick Business School who co-authored the study, said he found it "shocking" that such a significant change had been observed, given that all of the options were still available to consumers.
He told PA: "You can call it a cognitive trick. It's based on a bias, or a psychological blind spot we have when we are looking at the range of options in front of us or the world outside of us, because we focus our attention on things that are more salient, or are immediately coming in front of us - our eyes follow certain patterns when we are looking at the world, as when you're reading a page in a magazine."
The researchers initially worked with McDonald's, which funded the study, to make Coke Zero the "default" choice when ordering a meal.
But they were not able to do this because some people are allergic to the sweeteners used to replace sugar.
They also tried to get staff to prompt customers to choose healthier options but found that they did not always have time or remember, Dr Vlaev said.
He said that other retailers should take note of the "promising results", amid policy interventions such as the sugar tax and rules around fast food outlets by schools.
The study shows that changes can be made without affecting businesses' profits, and shows how industry can be a conduit for public health messages, he said.
He continued: "You can make a big difference by small changes at the right time and place.
"It doesn't need to be complex, you can replicate what we did with McDonald's.
"Menus in general are usually the medium that retailers communicate with customers, and we can make small changes in this medium such as reordering or making the healthy option more salient, and then make a big difference in people's lives, health-wise.
"It's a win-win if you do it the right way, and behavioural science could be the tool to find such a win-win situation."