In 2014, the American footballer Marshawn Lynch - a former NFL running back nicknamed 'Beast Mode' because he bulldozed and ran over would-be tacklers - signed an endorsement deal with Skittles. This was more than business. To Lynch, Skittles aren't just Skittles. Since he was young, the button-shaped candies have been his secret weapon.

As a rising football star in high school, Lynch was often struck by anxiety in advance of his games. It was often so extreme it caused an intense upset stomach. Young Lynch tried several over-the-counter remedies, but nothing seemed to work. Then one day, his mother, Delisa Lynch, told him that Skittles would settle his stomach. Not only that, but she said the Skittles would also make him play better: 'They're going to make you run fast, and they're going to make you play good.' And, somehow, they did.

No offence to Skittles lovers, but there's nothing special about them. They're mostly sugar, corn syrup and artificial flavours. Yet, throughout his college football and illustrious NFL career, Lynch held on to the belief that Skittles helped his game, and he always ate them before taking the field. You might assume that the Skittles were, for him, just a silly pre-game ritual. But by eating the Skittles and believing that they helped improve his performance, Lynch was taking advantage of a very real phenomenon: the placebo effect. [...]

Lynch's experience reflects an emerging research trend to study the possible beneficial effects of placebos given without deception, also known as 'open-label placebos' or 'non-deceptive placebos'. In a foundational study in 2010, researchers at Harvard Medical School randomised patients experiencing irritable bowel syndrome (IBS) symptoms into either an open-label placebo group or a no-treatment control group - and crucially, all the patients knew which group they were in. The researchers told patients in the open-label placebo group that the placebo effect is powerful, that the body can respond automatically to taking placebo pills (similar to the classic conditioning example of Pavlov's dogs, who salivated at the sound of the dinner bell), that a positive attitude helps but is not required, and that it is vital to take the pills faithfully for the entire 21-day study period, regardless of their belief in the pills. By the end of the study, even though the placebo pills contained no active ingredients, and despite the patients knowing they'd been taking placebos, they reported fewer IBS symptoms and more improvement in overall quality of life than patients in the no-treatment control group.

This paradigm of giving non-deceptive placebo pills as treatment has been repeated, including a recent replication of the benefit for IBS, while other trials have shown benefits for patients with ADHD and hay fever. Unsurprisingly, further research suggests that open-label placebos can also work in non-clinical settings. Together with colleagues, one of us (Darwin) showed in 2020 that an open-label placebo nasal spray reduced the distress provoked by looking at emotionally upsetting images. Like Lynch's Skittles, the open-label placebo we used helped our volunteers manage their feelings and anxiety, an effect that was even visible in their electrical brain activity.