A good nudge is like a GPS device: A small, low-cost intervention that tells you how to get where you want to go — and if you don't like what it says, you're free to ignore it. But when, exactly, will people do that? A new study sheds important light on that question, by showing the clear limits of nudging. Improbably, this research is also good news: It shows that when people feel strongly, it's not easy to influence them to make choices that they won't like.

The focus of this new research, as with much recent work on behavioral science, is on what people eat. Numerous studies suggest that if healthy foods are made more visible or convenient to find, more people will choose them. We tend to make purchasing decisions quickly and automatically; if certain foods or drinks — chocolate bars, apples, orange juice — are easy to see and grab, consumption will jump.

At a University of Pennsylvania salad bar, for example, researchers found that moving high-calorie foods like cheese just 25 cm farther away reduced intake of those foods as much as 16 percent. In another study, at the cafeteria in Boston's Massachusetts General Hospital, placing salads next to the pizza counter and putting green items at eye level nudged people to buy healthier food.