Sentence:  Manipulating behavior is old hat in the private sector, where advertisers and companies have been nudging consumers for decades. Just think of strategically placed chocolate bars at the checkout counter. But in public policy, <WSD>nudge</WSD> proponents study human behavior to try to figure out why people sometimes make choices that they themselves would consider poor. Then they test small changes in how those choices are presented, to see whether people can be steered toward better decisions — like putting apples, not chocolate bars, at eye level in school cafeterias. ... It is tricky to run perfectly controlled experiments in real-life situations, but proving the worth of <WSD>nudges</WSD> is a central principle of the program, Mr. Halpern said. 
Meaning of the word marked with <WSD> tags:  The use of positive reinforcement and indirect suggestions as ways to influence.