NUDGES are born of good intentions and clever ideas. Alas, that’s not enough. I once proposed a nudge to promote online civility. I suggested that magazines and newspapers should moderate comments using a variation of ToneCheck, an “emotional spell-checker” for email that prompts users to tone down angry messages. Richard Thaler, one of the chief architects of nudge, loved it, tweeting: “A Nudge dream come true.” But my students saw a problem: legitimate opinions getting censored or watered down. The lesson I learned is that nudge designers must always consider the possibility of unintended consequences. In fact, that is only one of many concerns about nudging. As I found, creating effective nudges is difficult. Thaler and Cass Sunstein’s influential book Nudge creates the impression that nearly anyone can do it. All you need is a basic understanding of how the mind works and a flair for creative design. This is highly misleading. Nudges are usually designed on the assumption that everyone has the same cognitive biases. But this is only valid in some situations. Consider a proposal to increase organ donations in the US by making it a “mandated choice” – everybody applying for a driver’s licence or vehicle registration has to either opt in or opt out. This sounds reasonable, but it overlooks negative perceptions many Americans have of the Department of Motor Vehicles (DMV), which may exert a profound influence on decision-making. After waiting in line for hours, fuming over DMV inefficiencies, you may be less inclined towards altruism.