Officials in Britain’s tax system had a problem: Lots of citizens weren’t paying on time. For years Her Majesty’s Revenue and Customs had sent letters to the late payers, using traditional threats of interest charges, late fees, and legal action to try to get people to mail in their checks. Some did—but many didn’t. So in a 2009 pilot study (for which I served as a consultant), HMRC tried a different approach: It changed the language in its dunning letters, drawing on psychological techniques to increase the odds that delinquent taxpayers would pay up.