In theory it works perfectly. Rather than oblige parents to send their children to the nearest state-run or –funded school, give them a voucher to be spent at a private school of their choice. “The adoption of such arrangements”, argued Milton Friedman in 1955, “would make for more effective competition among various types of schools and for a more efficient utilisation of their resources.” As part of its recovery from Hurricane Katrina, which destroyed many schools in New Orleans, Louisiana undertook one of America’s largest school-choice schemes. According to a new paper by Atila Abdulkadiroglu of Duke University, Parag Pathak of MIT and Christopher Walters of Berkeley, it has not gone well.