In a much-talked-about recent paper entitled “Superstar Cities,” economists Joseph Gyourko, Christopher Mayer, and Todd Sinai argue that such high-status cities – not only London, Paris, and New York, but also cities like Philadelphia and San Diego – may show an “ever-widening gap in housing values” when compared with other cities. The authors seem to be saying, in effect, that a housing boom in these areas can go on forever.