Confronting the Mystery of Urban Hierarchy

The size distribution of cities in the United States is startlingly well described by a simpler power law: the number of cities whose population exceedsSis proportional to 1/S. This simple regularity is puzzling; even more puzzling is the fact that it has apparently remained true for at least the past century. Standard models of urban systems offer no explanation of the power law. A random growth model proposed by Herbert Simon 40 years ago is the best try to date—but while it can explain a power law, it cannot reproduce one with the right exponent. At this point we are in the frustrating postion of having a striking empirical regularity with no good theory to account for it.J. Japan. Int. Econ., December 1996,10(4), pp. 399–418. Stanford University, Stanford, California 94305-6072