CONFRONTING THE MYSTERY OF URBAN HIERARCHY

The size distribution of cities in the United States is startlingly we ll described by a simpler power law: the number of cities whose popula tion exceeds S is proportional to 1/S. This simple regularity is puzzl ing; even more puzzling is the fact that it has apparently remained tr ue for at least the past century. Standard models of urban systems off er 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 ex plain a power law, it cannot reproduce one with the right exponent. At this point we are in the frustrating position of having a striking em pirical regularity with no good theory to account for it. (C) 1996 Aca demic Press, Inc.