Continuous vs. Discrete Urban Ranks: Explaining the Evolution in the Italian Urban Hierarchy over Five Decades

The reasons for changes in ranking within urban systems are a matter of a wide and long debate. Some focus on a continuous and smooth ordering of cities by their size within the urban system, in the tradition of Zipf’s law. Others focus on discrete, discontinuous ordering, as cities take on functions at different levels, such as specialized market places or high-level education, in the tradition of Christaller. We enter the debate by empirically evaluating whether the same determinants explain continuous or discrete changes in urban ranks in the evolution of the Italian urban hierarchy over the years 1971 to 2011. We empirically show that small, continuous changes of cities’ ranks have different drivers than large, discontinuous leaps. The presence of high-level functions in a city predicts major leaps across discrete ranks. Results are robust to the use of an instrumental variable strategy based on a shift–share argument.