process. This paper takes a historically comparative and demographic oriented approach to understanding some key features of urbanization in today’s developing world. We show that in many cities, mortality has fallen to low levels, due to the epidemiological transition of the 20th century, while fertility has remained high. The resulting high rate of natural increase in urban areas compounds the effects of migration on urban growth. Today’s “mushroom cities” are in contrast to the “killer cities” of Industrial Europe, where high urban mortality rates offset the (lower) urban birth rate, resulting in much lower urban natural increase. The concept of “urban push” (rather than “rural push” or “urban pull”) is used to describe this demographic mechanism of urbanization and its implications in a three step analysis.<sup>1</sup> In particular, we carefully document and decompose urban growth in industrializing Europe and developing countries. To do so, we created an extensive new data set on the crude rates of birth and death for urban and rural areas of 7 European (or Neo-European) countries in the 19th century (every forty years in 1800-1910) and 35 countries that were still developing countries in 1960 (every ten years in 1960-2010).<sup>2</sup> We show that the fast _absolute_ growth of cities in today’s developing world was also driven by urban natural increase, rather than solely by migration as in Europe. The difference in urban rates of natural increase then drove the urban population in today’s developing world to double every 18 years, compared with 35 years in Europe. Simulations further suggest that urban natural increase also accelerated the change in urbanization rates, i.e. the _relative_ growth of cities.

By way of extension, we explore whether the contributions of urban natural increase to urban growth and urbanization hold beyond an accounting sense using multivariate regressions. Decadal country level rates of urban growth and urbanization are regressed on urban natural increase, with controls for income growth, rural push and urban pull factors, and regional characteristics for the panel of 35 developing countries (1960-2010). Urban natural increase remains strongly associated with urban growth and urbanization. We then study whether these short-run decadal effects also hold in the longer run (a few decades later). Ruralto-urban migration could eventually slow down in response to urban natural increase or urban residents may adjust their fertility behavior. However, the observed long-run effects are not significantly different from the short-run effects. These results suggest that urban natural increase has compounded the effects of migration, accelerating urban growth and urbanization in the developing world vis-à-vis what was observed in Industrial Europe.

This finding also raises a number of additional questions, which the paper briefly reflects upon. First, do the speeds of urban growth and urbanization matter? Second, why do migration rates

> 1While the former two concepts imply that rural workers are pushed to (or pulled to) cities by changes in rural (urban) conditions, the latter suggests that cities growing internally are pushing their own boundaries. 2Economic historians have focused individually either on England or the U.S. in the 19th century (Williamson, 1990; Haines, 2008). Our vastly expanded dataset thus allows us to significantly extend their work.

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