not adjust (more) in response to this rapid urban natural increase and higher congestion? Finally why do urban fertility rates not come down faster? To shed some light on the first question, the link between the speed of urban growth and urban congestion is explored using a novel cross-sectional data set of urban congestion measures for 95 developing countries (1990-2010). Higher urban growth due to natural increase is associated with more congested cities today. Interestingly, the corresponding effects of migration on urban congestion tend to be much smaller. We then discuss various potential explanations for why rural migration may continue to, and fertility may remain high in, these congested urban areas.

The paper adds to the literature on urbanization and growth. The role of urban natural increase has been recognized by demographers (Rogers, 1978; Preston, 1979; Keyfitz, 1980; Fox, 2012), but is little discussed in economics (see surveys by Duranton (2008, 2013), Henderson (2010) and Desmet & Henderson (2014)).<sup>3</sup> First, urban natural increase creates a disconnect between urbanization and growth if urban areas expand without an increase in living standards.<sup>4</sup> Second, while there is an extensive literature measuring agglomeration effects in developing countries, little is known about the magnitude of congestion effects.<sup>5</sup> The speed of urban growth is an understudied dimension of the urbanization process. Third, whether urban growth is mainly driven by migration or natural increase has implications for policy making. When urban congestion is the result of excessive migration, investment in urban infrastructure may not be justified if it further fuels migration (see Feler & Henderson (2011) for a discussion of urban policies in Brazil). However, if urban growth is due to urban natural increase, the resulting immediate increase in the urban population may necessitate such investment, as well as stronger urban family planning policies.

The paper is organized as follows: Section 2 presents the data and the decompositions results. Sections 3 shows the econometric results. Section 4 discusses the results. Section 5 concludes.

> 3 In a more recent paper, Jedwab & Vollrath (2015 _a_ ) use a theoretical model to study the “Malthusian” effects of the epidemiological transition on the rise of poor mega-cities. They focus their analysis on the absolute population growth of the largest cities in the world, hence _mega-city growth_ , while here we look at both the absolute and relative growth of the total urban population, hence _urban growth_ and _urbanization rates_ .

> 4In countries where urban growth comes from migration, it is likely that urban wages are rising, which given low rural wages attract residents to the cities. Eventually, as the urban-rural income gap closes, rural residents cease migrating to the cities, and the urbanization rate and income stabilize at a higher level. In countries where urban growth comes from urban natural increase, urbanization occurs because poor urban families have high fertility rates. There is also migration if the countryside gets too congested due to rural natural increase. Eventually, there is no urban-rural gap, but the country is more urbanized, although income did not increase.

> 5Likewise, there are few papers about the role of slums in developing countries. Notable exceptions are Lall, Lundberg & Shalizi (2008), Brueckner & Selod (2009), Brueckner (2013) and Cavalcanti & Da Mata (2013). 3