Associate Professor of Economics, Tufts University
Abstract: Quality-adjusted population density (QAPD) is population divided by land area that has been adjusted for geographic characteristics. We derive weights on these geographic characteristics from a global regression of population density at the quarter-degree level with country fixed effects. We show, first, that while income per capita is uncorrelated with conventionally measured population density across countries, there is a strong negative correlation between income per capita and QAPD; second, that the magnitude of this relationship exceeds the plausible structural effect of density on income, suggesting a negative correlation between QAPD and productivity or factor accumulation; and third, that higher QAPD in poor countries is primarily due to population growth since 1820. We argue that these facts are best understood as results of the differential timings of economic takeoff and demographic transition across countries, and particularly the rapid transfer of health technologies from early to late developers.
Abstract: We develop a methodology to estimate robust city-level vehicular speed indices, exactly decomposable into uncongested speed and congestion. We apply it to 180 Indian cities using 57 million simulated trips measured by a web mapping service. We verify the reliability of our simulated trips using a number of alternative data sources, including data on actual trips. We find wide variation in speed across cities that is driven more by differences in uncongested speed than congestion. Denser and more populated cities are slower, only in part because of congestion. Urban economic development is correlated with faster speed despite worse congestion.
The Average and Heterogeneous Effects of Transportation Investments: Evidence from sub-Saharan Africa 1960-2010 (2022; with Rémi Jedwab), Journal of the European Economic Association 20(1):1-38 (BibTeX)
Abstract: Previous work on transportation investments has focused on average impacts in high- and middle-income countries. We estimate average and heterogeneous effects in a poor continent, Africa, using roads and cities data spanning 50 years in 39 countries. Using changes in market access due to distant road construction as a source of exogenous variation, we estimate a 30-year elasticity of city population with respect to market access of about 0.08-0.13. Our results suggest that this elasticity is stronger for small and remote cities, and weaker in politically favored and agriculturally suitable areas. Access to foreign cities besides international ports matters little. Additional evidence points suggestively to rural-urban migration as the primary source of this population increase, though we cannot fully rule out natural increase or reallocation across cities.
The Global Spatial Distribution of Economic Activity: Nature, History, and the Role of Trade (2018; with Vernon Henderson, Tim Squires and David N. Weil) Quarterly Journal of Economics 133(1): 357-406 (BibTeX; ungated version)
Abstract: We explore the role of natural characteristics in determining the worldwide spatial distribution of economic activity, as proxied by lights at night, observed across 240,000 grid cells. A parsimonious set of 24 physical geography attributes explains 47% of worldwide variation and 35% of within-country variation in lights. We divide geographic characteristics into two groups, those primarily important for agriculture and those primarily important for trade, and confront a puzzle. In examining within-country variation in lights, among countries that developed early, agricultural variables incrementally explain over 6 times as much variation in lights as do trade variables, while among late developing countries the ratio is only about 1.5, even though the latter group is far more dependent on agriculture. Correspondingly, the marginal effects of agricultural variables as a group on lights are larger in absolute value, and those for trade smaller, for early developers than for late developers. We show that this apparent puzzle is explained by persistence and the differential timing of technological shocks in the two sets of countries. For early developers, structural transformation due to rising agricultural productivity began when transport costs were still high, so cities were localized in agricultural regions. When transport costs fell, these agglomerations persisted. In late-developing countries, transport costs fell before structural transformation. To exploit urban scale economies, manufacturing agglomerated in relatively few, often coastal, locations. Consistent with this explanation, countries that developed earlier are more spatially equal in their distribution of education and economic activity than late developers.
Has climate change driven urbanization in Africa? (2017; with Vernon Henderson and Uwe Deichmann; formerly "50 years of urbanization in Africa: Examining the role of climate") Journal of Development Economics 124: 60-82. (BibTeX; ungated version)
Abstract: This paper documents strong but differentiated links between climate and urbanization in large panels of districts and cities in Sub-Saharan Africa, which has dried substantially in the past fifty years. The key dimension of heterogeneity is whether cities are likely to have manufacturing for export outside their regions, as opposed to being exclusively market towns providing local services to agricultural hinterlands. In regions where cities are likely to be manufacturing centers (25% of our sample), drier conditions increase urbanization and total urban incomes. There, urban migration provides an "escape" from negative agricultural moisture shocks. However, in the remaining market towns (75% of our sample), cities just service agriculture. Reduced farm incomes from negative shocks reduce demand for urban services and derived demand for urban labor. There, drying has little impact on urbanization or total urban incomes. Lack of structural transformation in Africa inhibits a better response to climate change.
The View from Above: Applications of Satellite Data in Economics (2016; with Dave Donaldson) Journal of Economic Perspectives 30(4): 171-198. (BibTeX)
Abstract: The past decade or so has seen a dramatic change in the way that economists can learn by watching our planet from above. A revolution has taken place in remote sensing and allied fields such as computer science, engineering, and geography. Petabytes of satellite imagery have become publicly accessible at increasing resolution, many algorithms for extracting meaningful social science information from these images are now routine, and modern cloud-based processing power allows these algorithms to be run at global scale. This paper seeks to introduce economists to the science of remotely sensed data, and to give a flavor of how this new source of data has been used by economists so far and what might be done in the future.
Farther on down the road: transport costs, trade and urban growth (2016) Review of Economic Studies 83(3): 1263-1295. (BibTeX; ungated version)
Abstract: How does isolation affect the economic activity of cities? Transport costs are widely considered an important barrier to local economic activity but their impact in developing countries is not well-studied. This paper investigates the role of inter-city transport costs in determining the income of sub-Saharan African cities. In particular, focusing on fifteen countries whose largest city is a port, I ask how important access to that city is for the income of hinterland cities. The lack of panel data on both local economic activity and transport costs has prevented rigorous empirical investigation of this question. I fill this gap with two new datasets. Satellite data on lights at night proxy for city economic activity, and new road network data allow me to calculate the shortest route between cities. Cost per unit distance is identified by plausibly exogenous world oil prices. The results show that an oil price increase of the magnitude experienced between 2002 and 2008 induces the income of cities near a major port to increase by 6.6 percent relative to otherwise identical cities one standard deviation farther away. Combined with external estimates, this implies an elasticity of city economic activity with respect to transport costs of -0.25 at that distance. Moreover, the effect differs by the surface of roads between cities. Cities connected to the port by paved roads are chiefly affected by transport costs to the port, while cities connected to the port by unpaved roads are more affected by connections to secondary centers.
Dowry Deaths: response to weather variability in India (2014; with Sheetal Sekhri) Journal of Development Economics 111: 212-223. (BibTeX; ungated version)
Abstract: We examine the effect of rainfall shocks on dowry deaths using data from 583 Indian districts for 2002-2007. We find that a one standard deviation decline in annual rainfall from the local mean increases reported dowry deaths by 7.8 percent. Wet shocks have no apparent effect. We examine patterns of other crimes to investigate if increase in general unrest during economic downturn explains the results but do not find supportive evidence. Women's political representation in the national parliament has no apparent mitigating effect on dowry deaths.
Abstract: We develop a statistical framework to use satellite data on night lights to augment official income growth measures. For countries with poor national income accounts, the optimal estimate of growth is a composite with roughly equal weights on conventionally measured growth and growth predicted from lights. Our estimates differ from official data by up to three percentage points annually. Using lights, empirical analyses of growth need no longer use countries as the unit of analysis; we can measure growth for sub- and supra-national regions.
Print: The New York Times, The Economist, New Scientist, The American, Seed Magazine, Frankfurter Allgemeine Sonntagszeitung [Germany], Les Echos [France], Tages-Anzeiger [Switzerland], Clarin iEco [Argentina], The Economic Times [India]
Abstract: Emerging infectious diseases (EIDs) are a significant burden on global economies and public health. Their emergence is thought to be driven largely by socio-economic, environmental and ecological factors, but no comparative study has explicitly analysed these linkages to understand global temporal and spatial patterns of EIDs. Here we analyse a database of 335 EID 'events' (origins of EIDs) between 1940 and 2004, and demonstrate non-random global patterns. EID events have risen significantly over time after controlling for reporting bias, with their peak incidence (in the 1980s) concomitant with the HIV pandemic. EID events are dominated by zoonoses (60.3% of EIDs): the majority of these (71.8%) originate in wildlife (for example, severe acute respiratory virus, Ebola virus), and are increasing significantly over time. We find that 54.3% of EID events are caused by bacteria or rickettsia, reflecting a large number of drug-resistant microbes in our database. Our results confirm that EID origins are significantly correlated with socio-economic, environmental and ecological factors, and provide a basis for identifying regions where new EIDs are most likely to originate (emerging disease 'hotspots'). They also reveal a substantial risk of wildlife zoonotic and vector-borne EIDs originating at lower latitudes where reporting effort is low. We conclude that global resources to counter disease emergence are poorly allocated, with the majority of the scientific and surveillance effort focused on countries from where the next important EID is least likely to originate.
Supplementary Information (ungated)
Broadcast: National Public Radio
SELECTED WORK IN PROGRESS
The fast, the slow, and the congested: Urban transportation in rich and poor countries (with Prottoy Akbar, Victor Couture, and Gilles Duranton)
Climate Change, Population Growth, and Population Pressure (with Vernon Henderson, Bo Yeon Jang, and David N. Weil)
On the last frontier of the demographic transition: urbanization and fertility change in sub-Saharan Africa (with Vernon Henderson and Vivian Liu)
Is urbanization in sub-Saharan Africa different? (with Vernon Henderson and Mark Roberts)