The Urban Century: Understanding the Magnetism of Cities

For the first time in history, most of humanity lives in urban areas, a trend that continues to accelerate. The Institute of Experimental Demography's urbanization research program seeks to understand the causes, patterns, and consequences of this great demographic shift. Why do people cluster in cities? What are the effects of extreme density on health, social networks, and fertility? How do urban planning policies shape demographic outcomes? We tackle these questions using spatial econometrics, natural experiments arising from urban policy changes, and novel data from sensors and satellite imagery. Our focus is not just on describing urban growth but on identifying the causal mechanisms that make cities engines of innovation, inequality, and demographic change.

Causal Analysis of Urban Drivers and Outcomes

A central line of inquiry examines place-based policies and their demographic effects. We treat the establishment of special economic zones, new transit lines, or large-scale housing projects as natural experiments. By comparing treated areas to carefully selected control areas before and after the intervention, we estimate the causal impact on population inflows, household composition, and neighborhood socioeconomic sorting. For example, a study on a new subway line in a major city found it not only increased residential density but also altered the age profile of neighborhoods, attracting younger, childless adults.

We also study environmental stressors in dense settings. Using quasi-experimental designs, we analyze how changes in local pollution levels (due to a factory closure or a vehicle restriction zone) affect infant mortality, respiratory disease rates, and even cognitive development in children. High-frequency mobility data allows us to measure actual exposure rather than just residential location, providing a more precise picture of the health burden of urban living.

Another key area is social interaction and fertility in cities. Urban areas are often characterized by lower fertility. Is this due to higher costs, changed preferences, or the diffusion of new norms through dense social networks? We use survey experiments and network analysis within cities to disentangle these factors. We also study how urban design—such as the availability of parks and communal spaces—influences social cohesion and mental health, which in turn may affect family formation decisions.

• Housing Shock Studies: Exploit sudden changes in housing supply (e.g., large-scale public housing construction) or demand (e.g., foreign investment regulations) to study effects on household formation, internal migration, and segregation.
• Urban Heat Island Mitigation Experiment: Partners with a city to randomly assign tree-planting and cool-roof interventions across neighborhoods and measures demographic and health outcomes.
• Digital Divide and Urban Mobility: Uses mobile phone data to map how access to digital services affects physical mobility and access to jobs and services for different demographic groups.
• Informal Settlement Dynamics Project: Tracks the growth and demographic composition of informal settlements using satellite imagery and ground surveys, evaluating the impact of upgrading programs.

Planning for Sustainable and Equitable Urban Futures

The ultimate goal of our urbanization research is to inform the creation of cities that are not only economically vibrant but also demographically sustainable and socially inclusive. We work directly with urban planners and municipal governments, providing evidence-based tools for forecasting neighborhood change, evaluating the equity impacts of proposed developments, and designing public spaces that foster community across age and ethnic groups. Our models help answer critical questions: How will an aging population change housing demand? Can transit-oriented development reduce spatial inequality? What are the consequences of work-from-home trends for urban density? By applying the rigorous causal toolkit of experimental demography to the urban context, we move beyond simplistic narratives of 'good' or 'bad' density. Instead, we provide a nuanced understanding of how specific policies and designs can harness the dynamism of cities while mitigating their downsides, ensuring that the urban future is one of opportunity and well-being for all residents.