Empirical research on cities starts with a spatial equilibrium condition: workers and firms are assumed to be indifferent across space. This condition implies that research on cities is different from research on countries, and that work on places within countries needs to consider population, income, and housing prices simultaneously. Housing supply elasticity will determine whether urban success reveals itself in the form of more people or higher incomes. Urban economists generally accept the existence of agglomeration economies, which exist when productivity rises with density, but estimating the magnitude of those economies is difficult. Some manufacturing firms cluster to reduce the costs of moving goods, but this force no longer appears to be important in driving urban success. Instead, modern cities are far more dependent on the role that density can play in speeding the flow of ideas. Finally, urban economics has some insights to offer related topics such as growth theory, national income accounts, public economics, and housing prices.

We study how a market uses temporary workers to accommodate extraordinary demand shocks. When COVID-19 surges, hospitals need additional nurses—especially in specialties central to COVID-19 care. By comparing markets for COVID-relevant and other specialties, we show that the market for travel nurses expands dramatically and estimate travel nurse labor supply across space. Supply is quite elastic, as workers can choose to travel where they are needed. Workers travel longer distances to temporary jobs when payment increases, suggesting that an integrated national market facilitates reallocation when demand spikes. But when national cases peak, travel distance is less responsive to local demand.

Popular discussions often treat the great housing boom of the 1996-2006 period as if it were a national phenomenon with similar impacts across locales, but across metropolitan areas, price growth was dramatically higher in warmer, less educated cities with less initial density and higher initial housing values. Within metropolitan areas, price growth was faster in neighborhoods closer to the city center. The centralization of price growth during the boom was particularly dramatic in those metropolitan areas where income is higher away from the city center. We consider a number of different explanations for this connection, and find that the connection between centralized price growth and decentralized income seems to be most explained by the faster price growth in central cities that use relatively more public transit.

Is government guiding the invisible hand at the top of the labor market? We use new administrative data to measure physicians’ earnings and estimate the influence of health care policies on these earnings, physicians’ labor supply, and the allocation of talent. Combining the administrative registry of U.S. physicians with tax data, Medicare billing records, and survey responses, we find that physicians’ annual earnings average $350,000 and make up 8.6% of national health care spending. Business income makes up one-quarter of earnings and is systematically underreported in survey data. Earnings increase steeply early in the career, and there are major differences across specialties, regions, and firm sizes. The geographic pattern of earnings is unusual compared with other workers. We argue that these patterns reflect policy choices to subsidize demand for physician care, amplified by restrictions on physician entry, especially in certain specialties. Health policy has a major impact on the margin: 25% of physician fee revenue driven by Medicare reimbursements accrues to physicians personally. Physicians earn 8% of public money spent on insurance expansion. These policies in turn affect the type and quantity of medical care physicians supply, retirement timing, and the allocation of talent across specialties.