「选题人」

《Journal of Urban Affairs》 最新综述文章The state of urban research: Views across the disciplines 采访了来自不同学科的五位学者🏊🏿‍♂️。他们从城市定义、研究问题和方法、学科研究进展以及和其他领域关系等视角阐述了各自学科学者的重要研究和对城市研究的贡献。本期推送经济学者JefferyLin的论述。

Jeffrey Lin是Federal Reserve Bank of Philadelphia研究部的副主席，是全球顶尖的城市经济学者。其发表见之于 American Economic Review, Journal of Political Economy, Quarterly Journal of Economics, Review of Economic Studies等经济学顶刊🏄🏽‍♀️🤴🏽。

Research questions and methodologies

What determines the uneven geographic distribution of economic activity? This is the core question of urban, regional, and spatial economics. If economics is the study of the allocation of scarce resources, then urban economics is the study of the allocation of land or geographic space. Urban economists understand spatial structure as the balance between agglomeration and dispersion forces. Agglomeration forces encourage the geographic concentration of economic activity. For example, firms may benefit from density by economizing on transportation costs while exploiting increasing returns. Households might cluster near other to take advantage of both exogenous amenities (beaches or views) or endogenous amenities (safety or schools). Dispersion forces encourage economic activity to spread out. For example, competition for the fixed supply of land increases its price, encouraging firms and households to seek out other locations.

There is broad agreement among leading economists about the “core” of urban economics. Glaeser (2008) sums up many of these themes: What are the costs and benefits of density? In other words, what are the important agglomeration and dispersion forces at work in cities? (See also surveys by Brueckner, 2011; Duranton & Puga, 2004, 2015; Proost & Thisse, 2019) At a regional scale, questions range from the basic (Why do cities exist? Why do lasting and sizable regional disparities exist?) to the applied (Why have we seen growing divergence between U.S. cities since 1980? What kinds of regional development policies work?). At a neighborhood scale, questions also range from the basic (Why do land and property prices vary within cities? What explains sorting and segregation patterns?) to the applied (Does new transportation infrastructure reduce inequality? Does new development lead to displacement? What are the long-run effects of growing up in a poor neighborhood? What kinds of neighborhood characteristics do people value?).

The economic approach to cities starts with two important foundations. One, people respond to incentives. Two, things add up. In other words, “$20 dollar bills don’t lie in plain view very long, and every sale is also purchase” (P. R. Krugman, 1997, p. 75). Economists formalize these foundations in mathematical models. In that context, peoples’ response to incentives is an optimization problem: somebody is trying to get as much as they can of something. “Things adding up” is a resource constraint. For example, the amount spent on housing must be the same as the amount of income from housing. The goal of this kind of modeling is to clarify our thinking about how decentralized decision-making by firms or households can lead to organized outcomes like markets—or even cities! A second goal is to make clear, precise assumptions and connect them to results are internally consistent. If we are confident in these models, they might even be used to quantify the effects of policies or the importance of specific mechanisms. And if we are skeptical about the precision of these models, it is a good idea to keep the basic foundations in mind, “for two opposing reasons—to remind yourself not to take any particular mathematical formalization too seriously, but also to remind yourself that the basic principles of mainstream economics are not at all silly or unreasonable” (P. R. Krugman, 1997, p. 75).

A third important foundation in urban economics is spatial equilibrium (e.g., Roback, 1982). In spatial equilibrium, welfare is equalized across space (at least for marginal migrants). Intuitively, if this were not true—if welfare were higher in some places compared with others—then more households would move to higher-welfare places, bidding up the price of land and housing. The intensifying dispersion force of higher land and housing prices would then tend to equilibrate welfare levels across space.

To see how these ingredients come together in urban economics, consider the urban land use models developed by Alonso (1964), Mills (1967), and Muth (1969).3 The goal is to explain the spatial structure of cities. Households must commute to the city center. (This is the agglomerating force.) They choose a location to commute from and an amount of housing to maximize utility (optimizing behavior). In spatial equilibrium, identical households obtain the same level of utility everywhere in the city (otherwise, they would respond to the incentive to move). With these few ingredients, the model predicts that housing prices fall, and housing consumption increases, as commuting costs increase (land scarcity at the city center is the dispersion force.) The model thus explains parsimoniously many features of real-world cities: increasing housing and population densities, increasing land and housing prices, and declining housing unit size with proximity to the city’s center reflect the trade-off between accessibility and housing consumption. The model can also accommodate extensions; for example, Baum-Snow (2007) uses a version of this model to analyze the effect of Interstate highway construction on the suburbanization of population.

I have focused on the theoretical foundations of urban economics, with less emphasis on empirics. Holmes (2010) classifies three general approaches to empirical work in economics: descriptive, structural, and experimentalist. Descriptive approaches develop new summary statistical relationships (e.g., correlations) to distinguish between alternative theories that yield qualitatively distinct predictions. Thus, descriptive approaches are less interested in precise quantitative estimates; instead, they use gross features of the world to validate (or reject) theoretical models. In contrast, structural and experimentalist approaches both seek to identify quantitative impacts of policy changes. The structural approach aims to estimate “deep” model parameters of preferences and technology within a fully-specified economic model. Then, the estimated model may be used to simulate the effects of policy changes. Holmes and Sieg (2015) and Redding and Rossi-Hansberg (2017) review recent developments in this approach. In the experimentalist approach, the goal is to estimate the causal impact of a policy or specific factor. A common feature of the experimentalist approach is the use of “natural experiments” that help to define appropriate “treatment” and “control” groups, so that the analysis closely resembles an experimental design. Baum-Snow and Ferreira (2015) review recent developments in causal inference in urban economics.

Relationships to other disciplines

How connected is urban economics to other disciplines outside economics? Based on citations, economics in general seems insular. Fourcade, Ollion and Algan (2015) analyze citations in flagship journals of economics, sociology and political science. Articles in the American Political Science Review cite top-25 economics journals more than five times as often as articles in the American Economic Review cite top-25 political science journals. Articles in the American Sociological Review cite top-25 economics journals more than seven times as often as articles in the AER cite top-25 sociology journals. And in an opinion survey, economists were the only group (among sociology, political science, psychology, finance, and history) among whom a substantial majority disagreed or strongly disagreed with the statement “in general, interdisciplinary knowledge is better than knowledge obtained from a single discipline.”

One important exception that is specific to urban economics is the influence of regional science, rooted in Walter Isard’s (1956) work. As Fujita et al. (1999) note, many ideas from regional science have been incorporated into mainstream urban economics, including the central place theory of Christaller (1933) and Losch (1940) and the market potential analysis of C. Harris (1954).

Urban economists pay attention to developments in the other social sciences. The most common way is by inspiration. Lin (2011) studies the occupational dynamics of cities, drawing on Jacobs (1969), and Bleakley and Lin (2012, 2015) study the evolution of portage cities, drawing on work by geographers such as Semple (1903) and Cronon (1991). Recent work on the long-run effects of redlining (Aaronson et al., 2021; Krimmel, 2018) relies on contributions by Jacobs (1961), Jackson (1980), and Rothstein (2017), and the data digitization work of the Mapping Inequality project (Nelson et al., 2020). Collinson et al. (2021) estimate the effect of eviction on poverty using the random assignment of judges to eviction cases in New York and Chicago, with close links to the work of Desmond (2016). Thus, it is not uncommon for urban economists to find inspiration in work from other disciplines. In fact, I would go further and say ideas from other social scientists have been a necessary factor in progress in urban economics by uncovering new facts or by pointing out blind spots.

There are examples of deeper interdisciplinary work. One is a collaboration between Assyriologist Gojko Barjamovic and economists Thomas Chaney, Kerem Coşar, and Ali Hortaçsu (2019). They analyze a large database of commercial records produced by Assyrian merchants in the 19th century BCE. They extract systematic information on commercial linkages between cities from these records. Then, they estimate a standard model of trade. This then allows them to estimate locations of “lost cities” of the Bronze Age! In many cases these estimates confirm conjectures by historians, and, where historians disagree, help to rule out certain conjectures. This example is a model for successful interdisciplinary collaboration.

Despite these interdisciplinary successes, it is still true that urban economists write mostly for other economists, in economics journals. Despite some recent examples to the contrary, Bertaud (2018) notes the isolation of urban economists from urban policy. A continuing challenge is that the individual rewards for interdisciplinary work, especially for tenure-track researchers, tends to be low. This isolation might be inefficient. Other disciplines may offer better or more complete policy solutions. One example is the case of land use regulations that restrict housing supply. Urban economists have long pointed to land use regulations as a main factor in rising housing prices, especially in coastal U.S. cities (Glaeser & Gyourko, 2018). But work by political scientists moves beyond these contributions by clarifying the challenges of designing political institutions and strategies for increasing housing supply in areas with high housing prices (e.g., Einstein et al., 2019).

Despite Bertaud’s (2018) lament about the isolation of urban economists from urban policy, more recent work brings economists closer to urban policy making. For example, a recent article by Michaels et al. (2021) finds that modest infrastructure investments in greenfield areas in seven Tanzanian cities resulted in neighborhoods that developed larger, more regularly laid-out buildings and better-quality housing. Some other recent examples of policy-oriented economics research are Tsivanidis (2020), who analyzes the distributional effects of Bogota’s bus rapid transit expansion, and Kreindler (2022), who analyzes congestion pricing policies in Bangalore.

Contributions to urban studies

Economists have insights to offer to other urban scholars. Two important ones are quantification and an emphasis on decentralized decision making. First, a primary goal of empirical economics is quantification. How big is something? Do things “add up”? Invariably, at least one economist at a workshop or seminar presentation will ask “why should we care?” The typical response format is “this has a big role in explaining X” or “X affects a lot of people.” Answering a “big question” is a strict filter at the top economics journals. Overall, this tendency may push economists to devote their time and energy to questions in proportion with their importance. Another virtue of this habit is that sometimes things that seem like they might be small or minor turn out to be very big (and vice versa). This can be a useful contribution to both scholarship and policy. For example, Brinkman and Lin (2019, 2022) estimate the harm done to central city quality of life from Interstate highways. This builds on a large social science literature concerned about the negative effects of highways (e.g., Crockett, 2018; DiMento & Ellis, 2013; Robinson, 1971; Wilson, 2008). We estimate that highways caused large declines in central city quality of life: the quality-of-life channel alone accounts for about one-third of the population loss of central cities and the welfare losses from quality-of-life declines are equivalent to about five percent of income. The large size of these losses is interesting because it suggests that targeted mitigation policies (e.g., the Big Dig) could be cost effective. They are also large relative to the estimated benefits of the Interstate system as a whole: Allen and Arkolakis (2014) estimate the gross welfare gains from the Interstate highway program due to increased goods trade was 1.1 to 1.4%. Quantification can add to our knowledge.

A second potential contribution of economists is to emphasize the important role that decentralized decision making plays in determining spatial structure. One example is the recent work by Shertzer and Walsh (2019) on the emergence of racial segregation in northern U.S. cities. Existing literature has emphasized collective action by whites to exclude blacks from certain neighborhoods as the driving increased segregation. The contribution of Shertzer and Walsh is not to deny the role of government policy or other forms of collective action, including coordinated violence against black arrivals. (Indeed, they likely had large effects, as emphasized by Rothstein, 2017.) Instead, they argue that white flight was also a significant factor in increased segregation. According to their estimates, segregation would have increased by about 30 to 50% as much as it did over the 1910s and 1920s due to white flight alone. Shertzer and Walsh’s work illustrates both economists’ focus on quantification and on market mechanisms.

Conclusion

In sum, the core question in urban economics is to understand the uneven spatial distribution of economic activity. Urban economists use optimizing behavior, equilibrium conditions, and agglomeration and dispersion forces to understand the geography of economic activity at all spatial scales. Urban economics is growing, driven by cross-fertilization of ideas with other subfields of economics. There is some interaction with other disciplines outside of economics, but mostly as inputs into research targeted to other economists. Economists’ focus on quantification and market forces may be useful for other social scientists studying cities.

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