E. Glen Weyl

When I was last living in Chicago, in the spring 2014, a regular visitor to the department of the University of Chicago and the editor of the Journal of Economic Literature, Steven Durlauf, asked me if I would be interested in writing something for the journal. For many years I had promised Gary Becker that I would write something to help clarify the meaning and role of price theory to my generation of economists, especially those with limited exposure to the Chicago environment, which did so much to shape my approach to economics. With Gary’s passing later that spring, I decided to use this opportunity to follow through on that promise. More than a year later I have posted on SSRN the result.

I have an unusual relationship to “price theory”. As far as I know I am the only economist under 40, with the possible exception of my students, who openly identifies myself as focusing my research on price theory. As a result I am constantly asked what the phrase means. Usually colleagues will follow up with their own proposed definitions. My wife even remembers finding me at our wedding reception in a heated debate not about the meaning of marriage, but of price theory.

The most common definition, which emphasizes the connection to Chicago and to models of price-taking in partial equilibrium, doesn’t describe the work of the many prominent economists today who are closely identified with price theory but who are not at Chicago and study a range of different models. It also falls short of describing work by those like Paul Samuelson who were thought of as working on price theory in their time even by rivals like Milton Friedman. Worst of all it consigns price theory to a particular historical period in economic thought and place, making it less relevant to the future of economics.

I therefore have spent many years searching for a definition that I believe works and in the process have drawn on many sources, especially many conversations with Gary Becker and Kevin Murphy on the topic as well as the philosophy of physics and the methodological ideas of Raj Chetty, Peter Diamond and Jim Heckman among others. This process eventually brought me to my own definition of price theory as analysis that reduces rich (e.g. high-dimensional heterogeneity, many individuals) and often incompletely specified models into ‘prices’ sufficient to characterize approximate solutions to simple (e.g. one-dimensional policy) allocative problems. This approach contrasts both with work that tries to completely solve simple models (e.g. game theory) and empirical work that takes measurement of facts as prior to theory. Unlike other definitions, I argue that mine does a good job connecting the use of price theory across a range of fields of microeconomics from international trade to market design, being consistent across history and suggesting productive directions for future research on the topic.

To illustrate my definition I highlight four distinctive characteristics of price theory that follow from this basic philosophy. First, diagrams in price theory are usually used to illustrate simple solutions to rich models, such as the supply and demand diagram, rather than primitives such as indifference curves or statistical relationships. Second, problem sets in price theory tend to ask students to address some allocative or policy question in a loosely-defined model (does the minimum wage always raise employment under monopsony?), rather than solving out completely a simple model or investigating data. Third, measurement in price theory focuses on simple statistics sufficient to answer allocative questions of interest rather than estimating a complete structural model or building inductively from data. Raj Chetty has described these metrics, often prices or elasticities of some sort, as “sufficient statistics”. Finally, price theory tends to have close connections to thermodynamics and sociology, fields that seek simple summaries of complex systems, rather than more deductive (mathematics), individual-focused (psychology) or inductive (clinical epidemiology and history) fields.

I trace the history of price theory from the early nineteenth to the late twentieth when price theory became segregated at Chicago and against the dominant currents in the rest of the profession. For a quarter century following 1980, most of the profession either focused on more complete and fully-solved models (game theory, general equilibrium theory, mechanism design, etc.) or on causal identification. Price theory therefore survived almost exclusively at Chicago, which prided itself on its distinctive approach, even as the rest of the profession migrated away from it.

This situation could not last, however, because price theory is powerfully complementary with the other traditions. One example is work on optimal redistributive taxation. During the 1980’s and 1990’s large empirical literatures developed on the efficiency losses created by income taxation (the elasticity of labor supply) and on wage inequality. At the same time a rich theory literature developed on very simple models of optimal redistributive income taxation. Yet these two literatures were largely disconnected until the work of Emmanuel Saez and other price theorists showed how measurements by empiricists were closely related to the sufficient statistics that characterize some basic properties of optimal income taxation, such as the best linear income tax or the optimal tax rate on top earners.

Yet this was not the end of the story; these price theoretic stimulated empiricists to measure quantities (such as top income inequality and the elasticity of taxable income) more closely connected to the theory and theorists to propose new mechanisms through which taxes impact efficiency which are not summarized correctly by these formulas. This has created a rich and highly productive dialog between price theoretic summaries, empirical measurement of these summaries and more simplistic models that suggest new mechanisms left out of these summaries.

A similar process has occurred in many other fields of microeconomics in the last decade, through the work of, among others, five of the last seven winners of the John Bates Clark medal. Liran Einav and Amy Finkelstein have led this process for the economics of asymmetric information and insurance markets; Raj Chetty for behavioral economics and optimal social insurance; Matt Gentzkow for strategic communication; Costas Arkolakis, Arnaud Costinot and Andrés Rodriguez-Clare in international trade; and Jeremy Bulow and Jon Levin for auction and market design. This important work has shown what a central and complementary tool price theory is in tying together work throughout microeconomics.

Yet the formal tools underlying these price theoretic approximations and summaries have been much less fully developed than have been analytic tools in other areas of economics. When does adding up “consumer surplus” across individuals lead to accurate measurements of social welfare? How much error is created by assumptions of price-taking in the new contexts, like college admissions or voting, to which they are being applied? I highlight some exciting areas for further development of such approximation tools complementary to the burgeoning price theory literature.

Given the broad sweep of this piece, it will likely touch on the interests of many readers of this blog, especially those with a Chicago connection. Your comments are therefore very welcome. If you have any, please email me at glenweyl@microsoft.com.