Tuesday, October 07, 2008

Do you take models (too) seriously?

The very wise Paul Romer suggests that the key divide among economists today is between fundamentalists and realists.

The financial crisis provoked three open letters to policy makers. Fundamentalists opposed the plan (here.) Realists supported the plan (here) or supported more discretionary powers for dealing with the crisis without endorsing any specific plan (here.)

A quick look through the lists of economists who signed the various letters shows that the camps do not separate cleanly along the familiar lines of left-versus-right or active-versus-limited government. The key difference lies in the relative weight each side gives to formal models as opposed to judgment.

Fundamentalists have an unswerving faith in models. Policies should always be derived from the best available model. Data should be filtered through a model. If an observation does not fit within the context of a model, it should be excluded from consideration.

Realists are more conscious of the limits of models and more comfortable with a division of labor between the researcher who improves the models and the clinician who makes policy decisions. They recognize that the power of models comes precisely from a commitment to abstraction that filters out potentially important complexity. They believe that useful evidence can accumulate with direct experience as well as through the research process of testing and refining models. They believe that researchers should consider the possibility that the fault lies with the model when its predictions diverge from clinical judgment and that policies should draw on both sources of evidence.

Many times, the confidence fundamentalists have had in abstract models turned out to be well founded and the objections raised by realists who were more focused on details were misplaced. The fundamentalists were right that an airline industry could still function even if airlines could set their own fares; that people could still talk to each other even if they purchased phone service from different companies. The realists pointed to all the complicated details that arise in such markets, details that simple models could not capture. Fundamentalists, correctly, ignored the detail and pushed prescriptions based on the textbook model of competition.

Other times, the models are missing something that is too important. In the study of macroeconomic fluctuations, real business cycle theorists and their descendants, the dynamic stochastic general equilibrium modelers, are the quintessential fundamentalists. Their models are a useful way to make research progress, but in macroeconomic policy making, the great depression, which these models cannot explain, is a decisive data point warning us that the models are incomplete and have to be supplemented by clinical judgment.

Thanks to Mark Thoma for the pointer.