"Fundamentalists versus Realists"

Paul Romer says some fundamentalists need to get real:

Fundamentalists versus Realists, by Paul Romer: Debate among economists about the $700 billion Paulson plan reveals a deep divide between realists and fundamentalists. ...

The formal, model-based approach of the fundamentalists has contributed much to progress in economic analysis. At key junctures, it has also made important contributions to policy. The challenge is to maintain an intellectual environment that leaves space for ... realists as well. In complicated policy contexts where models don't yet capture key forces, the realists have much to offer...

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.

In the current crisis, the astonishing and unexpected consequences of the Lehman Brothers bankruptcy should serve as a similarly decisive data point. On the Thursday and Friday before Lehman filed for protection, I was at a conference on the financial crisis. Everyone there expected them to file on Monday. We repeated for each other all the fundamentalist arguments: "Everyone had been given time to prepare." "The courts handle bankruptcies all the time." None of us expected that putting Lehman through a court managed bankruptcy would be much different from arranging a forced sale of Bear Stearns. We were all wrong. Within days, AIG was insolvent. Runs were developing on Goldman Sachs, Morgan Stanley, and the entire money market fund industry. Banks had stopped lending to each other in the Fed Fund market. Rates on Treasuries approached zero.

In response, the Treasury, Fed, and market regulators took drastic steps that the fundamentalists would surely have opposed had there been time for debate. ... Looking back, it appears that they had enough sand bags to hold back the flood and stop the panic, but perhaps just barely enough. This is not a data point that can be dismissed as an outlier. It is the kind of observation that should make the fundamentalists just a bit less confident in their models and a bit more willing to listen to the realists who are willing to defer to the policy makers on the front lines. ...