I’m currently working with Sightline Institute in Seattle, monitoring the economic analysis phase of the Western Climate Initiative. WCI, which consists of seven US states and three Canadian provinces, is cooking up a common carbon emissions plan, and a consulting firm has been brought onboard to help clarify the economic implications of different policy alternatives. There are many issues specific to this project I may return to later, but for now I’d like to put the spotlight on the methodology WCI will be relying on, CGE modeling. I think these models are so dubious theoretically and unreliable in practice that there is no case for using them. In particular, I am issuing a challenge to their defenders: if no one can answer it, the case is closed.
On a theoretical level, it is surprising that CGE modeling has become such a vibrant industry, since its underpinnings in general equilibrium theory have been systematically undermined over the past several decades. (1) CGE models use the technique of representative agents—vast numbers of households and firms are treated as if they were a single decision-making entity—when we now know that multiple agents cannot be modeled as if they were just one. (2) In particular, the Debreu-Sonnenschein-Mantel result demonstrates that full knowledge of all supply and demand relationships in an economy is not sufficient to predict the equilibrium the economy will arrive at when it is not there yet. (3) The behavioral assumptions of these models, typically resting on utility maximization or simple modifications of it, have been empirically falsified. (4) Production and utility functions are routinely chosen for their convexity properties, despite the widespread recognition that nonconvexities (that yield multiple equilibria) are rife. In short, if theory should inform practice, we shouldn’t be doing CGE.
Now for the challenge. As far as I know, there has never been a rigorous ex post evaluation of CGE models in practice, one that compares predicted to actual outcomes. Based on performance, is there any evidence that such models add value—that their predictions are any better than those derived from macro or sector-specific models, or even a random walk? Also, are CGE models employed by any private sector players who bet real money on the results, or is it only in academia and the public sector that CGE modeling is taken seriously?
My challenge is for those who think there is anything to CGE to come up with evidence that their forecasts add value—either careful retrospective analysis, market applications or both. If not, after more than three decades of experience to go on, why shouldn’t we draw the conclusion that this is a self-perpetuating enterprise promulgated by specialists who sell not an improved ability to make forecasts, but a patina of high-tech respectability for agencies with no stake in whether their policies will actually perform?