Monday, November 10, 2008

SDM: Path-Dependence and Instability

I recently read Peter Dorman's "Waiting for an Echo: The Revolution in General Equilibrium Theory and The Paralysis in Introductory Economics" (Review of Radical Political Economics, V. 33 (2001): pp. 325-333). Dorman claims that, in teaching introductory microeconomics, General Equilibrium Theory (GET) is "one of the back-of-the-book chapters we rarely get to." And if GET is taught, the teaching fails to reflect a "virtual revolution in GET during the past quarter-century". His thesis is that these developments in GET can and should be taught in introductory microeconomics classes.

The Sonnenschein-Debreu-Mantel theorem is one of these developments. This theorem states that almost any excess demand curves in markets for individual goods can be justified by aggregating over individual excess demands. Theory imposes only Walras' law, homogeneity of degree zero, and a technical continuity condition. No other restrictions need arise on the shape of aggregate demand curves.

Why are the SDM results exciting? They imply the general possibility of multiple equilibrium and instability. Or at least, that's what I have taken from the literature. I first thought idiosyncratic Dorman's take on the SDM results. He says that they show the "path-dependence instability of general equilibrium" and the indeterminancy of equilibrium:
"In general equilibrium, each action that alters the distribution of resources among agents (and that would be just about anything) also alters the equilibrium vector of prices. It is not possible to identify an equilibrium seperate from the actions individuals take either in pursuit of in utter ignorance of it."
And he writes:
"The first task facing a principles instructor is to ignore the scholarly debate that has surrounded S-D-M. The original authors demonstrated that out-of-equilibrium exchanges altered the distribution of resources, and, since different individuals have different preferences, also altered the general equilibrium itself. Since then, researchers have been investigating the exact extent of preference differentiation under which this result would hold. This, it seems to me, is an utterly arid line of investigation, and it has no meaningful implications for nonspecialists."

I have heard of indeterminacy, but had not thought of it in the context of the SDM. As I understand the instability implications of the SDM results, they are explored in the context of tâtonnement dynamics. How then, can one talk about path dependence here?

I did come up with some justification after some thought. The SDM results show that any dynamics is possible in GET. And I know of an interesting example of chaos in which the sensitive dependence on initial conditions is connected to a particular fractal structure. Newton's method is a numerical method for solving non-linear equations. One can think of Newton's method as a dynamical system for iteratively mapping a point in the complex plane to a root of an equation, when the method converges. Polynomials, for example, have multiple roots. Color the plane by the roots to which Newton's method maps each point. All points that map to a given root are the same color. For certain simple polynomials, you will have drawn a fractal. (Google also gave me this.) Thus, in certain regions, any infinitesimal change in the initial conditions can cause this dynamical method to tend towards a different equilibrium. This property is independent of any claim that multiple equilibrium lie along a continuum.

Since, according to the SDM results, any dynamics is possible, I guess that some sort of dynamics like I have described for Newton's method is possible in GET. And so one can say that the SDM results show the possibility of path-dependence in economics.

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