On the poverty of microfoundationalist fantasies
from Lars Syll
The microfoundationalist’s fantasy has a powerful hold on macroeconomists. They recognize that an agent-by-agent reconstruction of the economy is not feasible, but they argue that it is something that we could do “in principle,” and that the in-principle claim warrants a particular theoretical strategy. The strategy is to start with the analysis of a single agent and to build up through ever more complex analyses to a whole economy …
The implicit argument in favor of representative-agent models as empirically relevant to aggregate economic data runs something like this: a representative-agent model is not itself an acceptable representation of the whole economy … but it is a first step in a program which step by step will inevitably bring the model closer to the agent-by-agent microeconomic model of the whole economy … I call this argument eschatological justification: it is the claim that there is a plausible in-principle game plan for a reductionist program and that the conclusions of early stages of that program are epistemically warranted by the presumed, but undemonstrated, success of the future implementation of the program in the fullness of time …
Analysis using the representative-agent model employs an analogy between the behavior of a single agent and the agents collectively in a whole economy. For example, the representative-agent is typically endowed with a utility function from precisely the same family as those typically assigned to individual agents in microeconomic analysis. Do we have any good reason to accept the analogy? Microeconomists have long known that the answer is, no.
Exact aggregation requires that utility functions be identical and homothetic … Translated into behavioral terms, it requires that every agent subject to aggregation have the same preferences (you must share the same taste for chocolate with Warren Buffett) and those preferences must be the same except for a scale factor (Warren Buffet with an income of $10 billion per year must consume one million times as much chocolate as Warren Buffet with an income of $10,000 per year). This is not the world that we live in. The Sonnenschein-Mantel-Debreu theorem shows theoretically that, in an idealized general-equilibrium model in which each individual agent has a regularly specified preference function, aggregate excess demand functions inherit only a few of the regularity properties of the underlying individual excess demand functions: continuity, homogeneity of degree zero (i.e., the independence of demand from simple rescalings of all prices), Walras’s law (i.e., the sum of the value of all excess demands is zero), and that demand rises as price falls (i.e., that demand curves ceteris paribus income effects are downward sloping) … These regularity conditions are very weak and put so few restrictions on aggregate relationships that the theorem is sometimes called “the anything goes theorem.”
The importance of the theorem for the representative-agent model is that it cuts off any facile analogy between even empirically well-established individual preferences and preferences that might be assigned to a representative agent to rationalize observed aggregate demand. The theorem establishes that, even in the most favorable case, there is a conceptual chasm between the microeconomic analysis and the macroeconomic analysis. The reasoning of the representative-agent modelers would be analogous to a physicist attempting to model the macro- behavior of a gas by treating it as single, room-size molecule. The theorem demonstrates thatthere is no warrant for the notion that the behavior of the aggregate is just the behavior of the individual writ large: the interactions among the individual agents, even in the most idealized model, shapes in an exceedingly complex way the behavior of the aggregate economy. Not only does the representative-agent model fail to provide an analysis of those interactions, but it seems likely that that they will defy an analysis that insists on starting with the individual, and it is certain that no one knows at this point how to begin to provide an empirically relevant analysis on that basis.