Symmetries as Schelling Points
Game theorists know that games with symmetry in payoffs are easier to solve. Appendix B of The Strategy of Conflict by Thomas Schelling is titled "For the Abandonment of Symmetry in Game Theory." In this appendix, Schelling argues that
though symmetry is consistent with the rationality of the players, it cannot be demonstrated that asymmetry is inconsistent with their rationality (page 278)
In other words, symmetry is not a necessary condition to rationality. But as Schelling documents in Appendix B, this truth was not clear to all game theorists. In particular, Harsanyi referred to the symmetry axiom as the "fundamental postulate," and said that "the assumption underlying the axiom is that a rational bargainer will not expect a rational opponent to grant him larger concessions than he would make himself under similar conditions."
Schelling goes on to ask whether the symmetry postulate can be derived from rationality. He concludes that it cannot. Thus, he observes that
we must be careful not to make symmetry part of the definition of rationality; to do so would destroy the empirical relevance of the theory and simply make symmetry an independent axiom.
In other words, although we might want more than the assumption of rationality to predict the outcome of any game, we cannot rely on the symmetry postulate, which is violated by many empirical observations. But then what, aside from the assumption of rationality, could be as fundamental? Schelling does not give a direct answer. Rather, he observes that
a theory of strategy ... is inherently empirical; it depends on how people coordinate their expectations.
From here he goes on to make his famous hypothesis that rational people will coordinate their behavior through focal points -- i.e., the most obvious answer known by prior experience to both people. The theory of focal points resolves the paradox of symmetry and rationality by assuming that the Nash equilibrium will be the focal point for rational actors. In other words, where obvious symmetries exist in payoffs, rational actors will tend to focus on those symmetries.
How Schelling Points Led to an Attack on the Rational Hypothesis
The explanatory value of focal points in understanding conflict and cooperation has led to advances in many fields of economics and politics. Schelling later won a Nobel.
Inevitably, the theory of focal points has pushed many economists and political scientists to undertake a more serious study of psychology. If theories of strategy are inherently empirical afterall, then cognitive and behavioral limits must be important in determining focal points.
But somewhere along the way, some of the academics studying behavior forgot that the end goal was to observe and hypothesize focal points. Their focus instead fell upon the hypothesis of rationality. This isn't surprising given that the observed behavior of individual humans often seems to contradict the hypothesis of rationality. The hypothesis of rationality is not a psychology; rather, it is a prediction about how, on average, large groups of people will behave over long periods of time. It is thus an empirical hypothesis, not about individuals, but about groups -- very large groups, in fact.
Yet the attack on the hypothesis of rationality has been distracting for two reasons. First, and less importantly, in many cases the attack on the rational hypothesis has led to a misunderstanding about what, exactly, the hypothesis of rationality hypothesizes. I do not mean to impugn all research on the subject. Much of it is quite interesting and useful. But it has been misleading to non-specialists.
Second, and more importantly, the attack has distracted theorists and experimentalists from the task that Schelling gave them of identifying empirically useful focal points. In fact, theorists of econometrics have gotten so distracted that some have given up entirely on identifying focal points that could be used to model collective behavior as complicated as market prices. Their approach instead has been to substitute ad hoc models of psychology to limit the hypothesis of rationality. This, in turn, has led to intractable mathematics and insoluble debates over what approximations are reasonable as limits to rationality.
Broken Symmetries as Schelling Points
Theories of economics and political science are starved for a new focal point theory that is useful and general as an empirical observation and consistent with the hypothesis of rationality. There is such a focal point. And what should not be surprising is that it was overlooked for the same reason that Schelling points were overlooked by other academics before The Strategy of Conflict. Specifically, academics had reached a tacit agreement that symmetry would be the basis for theories of rationality. Schelling's ability to see that symmetry was only one focal point among many that could have been chosen is the reason why he is justly lauded for his deep insights into human social behavior.
And since Schelling understood how Schelling points could lead to tacit agreement, naturally he was able to identify symmetry as a Schelling point for academic economists, and ask, in effect, "Why symmetry?" The answer he suggests is that symmetric problems are easier to solve mathematically.
This is certainly true. To wit, to be soluble, mathematical models for market equilibrium (and hence price) provided by economists must assume that wealth is conserved (i.e., is not created or destroyed). Equilibrium cannot be defined when total wealth is unstable. What only few economists may realize, however, is that (under Emmy Noether's Theorem) conservation laws are mathematically equivalent to symmetry invariance. In this case, wealth conservation implies time invariance.
Given the empirical fact that symmetries are often broken, it is natural to ask, then, what hypothesis about the social behavior of large groups over long periods could be nearly as fundamental as the hypothesis of rationality -- remembering, of course, that the rules of the game are to find a hypothesis that is also consistent with the hypothesis of rationality.
My humble suggestion is the following:
The activities of people change in time according
to rhythms that, averaged over a population within a period of time, have a
characteristic distribution in frequency.
I call this the fundamental hypothesis of periodicity. The hypothesis can be demonstrated to be consistent with the fundamental hypothesis of rationality by observing how supply and demand curves can be integrated up from underlying frequency distributions. Nonetheless it provides new insights into how elasticity and price dynamics arise from the aggregation of rhythms in the behavior of large numbers of people over long periods of time.
Periodicity is a focal point that is unexplored by economists, political scientists, and others. It could serve as the foundation for a more complete empirical theory of the firm.
[First draft June 25, 2008. Revised on March 2, 2011. -MFM]
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