‘Rational expectations’ is wrong

3 October, 2016 at 17:53 | Posted in Economics | 2 Comments

Lynn Parramore: It seems obvious that both fundamentals and psychology matter. Why haven’t economists developed an approach to modeling stock-price movements that incorporates both?

Roman Frydman: It took a while to realize that the reason is relatively straightforward. Economists have relied on models that assume away unforeseeable change. As different as they are, rational expectations and behavioral-finance models represent the market with what mathematicians call a probability distribution – a rule that specifies in advance the chances of absolutely everything that will ever happen.

In a world in which nothing unforeseen ever happened, rational individuals could compute precisely whatever they had to know about the future to make profit-maximizing decisions. Presuming that they do not fully rely on such computations and resort to psychology would mean that they forego profit opportunities.

LP: So this is why I often hear that supporters of the Rational Expectations Hypothesis imagine people as autonomous agents that mechanically make decisions in order to maximize profits?

fubYes! What has been misunderstood is that this purely computational notion of economic rationality is an artifact of assuming away unforeseeable change.

Imagine that I have a probabilistic model for stock prices and dividends, and I hypothesize that my model shows how prices and dividends actually unfold. Now I have to suppose that rational people will have exactly the same interpretation as I do — after all, I’m right and I have accounted for all possibilities … This is essentially the idea underpinning the Rational Expectations Hypothesis …

LP: So the only truth is the non-existence of the one true model?

RF: It’s the genuine openness that makes our ideas – and education – more exciting. Students can think about things in an open, yet structured way. We don’t lose the structure; we just renounce the pretense of exact knowledge …

Economists may fear that acknowledging this limit would make economic analysis unscientific. But that fear is rooted in a misconception of what the social scientific enterprise should be. Scientific knowledge generates empirically relevant regularities that are likely to be durable. In economics, that knowledge can only be qualitative, and grasping this insight is essential to its scientific status.  Until now, we have been wasting time looking for a model that would tell us exactly how the market works.

LP: Chasing the Holy Grail?

RF: Yes. It’s an illusion. We’ve trained generation after generation in this fruitless task, and it leads to extreme thinking.

Huffington Post

2-format2010Roman Frydman is Professor of Economics at New York University and a long time critic of the rational expectations hypothesis. In his seminal 1982 American Economic Review article Towards an Understanding of Market Processes: Individual Expectations, Learning, and Convergence to Rational Expectations Equilibrium — an absolute must-read for anyone with a serious interest in understanding what are the issues in the present discussion on rational expectations as a modeling assumption — he showed that models founded on the rational expectations hypothesis are inadequate as representations of economic agents’ decision making.

Those who want to build macroeconomics on microfoundations usually maintain that the only robust policies and institutions are those based on rational expectations and representative actors. As yours truly has tried to show in On the use and misuse of theories and models in economics there is really no support for this conviction at all. On the contrary. If we want to have anything of interest to say on real economies, financial crisis and the decisions and choices real people make, it is high time to place macroeconomic models building on representative actors and rational expectations-microfoundations in the dustbin of pseudo-science.

For if this microfounded macroeconomics has nothing to say about the real world and the economic problems out there, why should we care about it? The final court of appeal for macroeconomic models is the real world, and as long as no convincing justification is put forward for how the inferential bridging de facto is made, macroeconomic modelbuilding is little more than hand waving that give us rather little warrant for making inductive inferences from models to real world target systems. If substantive questions about the real world are being posed, it is the formalistic-mathematical representations utilized to analyze them that have to match reality, not the other way around.

The real macroeconomic challenge is to accept uncertainty and still try to explain why economic transactions take place – instead of simply conjuring the problem away by assuming rational expectations and treating uncertainty as if it was possible to reduce it to stochastic risk. That is scientific cheating. And it has been going on for too long now.

Cassidy: What about the rational-expectations hypothesis, the other big theory associated with modern Chicago? How does that stack up now?

Heckman: I could tell you a story about my friend and colleague Milton Friedman. In the nineteen-seventies, we were sitting in the Ph.D. oral examination of a Chicago economist who has gone on to make his mark in the world. His thesis was on rational expectations. After he’d left, Friedman turned to me and said, “Look, I think it is a good idea, but these guys have taken it way too far.”

CarriedAwayIt became a kind of tautology that had enormously powerful policy implications, in theory. But the fact is, it didn’t have any empirical content. When Tom Sargent, Lard Hansen, and others tried to test it using cross equation restrictions, and so on, the data rejected the theories. There were a certain section of people that really got carried away. It became quite stifling.

Cassidy: What about Robert Lucas? He came up with a lot of these theories. Does he bear responsibility?

Heckman: Well, Lucas is a very subtle person, and he is mainly concerned with theory. He doesn’t make a lot of empirical statements. I don’t think Bob got carried away, but some of his disciples did. It often happens. The further down the food chain you go, the more the zealots take over.

John Cassidy/The New Yorker



  1. Noah used the same expression and linked to this paper

    Information Rigidity and the Expectations Formation Process: A Simple Framework and New Facts
    Olivier Coibion and Yuriy Gorodnichenko

    • He now has a post on it: Hunting the Rational Expectations whale

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