Toda teoria é uma supersimplificação da realidade ,e como tal, não é perfeita, mas não deve ser descartada. Aliás,a hipótese por si só não é testável,pois deve ser empiricamente testada em conjunto com algum modelo de precificação de ativo ou modelo de equilíbrio. Entretanto, o inverso também é verdadeiro. Ou seja, grande parte dos modelos de risco-retorno não pode ser testado sem considerar a eficiência do mercado. Tudo o que foi dito neste parágrafo é conhecido como
joint hypothesis problem.
Apesar de ser falha, controversa e com inúmeras limitações, a HME ainda é útil para o entendimento do mercado de capitas, o retorno dos ativos financeiros e a dinâmica dos preços. Vários estudos em economia e finanças tratam dessa hipótese, então para maiores detalhes técnicos e material bibliográfico consulte o seguinte site:
Efficient Markets Hypothesis.
Na
entrevista, Fama comenta sobre a Hipótese dos Mercados Eficientes, a evolução das finanças, as possíveis causas da crise financeira de 2008 , o artigo
My Life in Finance e alguns temas sobre macroeconomia. Destaquei alguns pontos, mas recomendo a leitura e/ou audição da entrevista na íntegra.
Inicialmente, Fama explica o que é um mercado de capitais eficiente e de forma clara e objetiva trata das três formas de testes que utilizou para testar a HME: fraca, semi-forte e forte.É interessante observar que ele se "arrependeu" de ter utilizado essa nomenclatura (taxonomia), pois estava apenas tentando categorizar os testes que realizava nos anos 70. Não obstante, essa taxonomia tornou-se clássica e ainda é citada em muitas pesquisas.
Russ: And so when we say markets are efficient, what do you mean by that?
Guest: What you mean is that prices at any point in time reflect all available information.
Russ: Now that idea--what's the distinction between the weak form and the strong form that people talk about?
Guest:
Two words that I used in 1970 that I came to regret. Because I was trying to categorize various tests that were done. So, I called weak form tests, tests that only used past prices and returns to predict future prices and returns. And I called semi-strong form tests, tests that used other kinds of public information to predict returns, like an earnings announcement or something like that. And then I called strong form tests, tests that look at all available information;
Russ: And empirically, where do we stand today, do you believe and what has been established about those various hypotheses?
Guest:
Well, believe it or not, the weak form one has been the one that has been subject to the most, what people call anomalies, in finance. Things that are inconsistent with either market efficiency or some model of risk and return. The big one at the moment is what people call momentum--prices seem to move in the same direction for short periods of time. So, the winners of last year tend to be winners for a few more months, and the losers tend to be losers for a few more months.
In the strong form tests, Ken French and I just published a paper called "Luck Versus Skill in Mutual Fund Performance," and basically looked at performance of the whole mutual fund industry--in the aggregate, together, and fund by fund, and try to distinguish to what extent returns are due to luck versus skill.
And the evidence basically says the tests it's skill in the extreme. But you've got skill in both extremes. That's something people have trouble accepting. But it comes down to a simple proposition, which is that active management in trying to pick stocks has to be a zero sum game, because the winners have to win at the expense of losers. And that's kind of a difficult concept.
But it shows up when you look at the cross section of mutual fund returns, in other words the returns for all funds over very long periods of time. What you find is, if you give them back all their costs, there are people in the left tail that look too extreme and there are people in the right tail that look too extreme, and the right tail and left tail basically offset each other. If you look at the industry as a whole; the industry basically holds a market portfolio. That's all before costs. If you look at returns to investors then there is no evidence that anybody surely has information sufficient to cover their costs.
Em seguida, comenta sobre a posição dos investidores acerca da eficiência do mercado. É de fato o trecho mais interessante.
Russ: A friend of mine
who is a hedge fund manager--before I made this call I asked him what he would ask you, and he said, well,
his assessment is that efficient markets explain some tiny proportion of volatility of stock prices but there's still plenty of opportunity for a person to make money before markets adjust. And of course in doing so, make that adjustment actually happen and bring markets to equilibrium. Somebody has to provide the information or act on the information that is at least public and maybe only semi-public. What's your reaction to that comment?
Guest:
That's the standard comment from an active manager. It's not true. Merton Miller always liked to emphasize that you could have full adjustment to information without trading. If all the information were available at very low cost, prices could adjust without any trading taking place. Just bid-ask prices. So, it's not true that somebody has to do it. But the issue is--this goes back to a famous paper by Grossman and Stiglitz--the issue really is what is the cost of the information? And I have a very simple model in mind. In my mind, information is available, available at very low cost, then the cost function gets very steep. Basically goes off to infinity very quickly. Russ: And therefore?
Guest: And therefore prices are very efficient because the information that's available is costless.
Russ: But what's the implication of that steep incline? That information is not very--
Guest: It doesn't pay to try to take advantage of additional information.
Russ: It's not very valuable.
Guest: No, it's very valuable. If you were able to perfectly predict the future, of course that would be very valuable. But you can't. It becomes infinitely costly to do that.
Russ: So, your assessment, that you just gave me of the state of our knowledge of this area, I would say remains what it's been for some time--that at the individual certainly there is no return to--
prices reflect all publicly available information for practical purposes for an individual investor.Guest: For an individual investor? Even for an institutional investor.
Russ: Correct. So, what proportion of the economics and finance areas do you think agree with that?
Guest:
Finance has developed quite a lot in the last 50 years that I've been in it. I would say the people who do asset pricing--portfolio theory, risk and return--those people think markets are pretty efficient. If you go to people in other areas who are not so familiar with the evidence in asset pricing, well, then there is more skepticism. I attribute that to the fact that finance, like other areas of economics, have become more specialized. And people just can't know all the stuff that's available.
Russ: Sure.
Guest:
There's an incredible demand for market inefficiency. The whole investment management business is based on the idea that the market is not efficient. I say to my students when they take my course: If you really believe what I say and go out and recruit and tell people you think markets are efficient, you'll never get a job. Russ: Yes, it's true. And so there's a certain bias, you are saying, to how people assess the evidence.
Guest: There's a bias.
The bias is based, among professional money managers, the bias comes from the fact that they make more money from portraying themselves as active managers.
Posteriormente, faz uma excelente crítica sobre as causas da crise financeira de 2008 e o fetiche contemporâneo por bolhas de ativos:
I was going to ask you about the current crisis.
Guest: I have some unusual views on that, too.
Russ: I'd say that the mainstream view--and I recently saw a survey that said--it was an esteemed panel of economists; you weren't on it but it was still esteemed, both in finance and out of finance. And they asked them whether prices reflected information and there was near unanimity. Some strongly agreed; some just agreed. But there was also near unanimity that the housing market had been a bubble.
Guest:
The nasty b-word.Russ: Yes; and was showing some form of what we might call irrationality.
Guest: Okay, so they had strong feelings about that, getting mad about the word bubble.
Russ: Why?
Guest:
Because I think people see bubbles with 20-20 hindsight. The term has lost its meaning. It used to mean something that had a more or less predictable ending. Now people use it to mean a big swing in prices, that after the fact is wrong. But all prices changes after the fact are wrong. Because new information comes out that makes what people thought two minutes ago wrong two minutes later. Housing bubble--if you think there was a housing bubble, there might have been; if you had predicted it, that would be fine; but the reality is, all markets did the same thing at the same time. So you have to really face that fact that if you think it was a housing bubble, it was a stock price bubble, it was a corporate bond bubble, it was a commodities bubble. Are economists really willing to live with a world where there are bubbles in everything at the same time?
Russ: And your explanation then of that phenomenon?
Guest: My explanation is you had a big recession. I think you can explain almost everything just by saying you had a big recession. A really big recession.
...Guest: Okay, but it wasn't just housing. That was my point when we started. The same thing was going on in all asset markets.
Russ: Well, the timing isn't quite identical for all asset markets, right? The stock market--the housing market starts to collapse I think around early-mid-2006. Guest: It stops rising, right.
Russ: And then begins a steady decline.
Guest: That decline was nothing compared to the stock market decline.
Russ: But when did that happen? Guest: I don't know the exact timing. Russ: It's not around then. It's later.
Guest:
The onset of the recession started with the collapse of the stock market. The recession and the collapse of the stock market, the corporate bond market, all of that basically coincides. But that also coincides with the collapse of the securitized bond market.
Russ: Mortgage-backed securities.
Guest: The subprime mortgages and all of that. Russ: Well, yes; that happens through 2007, 2008. I guess there is some parallel. So, you are going to reverse the causation.
Guest: I'm not saying I know. What I'm saying is I can tell the whole story just based on the recession. And I don't think you can come up with evidence that contradicts that. But I'm not saying I know I'm right. I don't know.
I'm just saying people read the evidence through a narrow lens.Russ: Yes, they do. Confirmation bias.
Guest:
And the rhetoric acquires a life of its own; so there are books written that basically all say the same thing about the crisis.Russ: And you are arguing that they have essentially cherry-picked the data.
Guest:
Well, they just look at pieces of the data and the fact that the housing market collapsed is taken to be the cause; but the housing market could collapse for other reasons. People don't just decide that prices aren't high any more. They have to look at supply and demand somewhere in the background.
Russ: We did have people holding second and third homes who didn't have the income and capability of repaying the first one.
Guest: Sure. Standards were relaxed. But then you have to look on the supply side, the lending side. The people who were lending to these people had the information.
Em determinado momento , o economista é questionado sobre as finanças comportamentais. É oportuno lembrar que esta faz parte das finanças "tradicionais" , pois fornece subsídios para sua melhor compreensão. Em outras palavras, é uma teoria complementar e não substituta do que já existia antes. Veja a crítica de Fama:
Russ: But let's go back to finance. There's been a big trend in recent years towards what's called behavioral finance. What's your assessment of that?
Guest:
I think the behavioral people are very good at describing microeconomic behavior--the behavior of individuals--that doesn't seem quite rational. I think they are very good at that. The jump from there to markets is much more shaky.
Russ: Explain.
Guest: There are two types of behavioral economists. There are guys like my friend and colleague Richard Thaler,
who are solidly based in psychology, reasoned economics but he's become a psychologist, basically, and he is coming from the research in psychology. Now there are
other finance people who are basically what I call anomaly chasers.
What they are doing is scouring the data for things that look like market inefficiency, and they classify that as behavioral finance.Russ: They don't tell you about the times they can't find the anomaly.
Guest: Exactly. In all economics research, there is a multiple comparisons problem that never gets stated.
Russ: A multiple what?
Guest: The fact that the data have been used by so many other people and the people using it now use it in so many different ways that they don't report, that you have no real statistical basis to evaluate and come to a conclusion.
Guest:
Right. I've had people say to me that the people who do this anomaly stuff, when they come and give a paper and I'll say, when you do this, that, or the other thing, and they'll say Yes. And I'll say, why don't you report it? And they'll say it wasn't interesting.
Em seguida, Fama diz o que pessoas inteligentes deveriam conhecer sobre finanças e comenta sobre a importância do paper e da equação de Black- Scholes para as ciências econômicas. Num recente artigo publicado no jornal
The Guardian, Ian Stewart discute a equação e suas possíveis consequências nefastas para a economia mundial. É uma análise interessante e muito questionável, que está presente em seu novo livro:
17 Equations That Changed the World .
Guest: I'm obviously going to be biased.
I think all of our stuff on efficient markets would qualify. I think there is a lot of stuff in the corporate area, corporate governance and all of that, a huge field--that has penetrated to the practical level. The Black-Scholes option pricing paper in view is the most important economics paper of the century.
Russ: Why?
Guest:
Because every academic, every economist whether he went into finance or not, read that paper. And it created an industry. In the applied financial domain. What else can claim that? So, I think we've learned a lot about risk and return. Some of it is intuitive. But there is a lot of stuff on which stocks are more or less risky. A lot of stuff on international markets.
Por fim, fala sobre suas perspectivas futuras para as pesquisas em finanças , dá um recado aos cientistas, fala um pouso sobre sua tese de doutorado e comenta sobre o artigo, autobiografia e bibliografia: My Life in Finance.
Guest: Oh, absolutely. What I say to my students is: I'm showing you the stuff that people have done in the last 30 years, but in 20 years, it may all be irrelevant;
so the best I can do is to train you about how to think about these things, so you can absorb stuff that comes along in the future that may overturn what's there now. That's what makes this profession fun, I think--the fact that stuff can get overturned.
Russ: Of course,
if we only have the illusion of understanding, or what Hayek called the pretense of knowledge, we could be doing some dangerous and stupid things under the guise of thinking we are making progress. So, you do have to be careful. Where do you think in the near future finance is going?
Guest: Oh, gee, I don't know. That's part of the fun of it. You just don't know. I wouldn't have been able to predict 30 years ago the stuff that evolved during those intervening 30 years. No way.
Russ: It's kind of a random walk.
Guest: I don't think it pays to think about it very much. There's so much serendipity in what happens in research.
My best stuff has always been--I didn't start thinking about writing a great paper. I started thinking about a little problem; it just kept working in circles into a bigger problem. Or had offshoots that were related. I've beaten many topics to death, with the consequence I've got a lot of recognition;
what started as a little thing developed into something much bigger. That's not a predictable process. Lots of little things end up as nothing.
Russ: And?
Guest:
A student comes to me, a Ph.D. student, and says: I want to write a great paper. You can't start out to do that. You have to pick a problem and hope it works out into something that will get you a job, and hopefully a good one. But if you start saying: I want to come up with a great topic, you won't come up with anything.Russ: You recently wrote a very nice essay, "My Life in Finance," that gives an overview of some of your contributions and some of your thinking along the way and all those little problems. You started out by talking about your thesis topic, where you had five ideas and Merton Miller said four of them weren't very good. Did you ever go back to any of those four?
Guest: No, actually. Merton was incredible. He had a great eye for stuff that would work and wouldn't work. I went to Belgium for two years to teach, and I came back and showed him the stuff I'd been working on, and I think he discarded like 8 out of 10 things. He was right on all of them.
Russ: Such is life.
Guest:
It taught me that nobody can work in a vacuum. You really need colleagues around you to enrich your work. You get credit for it in the end, but there are a lot of inputs from other people that go into it in the meantime.
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