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- In 1984, Amos and I and our friend Richard Thaler visited a Wall Street firm. Our host, a
- senior investment manager, had invited us to discuss the role of judgment biases in
- investing. I knew so little about finance that I did not even know what to ask him, but I
- remember one exchange. “When you sell a stock,” d nI asked, “who buys it?” He
- answered with a wave in the vague direction of the window, indicating that he expected
- the buyer to be someone else very much like him. That was odd: What made one person
- buy and the other sell? What did the sellers think they knew that the buyers did not?
- Since then, my questions about the stock market have hardened into a larger puzzle: a
- major industry appears to be built largely on an illusion of skill. Billions of shares are
- traded every day, with many people buying each stock and others selling it to them. It is
- not unusual for more than 100 million shares of a single stock to change hands in one day.
- Most of the buyers and sellers know that they have the same information; they exchange
- the stocks primarily because they have different opinions. The buyers think the price is too
- low and likely to rise, while the sellers think the price is high and likely to drop. The
- puzzle is why buyers and sellers alike think that the current price is wrong. What makes
- them believe they know more about what the price should be than the market does? For
- most of them, that belief is an illusion.
- In its broad outlines, the standard theory of how the stock market works is accepted
- by all the participants in the industry. Everybody in the investment business has read
- Burton Malkiel’s wonderful book A Random Walk Down Wall Street. Malkiel’s central
- idea is that a stock’s price incorporates all the available knowledge about the value of the
- company and the best predictions about the future of the stock. If some people believe that
- the price of a stock will be higher tomorrow, they will buy more of it today. This, in turn,
- will cause its price to rise. If all assets in a market are correctly priced, no one can expect
- either to gain or to lose by trading. Perfect prices leave no scope for cleverness, but they
- also protect fools from their own folly. We now know, however, that the theory is not quite
- right. Many individual investors lose consistently by trading, an achievement that a dartthrowing
- chimp could not match. The first demonstration of this startling conclusion was
- collected by Terry Odean, a finance professor at UC Berkeley who was once my student.
- Odean began by studying the trading records of 10,000 brokerage accounts of
- individual investors spanning a seven-year period. He was able to analyze every
- transaction the investors executed through that firm, nearly 163,000 trades. This rich set of
- data allowed Odean to identify all instances in which an investor sold some of his holdings
- in one stock and soon afterward bought another stock. By these actions the investor
- revealed that he (most of the investors were men) had a definite idea about the future of
- the two stocks: he expected the stock that he chose to buy to do better than the stock he
- chose to sell.
- To determine whether those ideas were well founded, Odean compared the returns of
- the stock the investor had sold and the stock he had bought in its place, over the course of
- one year after the transaction. The results were unequivocally bad. On average, the shares
- that individual traders sold did better than those they bought, by a very substantial margin:
- 3.2 percentage points per year, above and beyond the significant costs of executing the
- two trades.
- It is important to remember that this is a statement about averages: some individuals
- did much better, others did much worse. However, it is clear that for the large majority of
- individual investors, taking a shower and doing nothing would have been a better policy
- than implementing the ideas that came to their minds. Later research by Odean and his
- colleague Brad Barber supported this conclusion. In a paper titled “Trading Is Hazardous
- to Yourt-t Wealth,” they showed that, on average, the most active traders had the poorest
- results, while the investors who traded the least earned the highest returns. In another
- paper, titled “Boys Will Be Boys,” they showed that men acted on their useless ideas
- significantly more often than women, and that as a result women achieved better
- investment results than men.
- Of course, there is always someone on the other side of each transaction; in general,
- these are financial institutions and professional investors, who are ready to take advantage
- of the mistakes that individual traders make in choosing a stock to sell and another stock
- to buy. Further research by Barber and Odean has shed light on these mistakes. Individual
- investors like to lock in their gains by selling “winners,” stocks that have appreciated since
- they were purchased, and they hang on to their losers. Unfortunately for them, recent
- winners tend to do better than recent losers in the short run, so individuals sell the wrong
- stocks. They also buy the wrong stocks. Individual investors predictably flock to
- companies that draw their attention because they are in the news. Professional investors
- are more selective in responding to news. These findings provide some justification for the
- label of “smart money” that finance professionals apply to themselves.
- Although professionals are able to extract a considerable amount of wealth from
- amateurs, few stock pickers, if any, have the skill needed to beat the market consistently,
- year after year. Professional investors, including fund managers, fail a basic test of skill:
- persistent achievement. The diagnostic for the existence of any skill is the consistency of
- individual differences in achievement. The logic is simple: if individual differences in any
- one year are due entirely to luck, the ranking of investors and funds will vary erratically
- and the year-to-year correlation will be zero. Where there is skill, however, the rankings
- will be more stable. The persistence of individual differences is the measure by which we
- confirm the existence of skill among golfers, car salespeople, orthodontists, or speedy toll
- collectors on the turnpike.
- Mutual funds are run by highly experienced and hardworking professionals who buy
- and sell stocks to achieve the best possible results for their clients. Nevertheless, the
- evidence from more than fifty years of research is conclusive: for a large majority of fund
- managers, the selection of stocks is more like rolling dice than like playing poker.
- Typically at least two out of every three mutual funds underperform the overall market in
- any given year.
- More important, the year-to-year correlation between the outcomes of mutual funds is
- very small, barely higher than zero. The successful funds in any given year are mostly
- lucky; they have a good roll of the dice. There is general agreement among researchers
- that nearly all stock pickers, whether they know it or not—and few of them do—are
- playing a game of chance. The subjective experience of traders is that they are making
- sensible educated guesses in a situation of great uncertainty. In highly efficient markets,
- however, educated guesses are no more accurate than blind guesses.
- Some years ago I had an unusual opportunity to examine the illusion of financial skill up
- close. I had been invited to speak to a group of investment advisers in a firm that provided
- financial advice and other services to very wealthy clients. I asked for some data to
- prepare my presentation and was granted a small treasure: a spreadsheet summarizing the
- investment outcomes of some twenty-five anonymous wealth advisers, for each of eight
- consecutive years. Each adviser’s scoof re for each year was his (most of them were
- men) main determinant of his year-end bonus. It was a simple matter to rank the advisers
- by their performance in each year and to determine whether there were persistent
- differences in skill among them and whether the same advisers consistently achieved
- better returns for their clients year after year.
- To answer the question, I computed correlation coefficients between the rankings in
- each pair of years: year 1 with year 2, year 1 with year 3, and so on up through year 7 with
- year 8. That yielded 28 correlation coefficients, one for each pair of years. I knew the
- theory and was prepared to find weak evidence of persistence of skill. Still, I was
- surprised to find that the average of the 28 correlations was .01. In other words, zero. The
- consistent correlations that would indicate differences in skill were not to be found. The
- results resembled what you would expect from a dice-rolling contest, not a game of skill.
- No one in the firm seemed to be aware of the nature of the game that its stock pickers
- were playing. The advisers themselves felt they were competent professionals doing a
- serious job, and their superiors agreed. On the evening before the seminar, Richard Thaler
- and I had dinner with some of the top executives of the firm, the people who decide on the
- size of bonuses. We asked them to guess the year-to-year correlation in the rankings of
- individual advisers. They thought they knew what was coming and smiled as they said
- “not very high” or “performance certainly fluctuates.” It quickly became clear, however,
- that no one expected the average correlation to be zero.
- Our message to the executives was that, at least when it came to building portfolios,
- the firm was rewarding luck as if it were skill. This should have been shocking news to
- them, but it was not. There was no sign that they disbelieved us. How could they? After
- all, we had analyzed their own results, and they were sophisticated enough to see the
- implications, which we politely refrained from spelling out. We all went on calmly with
- our dinner, and I have no doubt that both our findings and their implications were quickly
- swept under the rug and that life in the firm went on just as before. The illusion of skill is
- not only an individual aberration; it is deeply ingrained in the culture of the industry. Facts
- that challenge such basic assumptions—and thereby threaten people’s livelihood and selfesteem—
- are simply not absorbed. The mind does not digest them. This is particularly true
- of statistical studies of performance, which provide base-rate information that people
- generally ignore when it clashes with their personal impressions from experience.
- The next morning, we reported the findings to the advisers, and their response was
- equally bland. Their own experience of exercising careful judgment on complex problems
- was far more compelling to them than an obscure statistical fact. When we were done, one
- of the executives I had dined with the previous evening drove me to the airport. He told
- me, with a trace of defensiveness, “I have done very well for the firm and no one can take
- that away from me.” I smiled and said nothing. But I thought, “Well, I took it away from
- you this morning. If your success was due mostly to chance, how much credit are you
- entitled to take for it?”
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