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The efficient-market hypothesis (EMH) is a hypothesis in
financial economics Financial economics, also known as finance, is the branch of economics characterized by a "concentration on monetary activities", in which "money of one type or another is likely to appear on ''both sides'' of a trade".William F. Sharpe"Financial ...
that states that
asset In financial accountancy, financial accounting, an asset is any resource owned or controlled by a business or an economic entity. It is anything (tangible or intangible) that can be used to produce positive economic value. Assets represent value ...
prices reflect all available information. A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since market prices should only react to new information. Because the EMH is formulated in terms of risk adjustment, it only makes testable predictions when coupled with a particular model of risk. As a result, research in financial economics since at least the 1990s has focused on market anomalies, that is, deviations from specific models of risk. The idea that financial market returns are difficult to predict goes back to Bachelier, Mandelbrot, and
Samuelson Samuelson is an English-language patronymic surname meaning "son of Samuel (name), Samuel". There are alternative spellings such as the Scandinavian language, Scandinavian-origin Samuelsson and Samuelsen. It is uncommon as a given name. Samuelson m ...
, but is closely associated with
Eugene Fama Eugene Francis "Gene" Fama (; born February 14, 1939) is an American economist, best known for his empirical work on portfolio theory, asset pricing, and the efficient-market hypothesis. He is currently Robert R. McCormick Distinguished Servi ...
, in part due to his influential 1970 review of the theoretical and empirical research. The EMH provides the basic logic for modern risk-based theories of asset prices, and frameworks such as consumption-based asset pricing and intermediary asset pricing can be thought of as the combination of a model of risk with the EMH. Many decades of empirical research on return predictability has found mixed evidence. Research in the 1950s and 1960s often found a lack of predictability (e.g. Ball and Brown 1968; Fama, Fisher, Jensen, and Roll 1969), yet the 1980s-2000s saw an explosion of discovered return predictors (e.g. Rosenberg, Reid, and Lanstein 1985; Campbell and Shiller 1988; Jegadeesh and Titman 1993). Since the 2010s, studies have often found that return predictability has become more elusive, as predictability fails to work out-of-sample (Goyal and Welch 2008), or has been weakened by advances in trading technology and investor learning (Chordia, Subrahmanyam, and Tong 2014; McLean and Pontiff 2016; Martineau 2021).


Theoretical background

Suppose that a piece of information about the value of a stock (say, about a future merger) is widely available to investors. If the price of the stock does not already reflect that information, then investors can trade on it, thereby moving the price until the information is no longer useful for trading. Note that this thought experiment does not necessarily imply that stock prices are unpredictable. For example, suppose that the piece of information in question says that a financial crisis is likely to come soon. Investors typically do not like to hold stocks during a financial crisis, and thus investors may sell stocks until the price drops enough so that the expected return compensates for this risk. How efficient markets are (and are not) linked to the
random walk In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some mathematical space. An elementary example of a random walk is the random walk on the integer number line \mathbb Z ...
theory can be described through the
fundamental theorem of asset pricing The fundamental theorems of asset pricing (also: of arbitrage, of finance), in both financial economics and mathematical finance, provide necessary and sufficient conditions for a market to be arbitrage-free, and for a market to be complete. An ...
. This theorem provides mathematical predictions regarding the price of a stock, assuming that there is no
arbitrage In economics and finance, arbitrage (, ) is the practice of taking advantage of a difference in prices in two or more markets; striking a combination of matching deals to capitalise on the difference, the profit being the difference between the ...
, that is, assuming that there is no risk-free way to trade profitably. Formally, if arbitrage is impossible, then the theorem predicts that the price of a stock is the discounted value of its future price and dividend: :P_t = E_t _ (P_+D_)/math> where E_ is the expected value given information at time t, M_ is the
stochastic discount factor The concept of the stochastic discount factor (SDF) is used in financial economics and mathematical finance. The name derives from the price of an asset being computable by "discounting" the future cash flow \tilde_i by the stochastic factor \tilde, ...
, and D_is the dividend the stock pays next period. Note that this equation does not generally imply a
random walk In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some mathematical space. An elementary example of a random walk is the random walk on the integer number line \mathbb Z ...
. However, if we assume the stochastic discount factor is constant and the time interval is short enough so that no dividend is being paid, we have :P_t = M E_t _/math>. Taking logs and assuming that the Jensen's inequality term is negligible, we have :\log P_t = \log M + E_t log P_/math> which implies that the log of stock prices follows a
random walk In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some mathematical space. An elementary example of a random walk is the random walk on the integer number line \mathbb Z ...
(with a drift). Although the concept of an efficient market is similar to the assumption that stock prices follow: E S_S_ which follows a martingale, the EMH does not always assume that stocks follow a martingale.


Empirical studies

Research by
Alfred Cowles Alfred Cowles III (September 15, 1891 – December 28, 1984) was an American economist, businessman and founder of the Cowles Commission. He graduated from Yale in 1913, where he was a member of Skull and Bones. He was the grandson of Alfred ...
in the 1930s and 1940s suggested that professional investors were in general unable to outperform the market. During the 1930s-1950s empirical studies focused on time-series properties, and found that US stock prices and related financial series followed a random walk model in the short-term. While there is some predictability over the long-term, the extent to which this is due to rational time-varying risk premia as opposed to behavioral reasons is a subject of debate. In their seminal paper, Fama, Fisher, Jensen, and Roll (1969) propose the event study methodology and show that stock prices on average react before a stock split, but have no movement afterwards.


Weak, semi-strong, and strong-form tests

In Fama's influential 1970 review paper, he categorized empirical tests of efficiency into "weak-form", "semi-strong-form", and "strong-form" tests. These categories of tests refer to the information set used in the statement "prices reflect all available information." Weak-form tests study the information contained in historical prices. Semi-strong form tests study information (beyond historical prices) which is publicly available. Strong-form tests regard private information.


Historical background

Benoit Mandelbrot Benoit B. Mandelbrot (20 November 1924 – 14 October 2010) was a Polish-born French-American mathematician and polymath with broad interests in the practical sciences, especially regarding what he labeled as "the art of roughness" of phy ...
claimed the efficient markets theory was first proposed by the French mathematician
Louis Bachelier Louis Jean-Baptiste Alphonse Bachelier (; 11 March 1870 – 28 April 1946) was a French mathematician at the turn of the 20th century. He is credited with being the first person to model the stochastic process now called Brownian motion, as part ...
in 1900 in his PhD thesis "The Theory of Speculation" describing how prices of commodities and stocks varied in markets. It has been speculated that Bachelier drew ideas from the
random walk model In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some Space (mathematics), mathematical space. An elementary example of a random walk is the random walk on the integer n ...
of
Jules Regnault Jules Augustin Frédéric Regnault (; 1 February 1834, Béthencourt – 9 December 1894, Paris) was a French stock broker's assistant who first suggested a modern theory of stock price changes i''Calcul des Chances et Philosophie de la Bourse''( ...
, but Bachelier did not cite him, and Bachelier's thesis is now considered pioneering in the field of financial mathematics. It is commonly thought that Bachelier's work gained little attention and was forgotten for decades until it was rediscovered in the 1950s by
Leonard Savage Leonard Jimmie Savage (born Leonard Ogashevitz; 20 November 1917 – 1 November 1971) was an American mathematician and statistician. Economist Milton Friedman said Savage was "one of the few people I have met whom I would unhesitatingly call a g ...
, and then become more popular after Bachelier's thesis was translated into English in 1964. But the work was never forgotten in the mathematical community, as Bachelier published a book in 1912 detailing his ideas, which was cited by mathematicians including Joseph L. Doob,
William Feller William "Vilim" Feller (July 7, 1906 – January 14, 1970), born Vilibald Srećko Feller, was a Croatian-American mathematician specializing in probability theory. Early life and education Feller was born in Zagreb to Ida Oemichen-Perc, a Croa ...
and
Andrey Kolmogorov Andrey Nikolaevich Kolmogorov ( rus, Андре́й Никола́евич Колмого́ров, p=ɐnˈdrʲej nʲɪkɐˈlajɪvʲɪtɕ kəlmɐˈɡorəf, a=Ru-Andrey Nikolaevich Kolmogorov.ogg, 25 April 1903 – 20 October 1987) was a Sovi ...
. The book continued to be cited, but then starting in the 1960s the original thesis by Bachelier began to be cited more than his book when economists started citing Bachelier's work. The concept of market efficiency had been anticipated at the beginning of the century in the dissertation submitted by Bachelier (1900) to the Sorbonne for his PhD in mathematics. In his opening paragraph, Bachelier recognizes that "past, present and even discounted future events are reflected in market price, but often show no apparent relation to price changes". The efficient markets theory was not popular until the 1960s when the advent of computers made it possible to compare calculations and prices of hundreds of stocks more quickly and effortlessly. In 1945,
F.A. Hayek Friedrich August von Hayek ( , ; 8 May 189923 March 1992), often referred to by his initials F. A. Hayek, was an Austrian–British economist, Jurisprudence, legal theorist and philosopher who is best known for his defense of classical lib ...
argued that markets were the most effective way of aggregating the pieces of information dispersed among individuals within a society. Given the ability to profit from private information, self-interested traders are motivated to acquire and act on their private information. In doing so, traders contribute to more and more efficient market prices. In the competitive limit, market prices reflect all available information and prices can only move in response to news. Thus there is a very close link between EMH and the random walk hypothesis. The efficient-market hypothesis emerged as a prominent theory in the mid-1960s.
Paul Samuelson Paul Anthony Samuelson (May 15, 1915 – December 13, 2009) was an American economist who was the first American to win the Nobel Memorial Prize in Economic Sciences. When awarding the prize in 1970, the Swedish Royal Academies stated that he "h ...
had begun to circulate Bachelier's work among economists. In 1964 Bachelier's dissertation along with the empirical studies mentioned above were published in an anthology edited by Paul Cootner. In 1965, Eugene Fama published his dissertation arguing for the random walk hypothesis. Also, Samuelson published a proof showing that if the market is efficient, prices will exhibit random-walk behavior. This is often cited in support of the efficient-market theory, by the method of affirming the consequent, however in that same paper, Samuelson warns against such backward reasoning, saying "From a nonempirical base of axioms you never get empirical results." In 1970, Fama published a review of both the theory and the evidence for the hypothesis. The paper extended and refined the theory, included the definitions for three forms of
financial market efficiency There are several concepts of efficiency for a financial market. The most widely discussed is informational or price efficiency, which is a measure of how quickly and completely the price of a single asset reflects available information about the ...
: weak, semi-strong and strong (see above).


Criticism

Investors, including the likes of
Warren Buffett Warren Edward Buffett ( ; born August 30, 1930) is an American business magnate, investor, and philanthropist. He is currently the chairman and CEO of Berkshire Hathaway. He is one of the most successful investors in the world and has a net w ...
,
George Soros George Soros ( name written in eastern order), (born György Schwartz, August 12, 1930) is a Hungarian-American businessman and philanthropist. , he had a net worth of US$8.6 billion, Note that this site is updated daily. having donated mo ...
, and researchers have disputed the efficient-market hypothesis both empirically and theoretically.
Behavioral economists Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as well ...
attribute the imperfections in financial markets to a combination of
cognitive bias A cognitive bias is a systematic pattern of deviation from norm or rationality in judgment. Individuals create their own "subjective reality" from their perception of the input. An individual's construction of reality, not the objective input, m ...
es such as
overconfidence Confidence is a state of being clear-headed either that a hypothesis or prediction is correct or that a chosen course of action is the best or most effective. Confidence comes from a Latin word 'fidere' which means "to trust"; therefore, having ...
, overreaction, representative bias, information bias, and various other predictable human errors in reasoning and information processing. These have been researched by psychologists such as
Daniel Kahneman Daniel Kahneman (; he, דניאל כהנמן; born March 5, 1934) is an Israeli-American psychologist and economist notable for his work on the psychology of judgment and decision-making, as well as behavioral economics, for which he was award ...
,
Amos Tversky Amos Nathan Tversky ( he, עמוס טברסקי; March 16, 1937 – June 2, 1996) was an Israeli cognitive and mathematical psychologist and a key figure in the discovery of systematic human cognitive bias and handling of risk. Much of his ...
and
Paul Slovic Paul Slovic (born 1938 in Chicago) is a professor of psychology at the University of Oregon and the president oDecision Research Decision Research is a collection of scientists from all over the nation and in other countries that study decision- ...
and economist
Richard Thaler Richard H. Thaler (; born September 12, 1945) is an American economist and the Charles R. Walgreen Distinguished Service Professor of Behavioral Science and Economics at the University of Chicago Booth School of Business. In 2015, Thaler was pr ...
. Empirical evidence has been mixed, but has generally not supported strong forms of the efficient-market hypothesis.Empirical papers questioning EMH: * Francis Nicholson. Price-Earnings Ratios in Relation to Investment Results. ''Financial Analysts Journal''. Jan/Feb 1968:105–109. * * Rosenberg B, Reid K, Lanstein R. (1985). Persuasive Evidence of Market Inefficiency. ''Journal of Portfolio Management'' 13:9–17. According to Dreman and Berry, in a 1995 paper, low P/E ( price-to-earnings) stocks have greater returns. In an earlier paper, Dreman also refuted the assertion by Ray Ball that these higher returns could be attributed to higher
beta Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...
,; Dreman's research had been accepted by efficient market theorists as explaining the anomaly Dreman D. (1998). ''Contrarian Investment Strategy: The Next Generation''. Simon and Schuster. in neat accordance with
modern portfolio theory Modern portfolio theory (MPT), or mean-variance analysis, is a mathematical framework for assembling a portfolio of assets such that the expected return is maximized for a given level of risk. It is a formalization and extension of diversificatio ...
.


Behavioral psychology

Behavioral psychology approaches to stock market trading are among some of the more promising alternatives to EMH (investment strategies such as momentum trading seek to exploit exactly such inefficiencies). But Nobel Laureate co-founder of the programme Daniel Kahneman —announced his skepticism of investors beating the market: "They're just not going to do it. It's just not going to happen." Indeed, defenders of EMH maintain that Behavioral Finance strengthens the case for EMH in that it highlights biases in individuals and committees and not competitive markets. For example, one prominent finding in Behavioral Finance is that individuals employ
hyperbolic discounting In economics, hyperbolic discounting is a time-''inconsistent'' model of delay discounting. It is one of the cornerstones of behavioral economics and its brain-basis is actively being studied by neuroeconomics researchers. According to the disco ...
. It is demonstrably true that bonds,
mortgages A mortgage loan or simply mortgage (), in civil law jurisdicions known also as a hypothec loan, is a loan used either by purchasers of real property to raise funds to buy real estate, or by existing property owners to raise funds for any pu ...
,
annuities In investment, an annuity is a series of payments made at equal intervals.Kellison, Stephen G. (1970). ''The Theory of Interest''. Homewood, Illinois: Richard D. Irwin, Inc. p. 45 Examples of annuities are regular deposits to a savings account, mo ...
and other similar obligations subject to competitive market forces do not. Any manifestation of
hyperbolic discounting In economics, hyperbolic discounting is a time-''inconsistent'' model of delay discounting. It is one of the cornerstones of behavioral economics and its brain-basis is actively being studied by neuroeconomics researchers. According to the disco ...
in the pricing of these obligations would invite
arbitrage In economics and finance, arbitrage (, ) is the practice of taking advantage of a difference in prices in two or more markets; striking a combination of matching deals to capitalise on the difference, the profit being the difference between the ...
thereby quickly eliminating any vestige of individual biases. Similarly,
diversification Diversification may refer to: Biology and agriculture * Genetic divergence, emergence of subpopulations that have accumulated independent genetic changes * Agricultural diversification involves the re-allocation of some of a farm's resources to n ...
, derivative securities and other hedging strategies assuage if not eliminate potential mispricings from the severe risk-intolerance (
loss aversion Loss aversion is the tendency to prefer avoiding losses to acquiring equivalent gains. The principle is prominent in the domain of economics. What distinguishes loss aversion from risk aversion is that the utility of a monetary payoff depends o ...
) of individuals underscored by behavioral finance. On the other hand, economists, behavioral psychologists and mutual fund managers are drawn from the human population and are therefore subject to the biases that behavioralists showcase. By contrast, the price signals in markets are far less subject to individual biases highlighted by the Behavioral Finance programme. Richard Thaler has started a fund based on his research on cognitive biases. In a 2008 report he identified
complexity Complexity characterises the behaviour of a system or model whose components interaction, interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence. The term is generall ...
and
herd behavior Herd behavior is the behavior of individuals in a group acting collectively without centralized direction. Herd behavior occurs in animals in herds, packs, bird flocks, fish schools and so on, as well as in humans. Voting, demonstrations, rio ...
as central to the
global financial crisis of 2008 Global means of or referring to a globe and may also refer to: Entertainment * ''Global'' (Paul van Dyk album), 2003 * ''Global'' (Bunji Garlin album), 2007 * ''Global'' (Humanoid album), 1989 * ''Global'' (Todd Rundgren album), 2015 * Bruno ...
. Further empirical work has highlighted the impact transaction costs have on the concept of market efficiency, with much evidence suggesting that any anomalies pertaining to market inefficiencies are the result of a cost benefit analysis made by those willing to incur the cost of acquiring the valuable information in order to trade on it. Additionally, the concept of
liquidity Liquidity is a concept in economics involving the convertibility of assets and obligations. It can include: * Market liquidity, the ease with which an asset can be sold * Accounting liquidity, the ability to meet cash obligations when due * Liqui ...
is a critical component to capturing "inefficiencies" in tests for abnormal returns. Any test of this proposition faces the joint hypothesis problem, where it is impossible to ever test for market efficiency, since to do so requires the use of a measuring stick against which abnormal returns are compared —one cannot know if the market is efficient if one does not know if a model correctly stipulates the required rate of return. Consequently, a situation arises where either the asset pricing model is incorrect or the market is inefficient, but one has no way of knowing which is the case. The performance of stock markets is correlated with the amount of sunshine in the city where the main exchange is located.


EMH anomalies and rejection of the Capital Asset Pricing Model (CAPM)

While event studies of
stock split A stock split or stock divide increases the number of shares in a company. For example, after a 2-for-1 split, each investor will own double the number of shares, and each share will be worth half as much. A stock split causes a decrease of mark ...
s are consistent with the EMH (Fama, Fisher, Jensen, and Roll, 1969), other empirical analyses have found problems with the efficient-market hypothesis. Early examples include the observation that small neglected stocks and stocks with high book-to-market (low price-to-book) ratios (value stocks) tended to achieve abnormally high returns relative to what could be explained by the
CAPM CAPM may refer to: * Capital asset pricing model, a fundamental model in finance * Certified Associate in Project Management, an entry-level credential for project managers {{Disambig ...
. Further tests of portfolio efficiency by Gibbons, Ross and Shanken (1989) (GJR) led to rejections of the CAPM, although tests of efficiency inevitably run into the joint hypothesis problem (see
Roll's critique Roll's critique is a famous analysis of the validity of empirical tests of the capital asset pricing model (CAPM) by Richard Roll. It concerns methods to formally test the statement of the CAPM, the equation :E(R_i) = R_f + \beta_ (R_m) - R_f\, T ...
). Following GJR's results and mounting empirical evidence of EMH anomalies, academics began to move away from the CAPM towards risk factor models such as the Fama-French 3 factor model. These risk factor models are not properly founded on economic theory (whereas CAPM is founded on
Modern Portfolio Theory Modern portfolio theory (MPT), or mean-variance analysis, is a mathematical framework for assembling a portfolio of assets such that the expected return is maximized for a given level of risk. It is a formalization and extension of diversificatio ...
), but rather, constructed with long-short portfolios in response to the observed empirical EMH anomalies. For instance, the "small-minus-big" (SMB) factor in the FF3 factor model is simply a portfolio that holds long positions on small stocks and short positions on large stocks to mimic the risks small stocks face. These risk factors are said to represent some aspect or dimension of undiversifiable systematic risk which should be compensated with higher expected returns. Additional popular risk factors include the "HML" value factor (Fama and French, 1993); "MOM" momentum factor (Carhart, 1997); "ILLIQ" liquidity factors (Amihud et al. 2002). See also Robert Haugen.


View of some journalists, economists, and investors

Economists Matthew Bishop and Michael Green claim that full acceptance of the hypothesis goes against the thinking of
Adam Smith Adam Smith (baptized 1723 – 17 July 1790) was a Scottish economist and philosopher who was a pioneer in the thinking of political economy and key figure during the Scottish Enlightenment. Seen by some as "The Father of Economics"——— ...
and
John Maynard Keynes John Maynard Keynes, 1st Baron Keynes, ( ; 5 June 1883 – 21 April 1946), was an English economist whose ideas fundamentally changed the theory and practice of macroeconomics and the economic policies of governments. Originally trained in ...
, who both believed irrational behavior had a real impact on the markets. Economist
John Quiggin John Quiggin (born 29 March 1956) is an Australian economist, a professor at the University of Queensland. He was formerly an Australian Research Council Laureate Fellow and Federation Fellow and a member of the board of the Climate Change Aut ...
has claimed that "
Bitcoin Bitcoin ( abbreviation: BTC; sign: ₿) is a decentralized digital currency that can be transferred on the peer-to-peer bitcoin network. Bitcoin transactions are verified by network nodes through cryptography and recorded in a public distr ...
is perhaps the finest example of a pure
bubble Bubble, Bubbles or The Bubble may refer to: Common uses * Bubble (physics), a globule of one substance in another, usually gas in a liquid ** Soap bubble * Economic bubble, a situation where asset prices are much higher than underlying fundame ...
", and that it provides a conclusive refutation of EMH. While other assets that have been used as currency (such as gold, tobacco) have value or utility independent of people's willingness to accept them as payment, Quiggin argues that "in the case of Bitcoin there is no source of value whatsoever" and thus Bitcoin should be priced at zero or worthless.
Tshilidzi Marwala Tshilidzi Marwala (born 28 July 1971) is a South African artificial intelligence engineer, a computer scientist, a mechanical engineer and a university administrator. Early life and education Marwala was born at Duthuni Village in the Li ...
surmised that
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
(AI) influences the applicability of the efficient market hypothesis in that the greater amount of AI-based market participants, the more efficient the markets become.
Warren Buffett Warren Edward Buffett ( ; born August 30, 1930) is an American business magnate, investor, and philanthropist. He is currently the chairman and CEO of Berkshire Hathaway. He is one of the most successful investors in the world and has a net w ...
has also argued against EMH, most notably in his 1984 presentation "
The Superinvestors of Graham-and-Doddsville "The Superinvestors of Graham-and-Doddsville" is an article by Warren Buffett promoting value investing, published in the Fall, 1984 issue of ''Hermes'', Columbia Business School magazine. It was based on a speech given on May 17, 1984, at the Col ...
". He says preponderance of value investors among the world's money managers with the highest rates of performance rebuts the claim of EMH proponents that luck is the reason some investors appear more successful than others. Nonetheless, Buffett has recommended
index funds An index fund (also index tracker) is a mutual fund or exchange-traded fund (ETF) designed to follow certain preset rules so that the fund can a specified basket of underlying investments.Reasonable Investor(s), Boston University Law Review, avail ...
that aim to track average market returns for most investors. Buffett's business partner
Charlie Munger Charles Thomas Munger (born January 1, 1924) is an American billionaire investor, businessman, and former real estate attorney. He is vice chairman of Berkshire Hathaway, the conglomerate controlled by Warren Buffett; Buffett has described Mung ...
has stated the EMH is "obviously roughly correct", in that a hypothetical average investor will tend towards average results "and it's quite hard for anybody to onsistentlybeat the market by significant margins". However, Munger also believes "extreme" commitment to the EMH is "bonkers", as the theory's originators were seduced by an "intellectually consistent theory that allowed them to do pretty mathematics etthe fundamentals did not properly tie to reality."
Burton Malkiel Burton Gordon Malkiel (born August 28, 1932) is an American economist and writer most noted for his classic finance book ''A Random Walk Down Wall Street'' (first published 1973, in its 12th edition as of 2019). He is a leading proponent of the ef ...
in his ''
A Random Walk Down Wall Street ''A Random Walk Down Wall Street'', written by Burton Gordon Malkiel, a Princeton University economist, is a book on the subject of stock markets which popularized the random walk hypothesis. Malkiel argues that asset prices typically exhibit s ...
'' (1973) argues that "the preponderance of statistical evidence" supports EMH, but admits there are enough "gremlins lurking about" in the data to prevent EMH from being conclusively proved. In his book ''
The Reformation in Economics ''The Reformation in Economics'' is a book written by the Irish economist Philip Pilkington. It is a book that aims to deconstruct contemporary neoclassical economic theory in order to determine to what extent it is scientific and to what extent ...
'', economist and financial analyst
Philip Pilkington Philip Pilkington is an Irish economist working in investment finance. He became well-known for his critiques of neoclassical economics on his blog ''Fixing the Economists''. Since then he has written a book entitled ''The Reformation in Econom ...
has argued that the EMH is actually a tautology masquerading as a theory. He argues that, taken at face value, the theory makes the banal claim that the average investor will not beat the market average—which is a tautology. When pressed on this point, Pinkington argues that EMH proponents will usually say that any ''actual investor'' will converge with the ''average investor'' given enough time and so no investor will beat the market average. But Pilkington points out that when proponents of the theory are presented with evidence that a small minority of investors do, in fact, beat the market over the long-run, these proponents then say that these investors were simply 'lucky'. Pilkington argues that introducing the idea that anyone who diverges from the theory is simply 'lucky' insulates the theory from falsification and so, drawing on the philosopher of science and critic of neoclassical economics
Hans Albert Hans Albert (born 8 February 1921) is a German philosopher. Born in Cologne, he lives in Heidelberg. His fields of research are Social Sciences and General Studies of Methods. He is a critical rationalist, paying special attention to rational ...
, Pilkington argues that the theory falls back into being a tautology or a pseudoscientific construct. Nobel Prize-winning economist
Paul Samuelson Paul Anthony Samuelson (May 15, 1915 – December 13, 2009) was an American economist who was the first American to win the Nobel Memorial Prize in Economic Sciences. When awarding the prize in 1970, the Swedish Royal Academies stated that he "h ...
argued that the stock market is "micro efficient" but not "macro efficient": the EMH is much better suited for individual stocks than it is for the aggregate stock market as a whole. Research based on regression and scatter diagrams, published in 2005, has strongly supported Samuelson's dictum.
Peter Lynch Peter Lynch (born January 19, 1944) is an American investor, mutual fund manager, and philanthropist. As the manager of the Magellan Fund at Fidelity Investments between 1977 and 1990, Lynch averaged a 29.2% annual return, consistently more th ...
, a mutual fund manager at
Fidelity Investments Fidelity Investments, commonly referred to as Fidelity, earlier as Fidelity Management & Research or FMR, is an American multinational financial services corporation based in Boston, Massachusetts. The company was established in 1946 and is on ...
who consistently more than doubled market averages while managing the
Magellan Fund The Fidelity Magellan Fund () is a U.S.-domiciled mutual fund from the Fidelity family of funds. It is perhaps the world's best-known actively managed mutual fund, known particularly for its record-setting growth under the management of Peter Lyn ...
, has argued that the EMH is contradictory to the random walk hypothesis—though both concepts are widely taught in business schools without seeming awareness of a contradiction. If asset prices are rational and based on all available data as the efficient market hypothesis proposes, then fluctuations in asset price are ''not'' random. But if the random walk hypothesis is valid, then asset prices are not rational. Joel Tillinghast, also a fund manager at Fidelity with a long history of outperforming a benchmark, has written that the core arguments of the EMH are "more true than not" and he accepts a "sloppy" version of the theory allowing for a margin of error. But he also contends the EMH is not completely accurate or accurate in all cases, given the recurrent existence of
economic bubble An economic bubble (also called a speculative bubble or a financial bubble) is a period when current asset prices greatly exceed their intrinsic valuation, being the valuation that the underlying long-term fundamentals justify. Bubbles can be c ...
s (when some assets are dramatically overpriced) and the fact that value investors (who focus on underpriced assets) have tended to outperform the broader market over long periods. Tillinghast also asserts that even staunch EMH proponents will admit weaknesses to the theory when assets are significantly over- or under-priced, such as double or half their value according to
fundamental analysis Fundamental analysis, in accounting and finance, is the analysis of a business's financial statements (usually to analyze the business's assets, liabilities, and earnings); health; and competitors and markets. It also considers the overall state ...
. In a 2012 book, investor
Jack Schwager Jack D. Schwager (born 1948) is an American trader and author. His books include ''Market Wizards'' (1989), ''The New Market Wizards'' (1992), ''Stock Market Wizards'' (2001) and ''Unknown Market Wizards: The best traders you've never heard of'' ...
argues the EMH is "right for the wrong reasons". He agrees it is "very difficult" to consistently beat average market returns, but contends it's not due to how information is distributed more or less instantly to all market participants. Information may be distributed more or less instantly, but Schwager proposes information may not be interpreted or applied in the same way by different people and skill may play a factor in how information is used. Schwager argues markets are difficult to beat because of the unpredictable and sometimes irrational behavior of humans who buy and sell assets in the stock market. Schwager also cites several instances of mispricing that he contends are impossible according to a strict or strong interpretation of the EMH.


Late 2000s financial crisis

The
financial crisis of 2007–08 Finance is the study and discipline of money, currency and capital assets. It is related to, but not synonymous with economics, the study of production, distribution, and consumption of money, assets, goods and services (the discipline of fina ...
led to renewed scrutiny and criticism of the hypothesis. Market strategist
Jeremy Grantham Robert Jeremy Goltho Grantham (born 6 October 1938) is a British investor and co-founder and chief investment strategist of Grantham, Mayo, & van Otterloo (GMO), a Boston-based asset management firm. GMO had more than US$118 billion in a ...
said the EMH was responsible for the current financial crisis, claiming that belief in the hypothesis caused financial leaders to have a "chronic underestimation of the dangers of asset bubbles breaking". Financial journalist Roger Lowenstein said "The upside of the current
Great Recession The Great Recession was a period of marked general decline, i.e. a recession, observed in national economies globally that occurred from late 2007 into 2009. The scale and timing of the recession varied from country to country (see map). At ...
is that it could drive a stake through the heart of the academic nostrum known as the efficient-market hypothesis." Former
Federal Reserve The Federal Reserve System (often shortened to the Federal Reserve, or simply the Fed) is the central banking system of the United States of America. It was created on December 23, 1913, with the enactment of the Federal Reserve Act, after a ...
chairman
Paul Volcker Paul Adolph Volcker Jr. (September 5, 1927 – December 8, 2019) was an American economist who served as the 12th chairman of the Federal Reserve from 1979 to 1987. During his tenure as chairman, Volcker was widely credited with having ended the ...
said "It should be clear that among the causes of the recent financial crisis was an unjustified faith in rational expectations, market efficiencies, and the techniques of modern finance." One financial analyst said "By 2007–2009, you had to be a fanatic to believe in the literal truth of the EMH." At the International Organization of Securities Commissions annual conference, held in June 2009, the hypothesis took center stage.
Martin Wolf Martin Harry Wolf (born 16 August 1946 in London) is a British journalist of Austrian-Dutch descent who focuses on economics. He is the associate editor and chief economics commentator at the ''Financial Times''. Early life Wolf was born in ...
, the chief economics commentator for the ''
Financial Times The ''Financial Times'' (''FT'') is a British daily newspaper printed in broadsheet and published digitally that focuses on business and economic current affairs. Based in London, England, the paper is owned by a Japanese holding company, Nik ...
'', dismissed the hypothesis as being a useless way to examine how markets function in reality. Economist
Paul McCulley Paul Allen McCulley (born March 13, 1957) is an American economist and former managing director at PIMCO. He coined the terms " Minsky moment" and "shadow banking system", which became famous during the Financial crisis of 2007–2009. He is curr ...
said the hypothesis had not failed, but was "seriously flawed" in its neglect of human nature. The financial crisis led economics scholar
Richard Posner Richard Allen Posner (; born January 11, 1939) is an American jurist and legal scholar who served as a federal appellate judge on the U.S. Court of Appeals for the Seventh Circuit from 1981 to 2017. A senior lecturer at the University of Chica ...
to back away from the hypothesis. Posner accused some of his Chicago School colleagues of being "asleep at the switch", saying that "the movement to deregulate the financial industry went too far by exaggerating the resilience—the self healing powers—of laissez-faire capitalism." Others, such as economist and Nobel laurete
Eugene Fama Eugene Francis "Gene" Fama (; born February 14, 1939) is an American economist, best known for his empirical work on portfolio theory, asset pricing, and the efficient-market hypothesis. He is currently Robert R. McCormick Distinguished Servi ...
, said that the hypothesis held up well during the crisis: "Stock prices typically decline prior to a recession and in a state of recession. This was a particularly severe recession. Prices started to decline in advance of when people recognized that it was a recession and then continued to decline. That was exactly what you would expect if markets are efficient." Despite this, Fama said that "poorly informed investors could theoretically lead the market astray" and that stock prices could become "somewhat irrational" as a result.


Efficient markets applied in securities class action litigation

The theory of efficient markets has been practically applied in the field of
Securities Class Action A securities class action (SCA), or securities fraud class action, is a lawsuit filed by investors who bought or sold a company's publicly traded securities within a specific period of time (known as a “class period”) and suffered economic in ...
Litigation. Efficient market theory, in conjunction with "
fraud-on-the-market theory The fraud-on-the-market theory is the idea that stock prices are a function of all material information about the company and its business. It applies to securities markets, where it can be assumed that all material information is available to inv ...
", has been used in
Securities Class Action A securities class action (SCA), or securities fraud class action, is a lawsuit filed by investors who bought or sold a company's publicly traded securities within a specific period of time (known as a “class period”) and suffered economic in ...
Litigation to both justify and as mechanism for the calculation of damages. In the Supreme Court Case, Halliburton v. Erica P. John Fund, U.S. Supreme Court, No. 13-317, the use of efficient market theory in supporting securities class action litigation was affirmed. Supreme Court Justice Roberts wrote that "the court's ruling was consistent with the ruling in '
Basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
' because it allows '
direct evidence Direct evidence supports the truth of an assertion (in criminal law, an assertion of guilt or of innocence) directly, i.e., without an intervening inference. A witness relates what they directly experienced, usually by sight or hearing, but also p ...
when such evidence is available' instead of relying exclusively on the efficient markets theory."


See also

*
Adaptive market hypothesis The adaptive market hypothesis, as proposed by Andrew Lo,Lo, 2004. is an attempt to reconcile economic theories based on the efficient market hypothesis (which implies that markets are efficient) with behavioral economics, by applying the principl ...
*
Dumb agent theory The dumb agent theory (DAT) states that many people making individual buying and selling decisions will better reflect true value than any one individual can. In finance this theory is predicated on the efficient-market hypothesis (EMH). One of th ...
*
Financial market efficiency There are several concepts of efficiency for a financial market. The most widely discussed is informational or price efficiency, which is a measure of how quickly and completely the price of a single asset reflects available information about the ...
*
Grossman-Stiglitz Paradox The Grossman-Stiglitz Paradox is a paradox introduced by Sanford J. Grossman and Joseph Stiglitz in a joint publication in ''American Economic Review'' in 1980 that argues perfectly informationally efficient markets are an impossibility since, if pr ...
*
Index fund An index fund (also index tracker) is a mutual fund or exchange-traded fund (ETF) designed to follow certain preset rules so that the fund can a specified basket of underlying investments.Reasonable Investor(s), Boston University Law Review, avail ...
*
Insider trading Insider trading is the trading of a public company's stock or other securities (such as bonds or stock options) based on material, nonpublic information about the company. In various countries, some kinds of trading based on insider information ...
*
Investment theory Investment is the dedication of money to purchase of an asset to attain an increase in value over a period of time. Investment requires a sacrifice of some present asset, such as time, money, or effort. In finance, the purpose of investing is ...
*
Noisy market hypothesis In finance, the noisy market hypothesis contrasts the efficient-market hypothesis in that it claims that the prices of securities are not always the best estimate of the true underlying value of the firm. It argues that prices can be influenced by ...
*
Perfect market In economics, specifically general equilibrium theory, a perfect market, also known as an atomistic market, is defined by several idealizing conditions, collectively called perfect competition, or atomistic competition. In theoretical models whe ...
*
Transparency (market) In economics, a market is transparent if much is known by many about: What products and services or capital assets are available, market depth (quantity available), what price, and where. Transparency is important since it is one of the theoretica ...
* Random walk hypothesis


References


Further reading

* Bogle, John (1994). ''Bogle on Mutual Funds: New Perspectives for the Intelligent Investor'', Dell, * * * * Lo, Andrew and MacKinlay, Craig (2001). ''A Non-random Walk Down Wall St.'' Princeton Paperbacks * Malkiel, Burton G. (1987). "efficient market hypothesis," ''The New Palgrave: A Dictionary of Economics'', v. 2, pp. 120–23. * Malkiel, Burton G. (1996). ''A Random Walk Down Wall Street'', W. W. Norton, * Pilkington, P (2017). The Reformation in Economics: A Deconstruction and Reconstruction of Economic Theory. Palgrave Macmillan. * Samuelson, Paul (1972). "Proof That Properly Anticipated Prices Fluctuate Randomly." ''Industrial Management Review'', Vol. 6, No. 2, pp. 41–49. Reproduced as Chapter 198 in ''Samuelson, Collected Scientific Papers'', Volume III, Cambridge, M.I.T. Press. * Sharpe, William F
"The Arithmetic of Active Management"
* *Martineau, Charles (2021)
"Rest in Peace Post-Earnings Announcement Drift"
Critical Finance Review, Forthcoming.


External links


e-m-h.org

"Earnings Quality and the Equity Risk Premium: A Benchmark Model"
abstract from Contemporary Accounting Research
"The Persistence of Pricing Inefficiencies in the Stock Markets of the Eastern European EU Nations"
abstract from Economic and Business Review

Remarks by John Bogle on the superior returns of passively managed index funds.

Paul Samuelson
Human Behavior and the Efficiency of the Financial System (1999) by Robert J. Shiller Handbook of Macroeconomics
{{DEFAULTSORT:Efficient-Market Hypothesis 1900 introductions Behavioral finance