Efficient Market Hypothesis

Efficient Market Hypothesis Efficient Market Hypothesis (EMH) is the theory behind efficient capital markets. An efficient capital market is one in which security prices reflect and rapidly adjust to all new information. The derivation of the EMH is mostly credited to the work of Fama. In 1965 the doctoral dissertation written by Fama was republished. In this Fama looks at the current literature on stock price behaviour and examines the distribution and dependence of stock price returns.

He concluded that, ‘it seems safe to say that this paper has presented strong and voluminous evidence in favour of the random walk hypothesis. Due to a better understanding of price formation in competitive markets, the random walk model was now seen as a set of observations that can be consistent with the efficient markets hypothesis. This switch began with observations published in a paper by Samuelson in 1965.

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Samuelson presented his proof in the general form, which helped in the understanding of the notion of a well-functioning market.His paper had the observation ‘in competitive markets there is a buyer for every seller. If one could be sure that a price would rise, it would have already risen. Samuelson stated that ‘arguments like this are used to deduce that competitive prices must display price changes…

that perform a random walk with no predictable bias. ‘ Following on by the work done by Samuelson, as mentioned in the previous paragraph, a paper was published by Fama in 1970. This paper consisted of a comprehensive review of the theory and evidence of market efficiency.

He defined an efficient market as ‘one in which trading on available information fails to provide an abnormal profit. ‘ This paper was one of the firsts to distinguish between the three forms of market efficiency.The three forms of market efficiency are the weak form, semi-strong form and strong form. He concluded that the results are strongly in support of the weak form of market efficiency and that in short, the evidence in support of the efficient markets model is extensive, and (somewhat uniquely in economics) contradictory evidence is sparse.

I will now summarise some papers that have been written on the criticism of the EMH. Although there has been a vast amount of literature published on the development and the support of the efficient market theory, there has also been various studies published criticising the EMH.This criticism comes about due to the fact that the EMH is difficult to test. A number of studies indicate anomalous behaviour, which appears to be inconsistent with market efficiency. Such anomalies include the small firm effect as talked about in a paper by Banz in 1981. Banz analysed monthly returns over the period 1931-75 on shares listed on the New York Stock Exchange. Over this interval, the fifty smallest stocks outperformed the fifty largest by an average of one percentage point per month, on a risk-adjusted basis.

After the publication of this paper, many other authors published their own papers examining the subject of the small firm effect. A paper by Ball in 1978 points out that the evidence could equally indicate the shortcomings of the models of expected return. A paper by Fama in 1998 concludes that further study should not be done on developing behavioural based theories of stock markets that take into account the apparent anomalies, but that search for better asset pricing models should take president. There is also the area of behavioural finance that criticises EMH.

I will look at this in more depth in the next section. Market Bubble While the EMH is generally regarded as the best theory that can describe the actions of market prices it is not perfect and sometimes events occur that contradict the EMH. One of these events is that of the bubble. A bubble is when a specific industry’s market prices do really well, so well that prices seem to rise higher than the EMH dictates. Eventually, the bubble bursts and prices return to a price more in line with EMH.

One famous bubble was that of the dot. com bubble. EMH does not explain why this bubble exists in the first place.This is one of the major criticisms of the EMH.

Many academics have turned to the relatively new theory of behavioural finance to explain the bubble. Behavioural Finance One area that has recently undermined the EMH is the work published looking at behavioural finance. As observed by Shleifer (2000) ‘At the most general level, behavioural finance is the study of human fallibility in competitive markets. ‘ Behavioural finance incorporates elements of cognitive psychology into finance in an effort to better understand how individuals and entire markets respond to different circumstances.

Behavioural finance is based on the principle that all investors are not rational. Some investors can be over-confident, while other less knowledgeable investors might be prone to herding effects. Shefrin (1999) was one such author to talk about behavioural finance. He is one author who argues that ‘a few psychological phenomena pervade the entire landscape of finance. ‘ Harrington (2003) agrees with the notion that overconfidence can lead to irrational behaviour.

She states that ‘investors can become irrational and their irrational behaviour affects their ability to profit from owning stocks and bonds. ‘ Of course, behavioural finance does have its draw backs. One of which is the fact that using instincts alone can result in a loss. This is due to human error. The person that is using their instincts in determining where to invest might not have the greatest financial knowledge in the first place.

Also, this person might be having a bad day or be under a great deal of stress or be distracted in some other way.This could result in the wrong decision being made. Therefore, it is a good idea to use both behavioural finance on top of the traditional theories already in use today. This view is supported by an article by Malkiel (1989) who agrees with the notion that behavioural aspects have a great importance in stock market valuation. He argues that behavioural factors play an important role in stock valuation alongside traditional valuation theories. This is summed up by the following quote, ‘market valuations rest on both logical and psychological factors.

The theory of valuation depends on the projection of a long-term stream of dividends whose growth rate is extraordinarily difficult to estimate. Moreover, the appropriate risk premiums for common equities are changeable and far from obvious either to investors or economists. Thus, there is room for the hopes, fears, and favourite fashions of market participants to play a role in the valuation process. ‘ Another article from the Banker (2004) also supports the view that behavioural finance has a role to play alongside the traditional views.

Author: Donald Jacobs


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