The efficient market hypothesis implies that it is not generally possible to make above-average returns in the stock market by trading (including market timing), except through luck or obtaining and trading on inside information. There are three common forms in which the efficient markets hypothesis is commonly stated - weak form efficiency, semi-strong form efficiency and strong form efficiency, each of which have different implications for how markets work.
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2 Semi-strong form efficiency 3 Strong-form efficiency 4 Arguments concerning the validity of the hypothesis 5 See also: |
Many observers dispute the assumption that market participants are rational, or that markets behave consistently with the efficient market hypothesis, especially in its stronger forms. Many economists, mathematicians and market practitioners cannot believe that man-made markets are strong-form efficient when there are prima facie reasons for inefficiency including the slow diffusion of information, the relatively great power of some market participants (e.g. financial institutions), and the existence of apparently sophisticated professional investors.
The efficient market hypothesis was introduced in the late 1960s and the prevailing view prior to that time was that markets were inefficient. Inefficiency was commonly believed to exist e.g. in the United States and United Kingdom stock markets. However, earlier work by Kendall (1953) suggested that changes in UK stock market prices were random. Later work by Brealey and Dryden, and also by Cunningham found that there were no significant dependences in price changes suggesting that the UK stock market was weak-form efficient.
Further to this evidence that the UK stock market is weak form efficient, other studies of capital markets have pointed toward them being semi strong-form efficient. Studies by Firth (1976, 1979 and 1980) in the United Kingdom have compared the share prices existing after a takeover announcement with the bid offer. Firth found that the share prices were fully and instantaneously adjusted to their correct levels, thus concluding that the UK stock market was semi strong-form efficient.
It may be that professional and other market participants who have discovered reliable trading rules or stratagems see no reason to divulge them to academic researchers; the academics in any case tend to be intellectually wedded to the efficient markets theory. It might be that there is an information gap between the academics who study the markets and the professionals who work in them. Within the financial markets there is knowledge of features of the markets that can be exploited e.g seasonal tendencies and divergent returns to assets with various characteristics. E.g. factor analysis and studies of returns to different types of investment strategies suggest that some types of stocks consistently outperform the market (e.g in the UK, the USA and Japan).
Opponents of the EMH sometimes cite examples of market movements that seem inexplicable in terms of conventional theories of stock price determination, for example the stock market crash of October 1987 where most stock exchanges crashed at the same time. It is virtually impossible to explain the scale of those market falls by reference to any news event at the time. The correct explanation seems to lie either in the mechanics of the exchanges (e.g. no safety nets to discontinue trading initiated by program sellers) or the peculiarities of human nature. It is certainly true that "behavioural psychology" approaches to stock market trading are amongst the most promising that there are (and some investment strategies seek to exploit exactly such inefficiencies).Weak-form efficiency
Semi-strong form efficiency
Strong-form efficiency
Arguments concerning the validity of the hypothesis