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In financial economics, the efficient-market hypothesis (EMH) states that asset prices fully reflect all available information. A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since markets prices should only react to news, and news by definition is random. The EMH was developed by Professor Eugene Fama who argued that stocks always trade at their fair value, making it impossible for investors to either purchase undervalued stocks or sell stocks for inflated prices. As such, it should be impossible to outperform the overall market through expert stock selection or market timing, and that the only way an investor can possibly obtain higher returns is by purchasing riskier investments.〔http://www.investopedia.com/articles/basics/04/022004.asp〕 There are three variants of the hypothesis: "weak", "semi-strong", and "strong" form. The weak form of the EMH claims that prices on traded assets (''e.g.,'' stocks, bonds, or property) already reflect all past publicly available information. The semi-strong form of the EMH claims both that prices reflect all publicly available information and that prices instantly change to reflect new public information. The strong form of the EMH additionally claims that prices instantly reflect even hidden "insider" information. Critics have blamed the belief in rational markets for much of the late-2000s financial crisis. In response, proponents of the hypothesis have stated that market efficiency does not mean having no uncertainty about the future, that market efficiency is a simplification of the world which may not always hold true, and that the market is practically efficient for investment purposes for most individuals. ==Historical background== Historically, the EMH is preceded by Hayek's (1945) argument that markets are the most effective way of aggregating the pieces of information dispersed amongst 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 which was discussed in 1863 by Jules Regnault, a French broker. Later another French mathematician, Louis Bachelier, applied probability theory in his 1900 PhD thesis, "The Theory of Speculation".〔Kirman, Alan. "(Economic theory and the crisis )." ''Voxeu''. 14 November 2009.〕 His work was largely ignored until the 1950s when financial economists began making heavy use of probability theory and statistics to model asset prices (in particular, options prices). Empirically, a number of studies indicated that US stock prices and related financial series followed a random walk model.〔See Working (1934), Cowles and Jones (1937), and Kendall (1953), and later Brealey, Dryden and Cunningham.〕 Research by Alfred Cowles in the ’30s and ’40s suggested that professional investors were in general unable to outperform the market. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Efficient-market hypothesis」の詳細全文を読む スポンサード リンク
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