Journalof Economic Perspectives—Volume 17,Number 1—Winter 2003— Pages 59 – 82 TheEf cient Market Hypothesisand Its Critics BurtonG. Malkiel generationago, theef cient market hypothesis was widelyaccepted by academic nancial economists; forexample, see Eugene Fama’ s (1970) A inuential survey article, “ Efcient Capital Markets.” It was generallybe- lievedthat securitiesmarkets were extremely ef cient in re ecting information about individualstocks and about thestock marketas awhole.The accepted view was that when informationarises, the news spreads veryquickly and isincorporated intothe prices of securitieswithout delay. Thus, neithertechnical analysis, which is thestudy ofpast stock pricesin an attemptto predict future prices, nor even fundamental analysis, which isthe analysis of nancial informationsuch as com- pany earningsand asset valuesto help investors select “ undervalued”stocks, would enablean investorto achieve returns greater than those that could beobtained by holdinga randomlyselected portfolio of individual stocks, at leastnot withcom- parablerisk. Theef cient market hypothesis isassociated withthe idea of a“random walk,” which isatermloosely used inthe nance literatureto characterizea priceseries whereall subsequent pricechanges representrandom departuresfrom previous prices.The logic of the random walkidea is that ifthe owof information is unimpededand informationis immediately re ected in stock prices,then tomor- row’s pricechange willre ect only tomorrow’ s newsand willbe independent of the pricechanges today. Butnews is by de nition unpredictable, and, thus, resulting pricechanges must beunpredictable and random. Asaresult,prices fully re ect all known information,and evenuninformed investors buying a diversied portfolio at thetableau of prices given by the market will obtain arateof return as generousas that achievedby the experts. y BurtonG. Malkielis Chemical Bank Chairman’ s Professorof Economics, Princeton University,Princeton, New Jersey.His e-mail addressis
[email protected] .