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1、CHAPTER21BLACKMONDAYANDBLACKSWANS∗JohnC.BogleInvestorsneedtobeawarethatrareeventswithanextremeimpactthat,afterwards,wethinkwecouldhavepredictedinshort,blackswanshappeninthemarkets.Thosewhoaretryingtomeasureriskinthefinancialmarketsneedtocarefullydis-tinguishrisk,withitsprobabilities,fromuncertainty
2、,whichcannotbemeasured.Wehavebecomeincreasinglyvulnerabletoblackswansbecauseourfinancialecon-omyhascometoplayanever-largerroleinourproductiveeconomy.The20thanniversaryofwhatcametobeknownasBlackMonday19October1987providesamemorableplatformforconsidering,yetagain,theroleofriskinourfinancialmarkets.Ont
3、hatsingleday,theDowJonesIndustrialAveragedroppedfrom2,246to1,738,anastonishingdeclineofalmost25percent,nearlytwicethelargestpreviousdailydeclineof13percent.The13percentdecline,whichtookplaceon24October1929knownasBlackThursdayprovedtobeadistantearlywarningthattheGreatDepressionlayahead.1Fromitsearl
4、ierhighinlateAugust1987untilthestockmarketatlastclosedonthatfatefulBlackMonday,some$1trillionwaserasedfromthetotalvalueofU.S.stocks.Thestunningdeclineshockednearlyallmarketparticipants,althoughsomeveteranswerenotsur-prised.Forexample,Alan(Ace)Greenberg,formerchairmanofBearStearnsCompanies,wasquote
5、dinthenewspapersassaying,Somarketsfluctuate.Whatelseisnew?AndonlyayearbeforeBlackMonday,IobservedtotheVanguardcrewthatevena100-pointdeclineintheDowsomethingthathadneverbeforeoccurredwaspossible.Why?Becauseinthestockmarket,anythingcanhappen.Notonlycananythinghappeninthestockmarket,butanythingdoeshap
6、pen.Whatismore,changesinthenatureandstructureofourequitymarketandaradicalshiftinitsparticipantsaremakingshockingandunexpectedmarketaberrationsevermoreprobable.Infact,during∗ReprintedfromtheFinancialAnalystsJournal(March/April2008):3040.Note:Theopinionsexpressedinthisarticledonotnecessarilyrepresen
7、ttheviewsoftheVanguardGroupspresentmanagement.301CH021.indd3018/28/108:34:03PM302PartII:MeasuringRisk2007,wewitnessedanunprecedentedseriesofamazingmarketswings.Whereasinthe1950sand1960s,thedailychangesinthelevelo