题目内容：
Questionsarebasedonthefollowingpassage.
Economistsworkontheassumptionthatpeopleactrationally.Ifonlylifewerethatsimple.Investorscertainlydon'talwaysactastheyshould.Amongotherunwiseacts,theytradetoomuch,donotdiversifytheirportfolios(投资组合)enoughandarereluctanttodiscardunderperformingstocks.Theymayhavebeenborntobehavethatway.ArecentstudyintoinvestmentbehaviorbyStephanSiegelofArizonaStateUniversity'sWPCareySchoolofBusiness,andHenrikCronqvistofClaremontMcKennaCollege,illuminatestherolethatgenesplayindetermininginvestmentdecisions.
Theauthorsexaminetheinvestmentbehaviorof15,208pairsofSwedishtwins,usingdatafromthecountry'stwinregistryanditstaxauthority,whichuntil2007keptcomprehensiverecordsoneveryfinancialtransaction.Controllingforvariousfactors,theyfindthatidenticaltwins,whosharealltheirgenes,weremoresimilarintheirinvestingbehaviorthanfraternaltwins(异卵双胞胎),whoshareabouthalftheirgenes.
Theauthorscalculatethatgeneticfactorsaccountforbetweenaquarterandhalfofthevariationsinirrationalinvestmentbehaviorbetweenindividuals.Thesefactorsareatworkacrossmoredimensionsthanjustinvesting.Twinswhoshowedabiastowardsbuyingfamiliarsharesratherthanunknownones,forexample,alsoshowedapreferenceforlivingclosertotheirplaceofbirthandformarryingaspousefromthesameregion.Investorswithlargeportfolioswereparticularlysusceptible(易受……影响的)togeneticinfluences.
Ifgenesexplainuptoahalfofthevariationsininvestmentbehavior,whatgovernstherest?Theauthorsalsocalculatedtheimpactofsharedenvironmentalinfluencesonthetwinsaswellastheeffectofexperiencesuniquetoonehalfofatwinpair.Commonchildhoodexperienceslikeschoolingwerefoundtohavealmostnoinfluenceoninvestmentbehavior.Butindividualexperiencesexplainhalfofthevariationsbetweentwinpairs--asmuchas,andoftenmorethan,genes.
Thestudyhasitslimitations.Itlooksatdatafromonlyonecountryduringalimitedperiodoftime,forexample.Butitsuggeststhatattemptstopersuadepeopletoinvestmorerationallyhavesomebiginbuilt(生来就要的)biasestoovercome.
Howdoeconomiststhinkaboutinvestors? A．Theydolotsofstupidthings.
B．Theytradeasmuchastheycan.
C．Theyarereluctanttochange.
D．Theyalwaysmakefightdecisions.
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