曼昆经济学 Thinking Like an Economist.docx
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曼昆经济学 Thinking Like an Economist.docx
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曼昆经济学ThinkingLikeanEconomist
INTHISCHAPTERYOUWILL...
Seehoweconomistsapplythemethods
ofscience
Considerhow
assumptionsandmodelscanshed
lightontheworld
Learntwosimple
models—thecircularflowandthe
production
possibilitiesfrontier
Distinguishbetweenmicroeconomicsandmacroeconomics
THINKINGLIKEANECONOMIST
Everyfieldofstudyhasitsownlanguageanditsownwayofthinking.Mathe-maticianstalkaboutaxioms,integrals,andvectorspaces.Psychologiststalkaboutego,id,andcognitivedissonance.Lawyerstalkaboutvenue,torts,andpromissoryestoppel.
Economicsisnodifferent.Supply,demand,elasticity,comparativeadvantage,consumersurplus,deadweightloss—thesetermsarepartoftheeconomist’slan-guage.Inthecomingchapters,youwillencountermanynewtermsandsomefa-miliarwordsthateconomistsuseinspecializedways.Atfirst,thisnewlanguagemayseemneedlesslyarcane.But,asyouwillsee,itsvalueliesinitsabilitytopro-videyouanewandusefulwayofthinkingabouttheworldinwhichyoulive.
Thesinglemostimportantpurposeofthisbookistohelpyoulearntheecon-omist’swayofthinking.Ofcourse,justasyoucannotbecomeamathematician,psychologist,orlawyerovernight,learningtothinklikeaneconomistwilltake
19
Learnthedifferencebetweenpositiveandnormativestatements
Examinetheroleofeconomistsin
makingpolicy
Considerwhyeconomists
sometimesdisagreewithoneanother
sometime.Yetwithacombinationoftheory,casestudies,andexamplesofeco-nomicsinthenews,thisbookwillgiveyouampleopportunitytodevelopandpracticethisskill.
Beforedelvingintothesubstanceanddetailsofeconomics,itishelpfultohaveanoverviewofhoweconomistsapproachtheworld.Thischapter,therefore,dis-cussesthefield’smethodology.Whatisdistinctiveabouthoweconomistsconfrontaquestion?
Whatdoesitmeantothinklikeaneconomist?
THEECONOMISTASSCIENTIST
Economiststrytoaddresstheirsubjectwithascientist’sobjectivity.Theyapproachthestudyoftheeconomyinmuchthesamewayasaphysicistapproachesthestudyofmatterandabiologistapproachesthestudyoflife:
Theydevisetheories,collectdata,andthenanalyzethesedatainanattempttoverifyorrefutetheirtheories.
Tobeginners,itcanseemoddtoclaimthateconomicsisascience.Afterall,economistsdonotworkwithtesttubesortelescopes.Theessenceofscience,
“I’masocialscientist,Michael.ThatmeansIcan’texplainelectricityoranythinglikethat,butifyoueverwanttoknowaboutpeopleI’myourman.”
however,isthescientificmethod—thedispassionatedevelopmentandtestingoftheoriesabouthowtheworldworks.Thismethodofinquiryisasapplicabletostudyinganation’seconomyasitistostudyingtheearth’sgravityoraspecies’evolution.AsAlbertEinsteinonceputit,“Thewholeofscienceisnothingmorethantherefinementofeverydaythinking.”
AlthoughEinstein’scommentisastrueforsocialsciencessuchaseconomicsasitisfornaturalsciencessuchasphysics,mostpeoplearenotaccustomedtolookingatsocietythroughtheeyesofascientist.Let’sthereforediscusssomeofthewaysinwhicheconomistsapplythelogicofsciencetoexaminehowanecon-omyworks.
THESCIENTIFICMETHOD:
OBSERVATION,THEORY,ANDMOREOBSERVATION
IsaacNewton,thefamousseventeenth-centuryscientistandmathematician,al-legedlybecameintriguedonedaywhenhesawanapplefallfromanappletree.ThisobservationmotivatedNewtontodevelopatheoryofgravitythatappliesnotonlytoanapplefallingtotheearthbuttoanytwoobjectsintheuniverse.Subse-quenttestingofNewton’stheoryhasshownthatitworkswellinmanycircum-stances(although,asEinsteinwouldlateremphasize,notinallcircumstances).BecauseNewton’stheoryhasbeensosuccessfulatexplainingobservation,itisstilltaughttodayinundergraduatephysicscoursesaroundtheworld.
Thisinterplaybetweentheoryandobservationalsooccursinthefieldofeco-nomics.Aneconomistmightliveinacountryexperiencingrapidincreasesinpricesandbemovedbythisobservationtodevelopatheoryofinflation.Thetheorymightassertthathighinflationariseswhenthegovernmentprintstoomuchmoney.(Asyoumayrecall,thiswasoneoftheTenPrinciplesofEconomicsinChapter1.)Totestthistheory,theeconomistcouldcollectandanalyzedataonpricesandmoneyfrommanydifferentcountries.Ifgrowthinthequantityofmoneywerenotatallrelatedtotherateatwhichpricesarerising,theeconomistwouldstarttodoubtthevalidityofhistheoryofinflation.Ifmoneygrowthandin-flationwerestronglycorrelatedininternationaldata,asinfacttheyare,theecon-omistwouldbecomemoreconfidentinhistheory.
Althougheconomistsusetheoryandobservationlikeotherscientists,theydofaceanobstaclethatmakestheirtaskespeciallychallenging:
Experimentsareoftendifficultineconomics.Physicistsstudyinggravitycandropmanyobjectsintheirlaboratoriestogeneratedatatotesttheirtheories.Bycontrast,economistsstudy-inginflationarenotallowedtomanipulateanation’smonetarypolicysimplytogenerateusefuldata.Economists,likeastronomersandevolutionarybiologists,usuallyhavetomakedowithwhateverdatatheworldhappenstogivethem.
Tofindasubstituteforlaboratoryexperiments,economistspaycloseattentiontothenaturalexperimentsofferedbyhistory.WhenawarintheMiddleEastin-terruptstheflowofcrudeoil,forinstance,oilpricesskyrocketaroundtheworld.Forconsumersofoilandoilproducts,suchaneventdepresseslivingstandards.Foreconomicpolicymakers,itposesadifficultchoiceabouthowbesttorespond.Butforeconomicscientists,itprovidesanopportunitytostudytheeffectsofakeynaturalresourceontheworld’seconomies,andthisopportunitypersistslongafterthewartimeincreaseinoilpricesisover.Throughoutthisbook,therefore,wecon-sidermanyhistoricalepisodes.Theseepisodesarevaluabletostudybecausethey
giveusinsightintotheeconomyofthepastand,moreimportant,becausetheyal-lowustoillustrateandevaluateeconomictheoriesofthepresent.
THEROLEOFASSUMPTIONS
Ifyouaskaphysicisthowlongitwouldtakeforamarbletofallfromthetopofaten-storybuilding,shewillanswerthequestionbyassumingthatthemarblefallsinavacuum.Ofcourse,thisassumptionisfalse.Infact,thebuildingissurroundedbyair,whichexertsfrictiononthefallingmarbleandslowsitdown.Yetthephysi-cistwillcorrectlypointoutthatfrictiononthemarbleissosmallthatitseffectisnegligible.Assumingthemarblefallsinavacuumgreatlysimplifiestheproblemwithoutsubstantiallyaffectingtheanswer.
Economistsmakeassumptionsforthesamereason:
Assumptionscanmaketheworldeasiertounderstand.Tostudytheeffectsofinternationaltrade,forex-ample,wemayassumethattheworldconsistsofonlytwocountriesandthateachcountryproducesonlytwogoods.Ofcourse,therealworldconsistsofdozensofcountries,eachofwhichproducesthousandsofdifferenttypesofgoods.Butbyas-sumingtwocountriesandtwogoods,wecanfocusourthinking.Onceweunder-standinternationaltradeinanimaginaryworldwithtwocountriesandtwogoods,weareinabetterpositiontounderstandinternationaltradeinthemorecomplexworldinwhichwelive.
Theartinscientificthinking—whetherinphysics,biology,oreconomics—isdecidingwhichassumptionstomake.Suppose,forinstance,thatweweredrop-pingabeachballratherthanamarblefromthetopofthebuilding.Ourphysicistwouldrealizethattheassumptionofnofrictionisfarlessaccurateinthiscase:
Frictionexertsagreaterforceonabeachballthanonamarble.Theassumptionthatgravityworksinavacuumisreasonableforstudyingafallingmarblebutnotforstudyingafallingbeachball.
Similarly,economistsusedifferentassumptionstoanswerdifferentquestions.Supposethatwewanttostudywhathappenstotheeconomywhenthegovern-mentchangesthenumberofdollarsincirculation.Animportantpieceofthisanalysis,itturnsout,ishowpricesrespond.Manypricesintheeconomychangeinfrequently;thenewsstandpricesofmagazines,forinstance,arechangedonlyeveryfewyears.Knowingthisfactmayleadustomakedifferentassumptionswhenstudyingtheeffectsofthepolicychangeoverdifferenttimehorizons.Forstudyingtheshort-runeffectsofthepolicy,wemayassumethatpricesdonotchangemuch.Wemayevenmaketheextremeandartificialassumptionthatallpricesarecompletelyfixed.Forstudyingthelong-runeffectsofthepolicy,how-ever,wemayassumethatallpricesarecompletelyflexible.Justasaphysicistusesdifferentassumptionswhenstudyingfallingmarblesandfallingbeachballs,econ-omistsusedifferentassumptionswhenstudyingtheshort-runandlong-runef-fectsofachangeinthequantityofmoney.
ECONOMICMODELS
Highschoolbiologyteachersteachbasicanatomywithplasticreplicasofthehu-manbody.Thesemodelshaveallthemajororgans—theheart,theliver,thekid-neys,andsoon.Themodelsallowteacherstoshowtheirstudentsinasimplewayhowtheimportantpartsofthebodyfittogether.Ofcourse,theseplasticmodels
arenotactualhumanbodies,andnoonewouldmistakethemodelforarealper-son.Thesemodelsarestylized,andtheyomitmanydetails.Yetdespitethislackofrealism—indeed,becauseofthislackofrealism—studying
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