城市的财富：美国的集聚经济与空间均衡

NBERWORKINGPAPERSERIESTHEWEALTHOFCITIES:AGGLOMERATIONECONOMIESANDSPATIALEQUILIBRIUMINTHEUNITEDSTATESEdwardL.GlaeserJoshuaD.GottliebWorkingPaper14806(s)anddonotnecessarilyreflecttheviewsoftheNationalBureauofEconomicResearch.©2009byEdwardL.GlaeserandJoshuaD.Gottlieb.Allrightsreserved.Shortsectionsoftext,nottoexceedtwoparagraphs,maybequotedwithoutexplicitpermissionprovidedthatfullcredit,including©notice,isgiventothesource.TheWealthofCities:AgglomerationEconomiesandSpatialEquilibriumintheUnitedStatesEdwardL.GlaeserandJoshuaD.GottliebNBERWorkingPaperNo.14806March2009JELNo.D0,D00,R0,R00ABSTRACTEmpiricalresearchoncitiesstartswithaspatialequilibriumcondition:workersandfirmsareassumedtobeindifferentacrossspace.Thisconditionimpliesthatresearchoncitiesisdifferentfromresearchoncountries,andthatworkonplaceswithincountriesneedstoconsiderpopulation,incomeandhousingpricessimultaneously.Housingsupplyelasticitywilldeterminewhetherurbansuccessshowsupinmorepeopleorhigherincomes.Urbaneconomistsgenerallyaccepttheexistenceofagglomerationeconomies,whichexistwhenproductivityriseswithdensity,butestimatingthemagnitudeofthoseeconomiesisdifficult.Somemanufacturingfirmsclustertoreducethecostsofmovinggoods,butthisforcenolongerappearstobeimportantindrivingurbansuccess.Instead,moderncitiesarefarmoredependentontherolethatdensitycanplayinspeedingtheflowofideas.Finally,urbaneconomicshassomeinsightstoofferrelatedtopicssuchasgrowththeory,nationalincomeaccounts,publiceconomicsandhousingprices.EdwardL.GlaeserDepartmentofEconomics315ALittauerCenterHarvardUniversityCambridge,[e-mail]omicsLittauerCenter200HarvardUniversityCambridge,MA02138~jdgottl/[e-mail]researchhasfocusedon“thewealthofnations.”Manyeconomistshavetriedtounderstandnationalbusinesscyclesandthehigherincomesandfasterincomegrowththatoccurinsomecountries.Yetthewithin-countrydifferencesinincomeandproductivityarealsoquitestriking.Theaverageincomepercapitain2007intheSanFranciscometropolitanareawasabovealmost60,000dollars;thecomparablefigureforBrownsville,Texas,isunder20,000dollars.Percapitagrossmetropolitanproduct(GMP)ismorethanthreetimeshigherinNewYorkthaninElPaso.2Thedifferencesinpopulationdensityacrossspacewithincountriesareevenmorestriking.Asofthelastcensus,68percentofAmericansoccupiedonly1.8percentofthecountry’slandarea.Factsofthissortmotivatethecentralquestionofurbaneconomics:Whydocitiesexist?Toanswerthisquestionwealsomustalsounderstandwhydenseareasaresomuchmoreproductive.Urbaneconomistsapproachthisquestionbystudying,amongotherthings,within-countryvariationinincomes,populationdensityandhousingprices.Thisallowsthemtotreatpopulationdensitiesappropriately—asoutcomesthataredeterminedjointlywithpricesandwages.Thefield’scentraltheoreticaltoolisthespatialequilibrium,whichassumesthatwelfareisequalizedacrossspace—atleastformarginalmigrants.Thespatialequilibriumconceptguidesurbanmodelsofhousingpricesandindustrialagglomerationaswellasempiricalworkoncitygrowthandtheurbanwagepremium.Inthispaper,wereviewrecentresearchontheeconomicsofcities.Webeginbypresentingaversionofthestandardspatialequilibriummodelthatguidesourinterpretationofempiricalwork.Themodelhasthreecoreequilibriumconditions.Workersmustbeindifferentbetweenlocations,firmsmustbeindifferentabouthiringmoreworkersandbuildersmustbeindifferentaboutsupplyingmorehousing.Thesethreeconditionsprovidethelaborsupplycurve,housingsupplycurveandlabordemandcurvethatcollectivelydetermineareapopulation,wagesandprices.Exogenousdifferencesacrossspaceinproductivity,amenitiesandtheconstructionsector2GrossMetropolitanProductisproducedbytheBureauofEconomicAnalysis,andismeanttobecomparabletoGrossDomesticProduct.3drivedifferencesindensity,incomesandhomeprices.Weallowforthepossibilityofagglomerationeconomies,whichexistwhenproductivityriseswithpopulation.WefirstusethismodeltodiscussthedramaticriseofSunbeltcities.Novariablecanbetterpredictcitygrowthoverthepast50yearsthanJanuarytemperature,yetitisunclearaprioriwhywarmplaceshavegrownsodramatically.ThespatialequilibriummodelenablesustoassesswhetherthisgrowthreflectsrisingSunbeltproductivity,oranincreasedwillingnesstopaytoenjoywarmth,orgreaterhousingsupplyintheSouth.Overthepast50years,Sunbeltproductivityhasincreased,butinthepastdecadeincomeshavefalleninwarmerareas.HousingpriceshavealsostayedlowandrealwageshaverisenintheSouth.Thesefacts,interpretedusingthespatialequilibriummodel,implythattherecentriseofcitieslikeAtlanta,DallasandHoustonowesmoretoelastichousingsupplythantoamenitiesorproductivity.Thespatialequilibriummodelisalsoneededtomakesenseofthedramaticconnectionbet