Consumption-and-Saving宏观经济学-多恩布什

1Chapter13ConsumptionandSavingItemItemItemEtc.McGraw-Hill/IrwinMacroeconomics,10e©2023TheMcGraw-HillCompanies,Inc.,AllRightsReserved.2IntroductionConsumptionaccountsforabout70%ofADFluctuationsinCareproportionatelysmallerthanthefluctuationsinGDPConsumptionisrelativelystableInthischapterweseektounderstandconsumptionandthelinkbetweenconsumptionandincome

ConsumptiontheoryThedebateoverdifferentconsumptiontheories=debateoverthesizeofthemarginalpropensitytoconsume(MPC)KeynesianmodelMPCishigh,moderntheoriesMPCislow3IntroductionFigure13-2plotschangesinpercapitaconsumptionandchangesinpercapitadisposableincomeConsumptionfollowsincomeswingsthatlast5-10yearsConsumptiondoesnotrespondtospikesinincome[InsertFigure13-1here]Longtermchangesinincomegeneratechangesinconsumption,butshorttermchangesinincomedonot.4IntroductionFigure13-2comparesconsumptionthisquartertothepreviousquarter:ConsumptionisalmostperfectlypredictedbypreviousconsumptionThisrelationshipisanoutcomeofthelinkbetweencurrentconsumptionandexpectedfutureconsumptionUsedinthemoderntheoriesofconsumption[InsertFigure13-2here]5IntroductionEarlyKeynesiantheoriesexplainedconsumptionasafunctionofcurrentdisposableincome(Figure13-3)NoseparationfortemporaryandpermanentincomeThisconsumptionfunctionisoftheform:(1),wherecistheMPCandisonthe[0,1]rangeModernconsumptiontheoriesincorporateintertemporaldynamics[InsertFigure13-3here]6LifeCycleTheoryThelifecyclehypothesisviewsindividualsasplanningtheirconsumptionandsavingsbehavioroverlongperiodswiththeintentionofallocatingtheirconsumptioninthebestpossiblewayovertheirentirelifetimesDifferentMPCoutofpermanentincome,transitoryincome,andwealthcomparedtotheKeynesiantheorywithasingleMPCKeyassumption:mostpeoplechoosestablelifestyles,orsmoothoutconsumptionovertheirlifetimeIndividualsdonotlikeconsumptiontochangedramaticallyfromyeartoyearThesimplestformofthisassumptionistoconsumethesameamountineveryyear7LifeCycleTheorySupposeanindividual:Startslifeatage20Planstoworkuntilage65Willdieatage80HasannuallaborincomeofYL=$30,000Lifetimeresourcesare$30,000x45=$1,350,000Spreadinglifetimeresourcesoverthenumberofyearsoflife(80-20=60)allowsforC=$1,350,000/60=$22,500ThegeneralformulaisThemarginalpropensitytoconsumeis

8LifeCycleTheoryOncewehaveatheoryofconsumption,wehaveatheoryofsavings(savingsisincomelessconsumption)Figure13-4tracesoutthepathofconsumptionandsavingusingthelifecycletheoryWealthpeaksatretirementWealthiszeroatdeathAccumulatesavingsinworkingyears,butdissavethroughretirementIncomeispositiveinworkingyears,andzeroinretirement[InsertFigure13-4here]9LifeCycleTheoryContinuingwiththeexample,cancomputedifferentmarginalpropensitiestoconsumeforvariousmeasuresofincome:permanentandtransitoryincomeSupposeincomeincreasespermanentlyby$3,000:Theextra$3,000foreach45yearsspreadoutover60yearsoflifeincreasesconsumptionbyThemarginalpropensitytoconsumeoutofpermanentincomeisSupposeincomeincreasedby$3,000foronlyoneyear:Theextra$3,000over60yearswouldincreaseconsumptionbyTheMPCoutoftransitoryincomeis10LifeCycleTheoryTheMPCoutofpermanentincomeislargeTheMPCoutoftransitoryincomeissmallandfairlyclosetozeroThelife-cycletheoryimpliesthattheMPCoutofwealthshouldequaltheMPCoutoftransitoryincome

WHY?SpendingoutofwealthisspreadoutoverremainingyearsoflifeTheMPCoutofwealthisusedtolinkchangesinthevalueofassetstocurrentconsumption11PermanentIncomeTheoryPermanentincometheoryofconsumptionislikethelifecyclehypothesisinthatcurrentconsumptionisnotdependentuponcurrentincome,butonalonger-termestimateofincomeMiltonFriedmancalledthispermanentincomePermanentincomeisthesteadyrateofexpenditureapersoncouldmaintainfortherestofhis/herlife,giventhepresentlevelofwealthandtheincomeearnednowandinthefutureTheconsumptionfunctionisthen:(2),whereYPispermanentdisposableincomeLifecyclehypothesisandpermanentincomehypothesisareverysimilar,andareoftencombinedasthePILCH.12ConsumptionUnderUncertaintyIfpermanentincomewereknown,accordingthePILCH,consumptionwouldneverchangeThemodernversionofPILCHemphasizesthelinkbetweenincomeuncertaintyandchangesinconsumptionandtakesamoreformalapproachtoconsumermaximizationUnderthisnewerversionofconsumptiontheory,changesinconsumptionarisefromsurprisechangesinincomeAbsentsuchsurprises,consumptionthisperiodisthesameaslastperiodandisthesameasnextperiodConsumptioncanbemodeledas:,whereconsumptiontomorrowisequaltoconsumptiontodayplusatrulyrandomerror(RobertHall)13LiquidityConstraintsandMyopiaWhymightthePILCHmissexplainingclosetohalfofconsumptionbehavior?

Twoexplanationsinclude:Liquidityconstraints:consumerunabletoborrowtosustaincurrentconsumptionintheexpectationofhigherfutureincomeWhenpermanentincomeishigherthancurrentincome,consumersareunabletoborrowtoconsumeatthehigherlevelpredictedby