宏观经济学（第十三版）课件：Consumption and Saving

ConsumptionandSaving

.IntroductionConsumptionaccountsforabout70%ofADFluctuationsinCareproportionatelysmallerthanthefluctuationsinGDPConsumptionisrelativelystableInthischapterweseektounderstandconsumptionandthelinkbetweenconsumptionandincome

ConsumptiontheoryThedebateoverdifferentconsumptiontheories=debateoverthesizeofthemarginalpropensitytoconsume(MPC)KeynesianmodelMPCishigh,moderntheoriesMPCislow14-2IntroductionFigure14-1plotschangesinpercapitaconsumptionandchangesinpercapitadisposableincomeConsumptionfollowsincomeswingsthatlast5-10yearsConsumptiondoesnotrespondtospikesinincome14-3Introduction

14-4Figure14-2comparesconsumptionthisquartertothepreviousquarter:

ConsumptionisalmostperfectlypredictedbypreviousconsumptionThisrelationshipisanoutcomeofthelinkbetweencurrentconsumptionandexpectedfutureconsumptionIntroduction

14-5EarlyKeynesiantheoriesexplainedconsumptionasafunctionofcurrentdisposableincomeNoseparationfortemporaryandpermanentincomeThisconsumptionfunctionisoftheform:(1),wherecistheMPCandisonthe[0,1]rangeModernconsumptiontheoriesincorporateintertemporaldynamicsLifeCycleTheoryThelifecyclehypothesisviewsindividualsasplanningtheirconsumptionandsavingsbehavioroverlongperiodswiththeintentionofallocatingtheirconsumptioninthebestpossiblewayovertheirentirelifetimesDifferentMPCoutofpermanentincome,transitoryincome,andwealthcomparedtotheKeynesiantheorywithasingleMPCKeyassumption:mostpeoplechoosestablelifestyles,orsmoothoutconsumptionovertheirlifetimeIndividualsdonotlikeconsumptiontochangedramaticallyfromyeartoyearThesimplestformofthisassumptionistoconsumethesameamountineveryyear14-6LifeCycleTheorySupposeanindividual:Startslifeatage20Planstoworkuntilage65Willdieatage80HasannuallaborincomeofYL=$30,000Lifetimeresourcesare$30,000x45=$1,350,000Spreadinglifetimeresourcesoverthenumberofyearsoflife(80-20=60)allowsforC=$1,350,000/60=$22,500Thegeneralformulais14-7LifeCycleTheory

14-8Oncewehaveatheoryofconsumption,wehaveatheoryofsavings(savingsisincomelessconsumption)Figure14-4tracesoutthepathofconsumptionandsavingusingthelifecycletheoryWealthpeaksatretirementWealthiszeroatdeathAccumulatesavingsinworkingyears,dissavethroughretirementIncomeispositiveinworkingyears,andzeroinretirementLifeCycleTheoryContinuingwiththeexample,cancomputedifferentmarginalpropensitiestoconsumeforvariousmeasuresofincome:permanentandtransitoryincomeSupposeincomeincreasespermanentlyby$3,000:Theextra$3,000foreach45yearsspreadoutover60yearsoflifeincreasesconsumptionbyThemarginalpropensitytoconsumeoutofpermanentincomeis14-9LifeCycleTheoryContinuingwiththeexample,cancomputedifferentmarginalpropensitiestoconsumeforvariousmeasuresofincome:permanentandtransitoryincomeSupposeincomeincreasedby$3,000foronlyoneyear:Theextra$3,000over60yearswouldincreaseconsumptionbyTheMPCoutoftransitoryincomeis14-10LifeCycleTheoryTheMPCoutofpermanentincomeislargeTheMPCoutoftransitoryincomeissmallandfairlyclosetozeroThelife-cycletheoryimpliesthattheMPCoutofwealthshouldequaltheMPCoutoftransitoryincome

WHY?14-11LifeCycleTheoryThelife-cycletheoryimpliesthattheMPCoutofwealthshouldequaltheMPCoutoftransitoryincomeWHY?SpendingoutofwealthisspreadoutoverremainingyearsoflifeTheMPCoutofwealthisusedtolinkchangesinthevalueofassetstocurrentconsumption14-12PermanentIncomeTheoryPermanentincometheoryofconsumptionislikethelifecyclehypothesisinthatcurrentconsumptionisnotdependentuponcurrentincome,butonalonger-termestimateofincomeMiltonFriedmancalledthispermanentincomePermanentincomeisthesteadyrateofexpenditureapersoncouldmaintainfortherestofhis/herlife,giventhepresentlevelofwealthandtheincomeearnednowandinthefutureTheconsumptionfunctionisthen:(2),whereYPispermanentdisposableincome14-13ConsumptionUnderUncertaintyIfpermanentincomewereknown,accordingthePILCH,consumptionwouldneverchangeThemodernversionofPILCHemphasizesthelinkbetweenincomeuncertaintyandchangesinconsumptionandtakesamoreformalapproachtoconsumermaximizationUnderthisnewerversionofconsumptiontheory,changesinconsumptionarisefromsurprisechangesinincomeAbsentsuchsurprises,consumptionthisperiodisthesameaslastperiodandisthesameasnextperiodConsumptioncanbemodeledas:,whereconsumptiontomorrowisequaltoconsumptiontodayplusatrulyrandomerror(RobertHall)14-14LiquidityConstraintsandMyopiaWhymightthePILCHmissexplainingclosetohalfofconsumptionbehavior?

Twoexplanationsinclude:Liquidityconstraints:consumerunabletoborrowtosustaincurrentconsumptionintheexpectationofhigherfutureincomeWhenpermanentincomeishigherthancurrentincome,consumersareunabletoborrowtoconsumeatthehigherlevelpredictedbyPILCHConsumptionmorecloselylinkedtocurrentincomeMyopia:Consumerssimplyareno