10 理性消费者

WHATYOUWILLLEARNINTHISCHAPTERchapter:10Krugman/WellsEconomics©2009WorthPublishersTheRationalConsumerWHATYOUWILLLEARNINTHISCHAPTERHowconsumerschoosetospendtheirincomeongoodsandservicesWhyconsumersmakechoicesbymaximizingutility,ameasureofsatisfactionfromconsumptionWhytheprincipleofdiminishingmarginalutilityappliestotheconsumptionofmostgoodsandservicesHowtousemarginalanalysistofindtheoptimalconsumptionbundleWhatincomeandsubstitutioneffectsareOpportunityCostandDecisionsTheutilityofaconsumerisameasureofthesatisfactiontheconsumerderivesfromconsumptionofgoodsandservices.Anindividual’sconsumptionbundleisthecollectionofallthegoodsandservicesconsumedbythatindividual.Anindividual’sutilityfunctiongivesthetotalutilitygeneratedbyhisorherconsumptionbundle.Theunitofutilityisautil.15131197531–10152839485560636463Totalutility(utils)Marginalutilityperclam(utils)0123456789QuantityofclamsUtilityfunctionMarginalUtilityCurve798654321070605040302010Totalutility(utils)Quantityofclams7986543211614121086420–2Marginalutilityperclams(utils)Quantityofclams(a)Cassie’sUtilityFunction(b)Cassie’sMarginalUtilityCurveCassie’sTotalUtilityandMarginalUtilityCassie’sTotalUtilityandMarginalUtilityCassie’stotalutilitydependsonherconsumptionoffriedclams.Itincreasesuntilitreachesitsmaximumutilitylevelof64utilsat8clamsconsumedanddecreasesafterthat.Themarginalutilitycurveslopesdownwardduetodiminishingmarginalutility;eachadditionalclamgivesCassielessutilitythanthepreviousclam.ThePrincipleofDiminishingMarginalUtilityThemarginalutilityofagoodorserviceisthechangeintotalutilitygeneratedbyconsumingoneadditionalunitofthatgoodorservice.Themarginalutilitycurveshowshowmarginalutilitydependsonthequantityofagoodorserviceconsumed.Theprincipleofdiminishingmarginalutilitysaysthateachsuccessiveunitofagoodorserviceconsumedaddslesstototalutilitythanthepreviousunit.BudgetsandOptimalConsumptionAbudgetconstraintrequiresthatthecostofaconsumer’sconsumptionbundlebenomorethantheconsumer’stotalincome.Aconsumer’sconsumptionpossibilitiesisthesetofallconsumptionbundlesthatcanbeconsumedgiventheconsumer’sincomeandprevailingprices.Aconsumer’sbudgetlineshowstheconsumptionbundlesavailabletoaconsumerwhospendsallofhisorherincome.102345108642ABCDEFAffordableconsumptionbundlesthatcostallofSammy'sincomeSammy’sBudgetLine,BLAffordableconsumptionbundlesUnaffordableconsumptionbundlesQuantityofclams(pounds)Quantityofpotatoes(pounds)ABCDEFConsumptionbundle012345Quantityofclams(pounds)1086420Quantityofpotatoes(pounds)TheBudgetLineSammy’sUtilityfromConsumptionOptimalConsumptionChoiceTheoptimalconsumptionbundleistheconsumptionbundlethatmaximizesaconsumer’stotalutilitygivenhisorherbudgetconstraint.Sammy’sBudgetandTotalUtilitySammy’stotalutilityisthesumoftheutilityhegetsfromclamsandtheutilityhegetsfrompotatoes.Quantityofpotatoes(pounds)543210108642AABBCCDDEEBLUtilityfunctionFF5432108070605040302010Quantityofclams(pounds)0246810(a)Sammy’sBudgetLine(b)Sammy’sUtilityFunctionTheoptimalconsumptionbundle…Quantityofclams(pounds)Quantityofpotatoes(pounds)…maximizestotalutilitygiventhebudgetconstraintTotalutility(utils)OptimalConsumptionBundleSammy’stotalutilityismaximizedatbundleC,whereheconsumes2poundsofclamsand6poundsofpotatoes.ThisisSammy’soptimalconsumptionbundle.SpendingtheMarginalDollarThemarginalutilityperdollarspentonagoodorserviceistheadditionalutilityfromspendingonemoredollaronthatgoodorservice.Sammy’sMarginalUtilityperDollarMarginalUtilityperDollarIfSammyhas,infact,chosenhisoptimalconsumptionbundle,hismarginalutilityperdollarspentonclamsandpotatoesmustbeequal.654321BCBPMUC/PCMUP/PPC5432100246810Totalutility(utils)Quantityofclams(pounds)Quantityofpotatoes(pounds)Attheoptimalconsumptionbundle,themarginalutilityperdollarspentonclamsisequaltothemarginalutilityperdollarspentonpotatoes.OptimalConsumptionRuleTheoptimalconsumptionrulesaysthatwhenaconsumermaximizesutility,themarginalutilityperdollarspentmustbethesameforallgoodsandservicesintheconsumptionbundle.FromUtilitytotheDemandCurveThemainreasonforstudyingconsumerbehavioristounderstandhowthedownwardslopeofthemarketdemandcurveisexplainedbytheutility-maximizingbehaviorofindividualconsumers.MarginalUtility,theSubstitutionEffect,andtheLawofDemandThesubstitutioneffectofachangeinthepriceofagoodisthechangeinthequantityconsumedofthatgoodastheconsumersubstitutesthegoodthathasbecomerelativelycheaperforthegoodthathasbecomerelativelymoreexpensive.TheIncomeEffectTheincomeeffectofachangeinthepriceofagoodisthechangeinthequantityconsumedofthatgoodthatresultsfromachangeintheconsumer’spurchasingpowerduetothechangeinthepriceofthegood.NormalGoodsInferiorGoodsGiffenGoodsSUMMARY1.Consumersmaximizeameasureofsatisfactioncalledutility.Eachconsumerhasautilityfunctionthatdeterminestheleveloftotalutilitygeneratedbyhisorherconsumptionbundle,thegoodsandservicesthatareconsumed.2.Agood’sorservice’smarginalutilityistheadditionalutilitygeneratedbyconsumingonemoreunitofthegoodorservice.Weusuallyassumethattheprincipleofdiminishingmarginalutilityholds:consumption