第三讲 需求理论

第三讲需求理论

需求需要作为既定或事实，不作道德判断。需要的相对性：需要导致资源配置，一种需要是为了更多的需要；全部需要满足是不可能的（最低生活标准的制定，“科学”与主观）。稀缺导致需要满足先后的权衡。与欲望（desire）区别需求表与需求量：价格上升因而需求减少？需求增加因而价格上升？需求曲线：PoQD需求曲线与时间横轴的时间段瞬间快照长期、中期与短期弹性需求定律Thelawofdemand:价格下降，需求量上升需求定律在经济学中的地位。价重新定义使用价值交换价值钻石与水的悖论没有市场价，“替代”概念现值的价量成交量与需求量有质的量与委托的量（市场区分）价与量的对应消费者剩余什么是消费者剩余五个苹果最高用值30（10+8+6+4+2），总换值10（2×5），消费者剩余20三种收费办法：单独定价；入场费或两部定价；全部或零其他情况不变Otherthingsequalorunchanged或ceterisparibus其他商品的价格（互补品和替代品）；收入；偏好参看弗里德曼《价格理论》第二章和张五常《经济解释》第一卷偏好不变的假设品味的变化。学术分工，品味来源及其变化留给心理学力求证伪的可能性统计上的需求曲线

例证穿着打扮周末经济、假日经济生命与健康的选择犯罪与惩罚Alchian的苹果向上倾斜的需求曲线？价格越贵越有人买？价格与质量；炫耀品；投机预期效用什么是效用？边沁：效用是快乐或享受的指数。人们追求指数最大。收入的边际效用递减。Stilger纵论悲哀的效用理论发展。帕累托和费雪的贡献。效用是选择的序数排列。其量度可以是任何东西（数字的三种用途）。痛苦与快乐的计较？无差异曲线替代定理（“替代”概念在经济学中的重要性）内凸定理（多样化）不相交U1CombinationB,A,&DyieldthesamesatisfactionEispreferredtoU1U1

ispreferredtoH&GConsumerPreferencesFood(unitsperweek)1020304010203040Clothing(unitsperweek)50GDAEHB预算约束瓦尔拉斯预算集瓦尔拉斯定律预算集的决定及其变化BudgetLineF+2C=$801020(I/PC)=40BudgetConstraintsFood(unitsperweek)406080=(I/PF)201020300ABDEGClothing(unitsperweek)Pc

=$2Pf=$1I=$80EffectofaPriceChangeFood(unitspermonth)Clothing(unitspermonth)456U2U3ABDU141220Threeseparateindifferencecurvesaretangenttoeachbudgetline.Assume:I=$20PC=$2PF

=$2,$1,$.5010Price-ConsumptionCurveEffectofaPriceChangeFood(unitspermonth)Clothing(unitspermonth)456U2U3ABDU141220Theprice-consumptioncurvetracesouttheutilitymaximizingmarketbasketforthevariouspricesforfood.EffectofaPriceChangeDemandCurveIndividualDemandrelatesthequantityofagoodthataconsumerwillbuytothepriceofthatgood.Food(unitspermonth)PriceofFoodHEG$2.0041220$1.00$.50EffectsofIncomeChangesFood(unitspermonth)Clothing(unitspermonth)Anincreaseinincome,withthepricesfixed,causesconsumerstoaltertheirchoiceofmarketbasket.Income-ConsumptionCurve34AU1510BU2D716U3Assume:Pf=$1

Pc=$2

I=$10,$20,$30EffectsofIncomeChangesFood(unitspermonth)PriceoffoodAnincreaseinincome,from$10to$20to$30,withthepricesfixed,shiftstheconsumer’sdemandcurvetotheright.$1.004D1E10D2G16D3HAnInferiorGoodHamburger(unitspermonth)Steak(unitspermonth)1530U3CIncome-ConsumptionCurve…buthamburgerbecomesaninferiorgoodwhentheincomeconsumptioncurvebendsbackwardbetweenBandC.10520510AU1BU2Bothhamburgerandsteakbehaveasanormalgood,betweenAandB...EngelCurvesFood(unitspermonth)30481210Income($permonth)20160Engelcurvesslopeupwardfornormalgoods.EngelCurvesEngelcurvesslopebackwardbendingforinferiorgoods.InferiorNormalFood(unitspermonth)30481210Income($permonth)20160IncomeandSubstitution

Effects:NormalGoodFood(unitspermonth)OClothing(unitspermonth)RF1SC1AU1Theincomeeffect,EF2,(fromDtoB)keepsrelativepricesconstantbutincreasespurchasingpower.IncomeEffectC2F2TU2BWhenthepriceoffoodfalls,consumptionincreasesbyF1F2

astheconsumermovesfromAtoB.ETotalEffectSubstitutionEffectDThesubstitutioneffect,F1E,(frompointAtoD),changestherelativepricesbutkeepsrealincome(satisfaction)constant.Food(unitspermonth)ORClothing(unitspermonth)F1SF2TAU1ESubstitutionEffectDTotalEffectSincefoodisaninferiorgood,theincomeeffectisnegative.However,thesubstitutioneffectislargerthantheincomeeffect.BIncomeEffectU2IncomeandSubstitution

Effects:InferiorGood从无差异曲线到需求曲线收入效应正常物品与贫穷物品（inferiorgoods）吉芬物品的悖论争论：吉芬物品是否实际存在放弃效用理论？三种需求曲线马歇尔需求曲线希克斯需求曲线斯