1 1 第4章 Chapter 4 供给与需求的 市场力量 The Market Forces of Supply and Demand 2 供给与需求的市场力量 The Market Forces of Supply and Demand ?供给与需求是经济学家最常用的两个术语。 Supply and demand are the two words that economists use most often. ?供给与需求是推动市场经济运转的力量所在。 Supply and demand are the forces that make market economies work. ?现代微观经济学研究供给、需求和市场均衡。 Modern microeconomics is about supply, demand, and market equilibrium. 3 市场 Markets ?一个市场就是由某种商品或服务的买者和 卖者组成的群体。 A market is a group of buyers and sellers of a particular good or service. ?术语供给和需求指人们的行为……当他们 在市场中相互作用时。 The terms supply and demand refer to the behavior of people . . . as they interact with one another in markets. 4 市场 Markets ?买方决定需求。 Buyers determine demand. ?卖方决定供给。 Sellers determine supply. 5 市场类型：竞争市场 Market Type: A Competitive Market 竞争性市场是这样一个市场…… A competitive market is a market. . . ……拥有众多买者和卖者 with many buyers and sellers. ……不受任何一个人操纵 that is not controlled by any one person. ……买者和卖者所遵循的价格被约束在窄小的范围内。 in which a narrow range of prices are established that buyers and sellers act upon. 6 竞争：完全竞争与否 Competition: Perfect and Otherwise ?物品完全相同。 Products are the same ?买者、卖者如此众多以至于个人无法对价格施 加影响。 Numerous buyers and sellers so that each has no influence over price ?买者、卖者都是价格接收者。 Buyers and Sellers are price takers 完全竞争Perfect Competition 2 7 竞争：完全竞争与否 Competition: Perfect and Otherwise ?垄断Monopoly ?只有一个卖者，它控制价格 One seller, and seller controls price ?寡头Oligopoly ?少数几个卖者Few sellers 8 竞争：完全竞争与否 Competition: Perfect and Otherwise ?垄断竞争垄断竞争Monopolistic Competition ?许多卖者许多卖者 Many sellers ?存在微小差别的产品存在微小差别的产品 Slightly differentiated products ?每个卖者都能为自己的产品设定价格每个卖者都能为自己的产品设定价格 Each seller may set price for its own product 9 需求 Demand 需求量是买者愿意而且能够购买的 商品数量。 Quantity demanded is the amount of a good that buyers are willing and able to purchase. 10 需求的决定 Determinants of Demand ?市场价格Market price ?消费者收入Consumer income ?相关商品的价格 Prices of related goods ?嗜好Tastes ?预期Expectations 11 需求定律 Law of Demand 需求定律：价格与需求量负相关。 The law of demand states that there is an inverse relationship between price and quantity demanded. 12 需求表 Demand Schedule 需求表显示了商品价格与需求量之间 的关系。 The demand schedule is a table that shows the relationship between the price of the good and the quantity demanded. 3 13 需求表 Demand Schedule 价格 Price 数量 Quantity $0.00 12 0.50 10 1.00 8 1.50 6 2.00 4 2.50 2 3.00 0 14 需求曲线 Demand Curve 需求曲线：是将价格与需求量相联系的向 右下方倾斜的曲线。 The demand curve is the downward- sloping line relating price to quantity demanded. 15 需求曲线 Demand Curve $3.00 2.50 2.00 1.50 1.00 0.50 21 345678910 1211 冰激凌蛋卷的价格 Price of Ice- Cream Cone 冰激凌蛋卷的需求数量 Quantity of Ice- Cream Cones 0 价格 Price 数量 Quantity $0.00 12 0.50 10 1.00 8 1.50 6 2.00 4 2.50 2 3.00 0 16 “其他条件不变” Ceteris Paribus Ceteris paribus是一个拉丁习惯用语，意思是除了 被研究的诸变量之外，其余所有变量均假定为恒 定。字面意义是“其他条件不变（相同）”。 Ceteris paribus is a Latin phrase that means all variables other than the ones being studied are assumed to be constant. Literally, ceteris paribus means “other things being equal.” 需求曲线向下倾斜是因为，在其他条件不变 的情况下，较低的价格意味着较大的需求量 The demand curve slopes downward because, ceteris paribus, lower prices imply a greater quantity demanded! 17 市场需求 Market Demand ?市场需求指的是所有个人对某种商品或劳务的 需求总和。 Market demand refers to the sum of all individual demands for a particular good or service. ?从图形上看，个人需求曲线在水平上加总便得 到市场需求曲线。 Graphically, individual demand curves are summed horizontally to obtain the market demand curve. 18 需求的变化与需求量的变化 Change in Quantity Demanded versus Change in Demand 需求量的变化 Change in Quantity Demanded ?沿着需求曲线的移动。 Movement along the demand curve. ?由该产品价格的变动所引起。 Caused by a change in the price of the product. 4 19 需求量的变动 Changes in Quantity Demanded 0 D 1 每包香烟 的价格 Price of Cigarettes per Pack 每天抽烟的数量 Number of Cigarettes Smoked per Day 税收抬高了香烟价格，引致 了需求量由A到C沿着需求曲 线运动。A tax that raises the price of cigarettes results in a movement along the demand curve. A C 20 2.00 $4.00 12 20 需求量变化与需求变化 Change in Quantity Demanded versus Change in Demand 需求变化Change in Demand ?需求曲线发生移动，要么左移，要么右移 A shift in the demand curve, either to the left or right. ?由该物品价格之外的其他决定因素引起。 Caused by a change in a determinant other than the price. 21 需求的变化 Changes in Demand 0 D 1 价格 Price of Ice-Cream Cone 数量 Quantity of Ice-Cream Cones D 3 D 2 需求增加 Increase in demand 需求减少 Decrease in demand 22 消费者收入 Consumer Income ?随着收入的增加，对正常品的需求会增 加。 As income increases the demand for a normal good will increase. ?随着收入的增加，对劣等品（低档品） 的需求会减少。 As income increases the demand for an inferior good will decrease. 23 消费者收入：正常品 Consumer Income：Normal Good $3.00 2.50 2.00 1.50 1.00 0.50 21 345678910 1211 Price of Ice-Cream Cone Quantity of Ice-Cream Cones 0 需求增加 Increase in demand 收入增加… An increase in income... D 1 D 2 24 消费者收入：劣等品 Consumer Income：Inferior Good $3.00 2.50 2.00 1.50 1.00 0.50 21 345678910 1211 Price of Ice-Cream Cone Quantity of Ice-Cream Cones 0 需求减少 Decrease in demand 收入增加… An increase in income... D 1 D 2 5 25 相关物品的价格 Prices of Related Goods 替代品和互补品 Substitutes & Complements ?当一种商品的价格下跌减少了对另一种商 品的需求，这两种商品被称为替代品。 When a fall in the price of one good reduces the demand for another good, the two goods are called substitutes. ?当一种商品的价格下跌增加了对另一种商 品的需求，这两种商品被称为互补品。 When a fall in the price of one good increases the demand for another good, the two goods are called complements. 需求量变动与需求变动 Change in Quantity Demanded versus Change in Demand 影响需求量的变量 Variables that Affect Quantity Demanded 这些变量的变动将 A Change in This Variable . . . 价格Price 代表沿需求曲线的移动 Represents a movement along the demand curve 收入Income 移动需求曲线 Shifts the demand curve 相关商品价格 Prices of related goods 嗜好Tastes 预期Expectations 移动需求曲线 购买者人数 Number of buyers 移动需求曲线 移动需求曲线 移动需求曲线 27 供给 Supply 供给量是卖者愿意而且能够出售的数量。 Quantity supplied is the amount of a good that sellers are willing and able to sell. 28 决定供给的因素 Determinants of Supply ?市场价格Market price ?投入品价格Input prices ?技术Technology ?预期Expectations ?生产商的数量Number of producers 29 供给定律 Law of Supply 供给定律表述了价格与供给量之间的 正相关性。 The law of supply states that there is a direct (positive) relationship between price and quantity supplied. 30 供给表 Supply Schedule 供给表显示了商品价格与供给量的关系。 The supply schedule is a table that shows the relationship between the price of the good and the quantity supplied. 6 31 供给表 Supply Schedule 价格 Price 数量 Quantity $0.00 0 0.50 0 1.00 1 1.50 2 2.00 3 2.50 4 3.00 5 32 供给曲线 Supply Curve 供给曲线是将价格与供给量相联系的向上 倾斜的曲线。 The supply curve is the upward-sloping line relating price to quantity supplied. 33 供给曲线 Supply Curve $3.00 2.50 2.00 1.50 1.00 0.50 21 345678910 1211 Price of Ice-Cream Cone Quantity of Ice-Cream Cones 0 Price Quantity $0.00 0 0.50 0 1.00 1 1.50 2 2.00 3 2.50 4 3.00 5 34 市场供给 Market Supply ?市场供给是指某种商品或劳务的所有卖者 的供给量之和。 Market supply refers to the sum of all individual supplies for all sellers of a particular good or service. ?从图形上看，市场供给曲线是由单个供给 曲线在水平上的加总而得到。 Graphically, individual supply curves are summed horizontally to obtain the market supply curve. 35 供给量变化与供给变化 Change in Quantity Supplied versus Change in Supply 供给量变化Change in Quantity Supplied ?沿着供给曲线移动。 Movement along the supply curve. ?由该商品的市场价格变动引起。 Caused by a change in the market price of the product. 36 供给量的变动 Change in Quantity Supplied 15 Price of Ice-Cream Cone Quantity of Ice-Cream Cones 0 S 1.00 A C $3.00冰激凌蛋卷价格上扬 引致了供给量沿供给 曲线由A到C的运动 A rise in the price of ice cream cones results in a movement along the supply curve. 7 37 供给量变化与供给变化 Change in Quantity Supplied versus Change in Supply 供给变动Change in Supply ?供给曲线移动：左移或右移。 A shift in the supply curve, either to the left or right. ?由不同于该商品价格的其他决定因素引起的 变动。 Caused by a change in a determinant other than price. 38 供给变化 Change in Supply Price of Ice-Cream Cone Quantity of Ice-Cream Cones 0 S 1 S 2 S 3 供给增加 Increase in Supply 供给减少 Decrease in Supply 供给量变化与供给变化 Change in Quantity Supplied versus Change in Supply 影响供给量的变量 Variables that Affect Quantity Supplied 这种变量的变化将 A Change in This Variable . . . 价格 Price 代表沿供给曲线的移动 Represents a movement along the supply curve 要素价格 Input prices 移动供给曲线 Shifts the supply curve 技术 Technology 移动供给曲线 Shifts the supply curve 预期 Expectations 移动供给曲线 Shifts the supply curve 卖方人数 Number of sellers 移动供给曲线 Shifts the supply curve 40 供给与需求的结合 Supply and Demand Together 均衡价格Equilibrium Price ?使得供给和需求平衡的价格。在图形上，对应于供给曲 线与需求曲线交点处的价格。 The price that balances supply and demand. On a graph, it is the price at which the supply and demand curves intersect. 均衡数量Equilibrium Quantity ?使得供给和需求平衡的数量。在图形上，对应于供给曲 线和需求曲线交点处对应的数量。 The quantity that balances supply and demand. On a graph it is the quantity at which the supply and demand curves intersect. 41 供给与需求的结合 Supply and Demand Together Price Quantity $0.00 0 0.50 0 1.00 1 1.50 4 2.00 7 2.50 10 3.00 13 Price Quantity $0.00 19 0.50 16 1.00 13 1.50 10 2.00 7 2.50 4 3.00 1 需求表Demand Schedule供给表Supply Schedule 价格为$2时，需求量等于供给量！ At $2.00, the quantity demanded is equal to the quantity supplied! 42 供给Supply 需求Demand Price of Ice-Cream Cone Quantity of Ice-Cream Cones 供给与需求的均衡 Equilibrium of Supply and Demand 21 345678910 12110 $3.00 2.50 2.00 1.50 1.00 0.50 均衡点 Equilibrium 8 43 供给与需求的均衡 Equilibrium of Supply and Demand ?在均衡价格下，卖者想要和能够购买的物品数 量恰好等于卖者想要和能够出售的数量。 At the equilibrium price, the quantity of the good that buyers are willing and able to buy exactly balances the quantity that sellers are willing and able to sell. ?均衡价格有时也被称为市场出清价格，因为在 这种价格下，市场上的每一个人都得到满足。 The equilibrium price is sometimes called the market-clearing price because, at the price, everyone in the market has been satisfied. 44 供给与需求的均衡 Equilibrium of Supply and Demand ?均衡价格和均衡数量代表了经济学家对市场运 行结果的一种理论预测（prediction）。 ?为什么这种预测会是有道理的？因为： ?买者和卖者的行动会自然地推动市场向供求均 衡的方向移动。 The actions of buyers and sellers naturally move markets toward the equilibrium of supply and demand. 45 Price of Ice-Cream Cone Quantity of Ice-Cream Cones 21 345678910 12110 $3.00 2.50 2.00 1.50 1.00 0.50 供给Supply 需求Demand 过剩Surplus 超额供给 Excess Supply 46 过剩 Surplus 当价格高出均衡价格，供给量超出需求量，便 产生了超额供给或过剩。供应商将降低价格以 增加销售额，由此市场趋向均衡。 When the price is above the equilibrium price, the quantity supplied exceeds the quantity demanded. There is excess supply or a surplus. Suppliers will lower the price to increase sales, thereby moving toward equilibrium. 47 超额需求 Excess Demand Quantity of Ice-Cream Cones Price of Ice-Cream Cone $2.00 01234 5 6 7 8 9 10111213 供给Supply 需求Demand $1.50 短缺Shortage 48 短缺 Shortage 当价格低于均衡价格，需求量超出供给量，便产 生了超额需求或者短缺。由于过多的买者抢购过 少的物品，供应商将抬高价格，由此市场趋向于 均衡。 When the price is below the equilibrium price, the quantity demanded exceeds the quantity supplied. There is excess demand or a shortage. Suppliers will raise the price due to too many buyers chasing too few goods, thereby moving toward equilibrium. 9 49 分析均衡点的变动：比较静态 Analyzing Changes in Equilibrium： Comparative Statics ?三个步骤Three steps: （１）确定该事件是移动供给曲线还是需求曲线（或者 两者都移动）。 Decide whether the event shifts the supply or demand curve (or both). （２）确定曲线是左移还是右移。 Decide whether the curve(s) shift(s) to the left or to the right. （３）用供求图说明这种移动如何改变均衡价格和均衡 数量。 Examine how the shift affects equilibrium price and quantity. 需求的增长如何影响均衡 How an Increase in Demand Affects the Equilibrium 冰激凌蛋卷价格 Price of Ice-Cream Cone 2.00 0 7 冰激凌蛋卷数量 Quantity of Ice-Cream Cones 供给Supply 初始均衡点 Initial equilibrium D 1 1.天气炎热，冰激凌的需求增长 Hot weather increases the demand for ice cream... D 2 2. 价格上涨 ...resulting in a higher price... $2.50 10 3. 销售额增加 ...and a higher quantity sold. 新的均衡点New equilibrium 51 曲线移动与沿曲线移动 Shifts in Curves versus Movements along Curves ?供给曲线移动被称之为供给变动 A shift in the supply curve is called a change in supply. ?沿一固定供给曲线移动被称之为供给量变动 A movement along a fixed supply curve is called a change in quantity supplied. ?需求曲线移动被称之为需求变动 A shift in the demand curve is called a change in demand. ?沿一固定需求曲线移动被称之为需求量变动 A movement along a fixed demand curve is called a change in quantity demanded. S 2 供给减少如何影响均衡 How a Decrease in Supply Affects the Equilibrium 冰激凌蛋卷价格 Price of Ice-Cream Cone 2.00 0 1 2 3 4 7 8 9 11 12冰激凌蛋卷数量 Quantity of Ice-Cream Cones 13 需求Demand 初始均衡Initial equilibrium S 1 10 1. 地震，冰激凌供给减少 An earthquake reduces the supply of ice cream... 新的均衡 New equilibrium 2. 引致价格上扬 ...resulting in a higher price... $2.50 3.销售额下跌 ...and a lower quantity sold. 53 当供给或需求发生移动，价格和数量情况如何 What Happens to Price and Quantity When Supply or Demand Shifts? No Change In Supply An Increase In Supply A Decrease In Supply No Change In Demand P same Q same P down Q up P up Q down An Increase In Demand P up Q up P ambiguous Q up P up Q ambiguous A Decrease In Demand P down Q down P down Q ambiguous P ambiguous Q down 54 当供给或需求发生移动，价格和数量情况如何 What Happens to Price and Quantity When Supply or Demand Shifts? ?某同学通过观察美国资本市场的历史数 据发现，当利率较高时，投资额也较高 。他认为这明显违反了需求定律。他的 看法正确吗？ ?禽流感爆发时，照澜院的鸡蛋价格没有 变化，但销售量下降了。这是什么原因 ？ 10 55 当供给或需求发生移动，价格和数量情况如何 What Happens to Price and Quantity When Supply or Demand Shifts? ?我们通常观察到的市场价格和数量的变化都是 均衡价格和数量的变化。 ?决定均衡数量和价格变化的，是供给和需求两 种力量的共同作用。 ?因此，简单观察均衡价格和数量的相关关系， 并不能分辨（识别）出（identify）单纯的需求 曲线（需求定律）或供给曲线（供给定律）。 56 当供给或需求发生移动，价格和数量情况如何 What Happens to Price and Quantity When Supply or Demand Shifts? ?不过，在特定情形下，市场均衡价格和数量的 变化关系，反映了供给定律或需求定律规定的 数量－价格因果关系。 ?当市场上仅有供给发生变动（供给曲线移 动）时，我们观察到的均衡价格与数量的关 系就是需求曲线。 ?当市场上仅有需求发生变动（需求曲线移 动）时，我们观察到的均衡价格与数量的关 系就是供给曲线。 57 均衡分析的扩展与应用 ?供求均衡分析（特别是其比较静态分析 的方法）体现了一种基本的分析思想— —均衡分析。 ?均衡分析认为，人们观察到的许多经济 变量，是由不同性质的力量（关系）相 互作用的均衡结果。 ?当需要根据这些经济变量的变化来识别 这些不同性质的力量时，必须寻找适合 确定这一力量的其他力量的变化。 58 均衡分析举例：警察与犯罪 ?人们通常认为，一个地方警察人数的增 加，会遏制犯罪从而减少犯罪率。 ?然而，现实中往往观察到，一个地方的 警察人数与犯罪率可能是正相关的。 ?人们会问：是警察的增加导致犯罪的增 加吗？ 警察与犯罪 犯罪人数 （万人） 0警察人数 （万人） 警察的反应曲线P 罪犯的反应曲线C 1 初始均衡点 Initial equilibrium 1.贫困人口增多，导致犯罪增加 C 2 2. 犯罪人数增加 2.50 10 3. 警察人数也增加 2.00 7 新的均衡点New equilibrium 60 警察与犯罪 ?如何寻找证据，证明警察的增加是否会导致犯 罪率的下降？ ?一般说来，这并不容易。因为由于其他原因的 影响，犯罪总是在变动，这导致犯罪反应曲线 本身的移动。 ?但是，在某些特殊条件下，仅有警察反应曲线 变动，这一关系就被识别出来了。 11 警察反应曲线P 1 警察与犯罪 普通犯罪人数 （万人） 04警察人数（万人） 犯罪反应曲线C 初始均衡 Initial equilibrium P 2 7 2.00 1. 纽约市在“9.11”恐怖袭击之 后，增加警力保卫某些重 点区域 最新均衡 New equilibrium 3. 犯罪率下降 2.50 2.这些区域的警察人数增加 这一实际观察到的变化反应出犯罪反应曲线 为负斜率，说明警察对犯罪有阻遏作用。 62 总结 Summary ?经济学家们使用供求模型来分析竞争性市场。 Economists use the model of supply and demand to analyze competitive markets. ?需求曲线显示了一种物品的需求量如何依赖于 价格。 The demand curve shows how the quantity demanded of a good depends upon the price. 63 总结 Summary ?根据需求定律，当一种商品的价格上涨时，需 求数量下跌。 According to the law of demand, as the price of a good rises, the quantity demanded falls. ?除了价格外，决定需求量的因素还包括收入、 税收、预期以及互补品和替代品的价格。 In addition to price, other determinants of quantity demanded include income, tastes, expectations, and the prices of complements and substitutes. 64 总结 Summary ?供给曲线显示了一种物品的供给量如何依赖于 价格。 The supply curve shows how the quantity supplied of a good supplied depends upon the price. ?根据供给定律，随着一种商品的价格上涨，供 给的数量增加。 According to the law of supply, as the price of a good rises, the quantity supplied rises. 65 总结 Summary ?除了价格，决定供给量的其他因素还包括投入 要素价格、技术和预期。 In addition to price, other determinants of quantity supplied include input prices, technology, and expectations. ?市场均衡决定于供给曲线、需求曲线的交点。 Market equilibrium is determined by the intersection of the supply and demand curves. 66 总结 Summary ?供给和需求共同决定经济中商品和服务的价格 Supply and demand together determine the prices of the economy’s goods and services. ?在市场经济中，价格是引导资源配置的信号。 In market economies, prices are the signals that guide the allocation of resources.