北大中级微观经济学-第一章第2节

北京大学经济学院 张元鹏
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Readings about this chapter
Nicholson: Chapter 2-5,7 Zhang(2007): Chapter 3
北京大学经济学院 张元鹏
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Outline of Today’s Class
2.1 Preference and Utility
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4. Assumptions of Consumer’s Preference
Assumption 3: Reflexive.（自反性假设）
( x1 , x2 )f( x1 , x2 )
Assumption 4: Non Satiation or More is good.（非饱和性假设） An economic good is one that yields positive benefits to people. Thus, more of a good is, by definition, better. (此假设意味着多多益善)
2.1.1 Assumption of Consumer’s Preference 2.1.2 From Preference to Utility Function 2.1.3 Indifference Curve（无差异曲线） 2.1.4 Special Utility Functions and Their Indifference Curves
So, to model consumer choice must model consumer preferences.
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1. Defining Preference
消费者偏好(consumer preferences) 是指消费者根据自己的意愿对其 所消费的可能的商品组合所进行的喜 好顺序的排列。
Note: X, or Y is called a bundle of consumption goods (消费品束或组合)
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2. How to Express Preference Relations (1) Bundles of consumption goods:
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2.1.1 Assumption of Consumer’s Preference
Consumer’s Rationality in Economics
Basic behavioral postulate: Maximizing his well-being.(基本行为假定：最大化自己福利水平) Consumer’s optimization is constrained by his income or time. （消费者的最优选择受其收入和时 间的约束） Decision-maker chooses best available alternative（消费者选择最优的可供消费的产品组 合）.
If then
(x1, x2 )f( y1, y2 )
(x1, x2 )f(z1, z2 )
, and
( y1, y2 )f(z1, z2 )
Transitivity means a consumers choices must be logically consistent（转递性假设意味着消费者 选择的逻辑一致性）
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2.1.2 From Preference to Utility Ferences are
Complete Reflexive Transitive Non-satiation Continuous (no jumps)
Then, can be represented by a continuous utility function. （如果消费者偏好能够满足上述五个消费者偏好假 设，则一定会有一个连续的效用函数来表达这种偏好 关系）
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1.Utility Functions（效用函数）
A utility function U(x) represents a preference relation if and only if(当且仅当): x’ > x” x’ < x” U(x’) > U(x”) U(x’) < U(x”)
X = ( x1 , x2 )
If bundle X is preferred to Y If bundle Y is preferred to X
Y = ( y1 , y 2 )
( x1 , x2 ) f ( y1 , y2 ) ( y1 , y2 ) f ( x1 , x2 ) ( y1 , y2 ) ~ ( x1 , x2 )
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4. Assumptions of Consumer’s Preference
Assumption 5: Continuity （连续性） If an individual reports that “X is preferred to Y” then situations suitably “close to” X must also be preferred to Y” Continuity: small bundle changes cause only small changes to the preference level（消费 组合的任何微小的变化都会导致偏好水平的微 小变化）
Utility = U ( X 1, X 2 L X n ; other things).
The other things that appear after the semicolon are assumed to be held constant.
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3. Argument of Utility Function（效用 函数的自变量）
1. U=U(W) W presents an individual real wealth. 2. U=U(I, H) I presents an individual’s income, H presents hours of nonwork time 3. U=U(C1,C2) C1 is consumption in this period and C2 is consumption in the next period.
If consumer indifferent between X and Y
For example, x1=5 bananas, x2=6 oranges, y1=3 coconuts and y2=2 packets of biscuits
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2. How to Express Preference Relations(2)
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4.Assumptions of Consumer’s Preference
Assumption 1:Completeness （完备性假设）
If X and Y are any situations, the individual can always specify exactly one of the following three possibilities:
“X is preferred to Y”, i.e. ( x1 , x2 )f( y1 , y2 ) “Y is preferred to X”, i.e. ( x 1 , x 2 ) p ( y 1 , y 2 ) “X and Y are equally attractive”, i.e.( x1 , x2 ) ~ ( y1 , y2 )
Chapter 2
Consumer Behavior and Demand Theory (消费者行为与需求理论)
© 2007 MOL
This Chapter will Include:
2.1 Preference and Utility(偏好与效用) 2.2 Utility Maximization and Choice 2.3 Income and Substitution Effects （收入效应与替代效应） 2.4 Market Demand and Elasticity
Consumers can compare two different consumption bundles, X and y: strict preference（严格偏好）: X f Y weak preference（弱偏好）: X fY
indifference: X ~ Y
Preference Relations If X > Y and Y > X, then X~Y If X > Y but not Y> X, then X > Y
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2.1 Preference and Utility
2.1.1 Assumption of Consumer’s Preference 2.1.2 From Preference to Utility Function 2.1.3 Indifference Curve（无差异曲线） 2.1.4 Special Utility Functions and Their Indifference Curves
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1. Defining Preference
When an individual reports that “X is preferred to Y,” it is taken to mean that all things considered, he or she feel better off under situation X than under situation Y.