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1、Chapter Three,Preferences 偏好,Structure,Describe preferences Indifference curves (无差异曲线) Well-behaved preferences Marginal rate of substitution (边际替代率),Rationality in Economics,Behavioral Postulate: A decisionmaker always chooses its most preferred alternative from its set of available alternatives.
2、So to model choice we must model decision-makers preferences.,Preference Relations,Comparing two different consumption bundles, x and y: strict preference (严格偏好): x is more preferred than is y (x y). Indifference (无差异): x is exactly as preferred as is y (xy). weak preference (弱偏好): x is as at least
3、as preferred as is y (x y).,p,Preference Relations,Preference relations are ordinal relations; i.e. they state only the order in which bundles are preferred.,Preference Relations,x y and y x imply x y.,Preference Relations,x y and y x imply x y. x y and (not y x) imply x y.,p,Assumptions about Prefe
4、rence Relations,Completeness (完备性): For any two bundles x and y it is always possible to make the statement that either x y or y x.,Assumptions about Preference Relations,Reflexivity (反身性): Any bundle x is always at least as preferred as itself; i.e. x x.,Assumptions about Preference Relations,Trans
5、itivity (传递性): If x is at least as preferred as y, and y is at least as preferred as z, then x is at least as preferred as z; i.e. x y and y z x z.,Indifference Curves,Take a reference bundle x. The set of all bundles equally preferred to x is the indifference curve containing x; the set of all bund
6、les y x. Since an indifference “curve” is not always a curve a better name might be an indifference “set”.,Indifference Curves,x2,x1,x”,x”,x x” x”,x,Indifference Curves,x2,x1,z x y,p,p,x,y,z,Indifference Curves,x2,x1,x,All bundles in I1 are strictly preferred to all in I2.,y,z,All bundles in I2 are
7、strictly preferred to all in I3.,I1,I2,I3,x2,x1,I(x),x,I(x),WP(x), the set of bundles weakly preferred to x.,Weakly Preferred Set (弱偏好集),Weakly Preferred Set (弱偏好集),x2,x1,WP(x), the set of bundles weakly preferred to x.,WP(x) includes I(x).,x,I(x),Strictly Preferred Set (严格偏好集),x2,x1,SP(x), the set
8、of bundles strictly preferred to x, does not include I(x).,x,I(x),Indifference Curves Cannot Intersect,x2,x1,x,y,z,I1,I2,From I1, x y. From I2, x z. Therefore y z. But from I1 and I2 we see y z, a contradiction.,p,Slopes of Indifference Curves,When more of a commodity is always preferred, the commod
9、ity is a good. If every commodity is a good then indifference curves are negatively sloped.,Slopes of Indifference Curves,Better,Worse,Good 2,Good 1,Two goods a negatively sloped indifference curve.,Slopes of Indifference Curves,If less of a commodity is always preferred then the commodity is a bad.
10、,Slopes of Indifference Curves,Better,Worse,Good 2,Bad 1,One good and one bad a positively sloped indifference curve.,Examples,Perfect substitutes (完全替代) Perfect complements (完全互补) Satiation (餍足) Discrete goods (离散商品),Extreme Cases of Indifference Curves; Perfect Substitutes,If a consumer always reg
11、ards units of commodities 1 and 2 as equivalent, then the commodities are perfect substitutes and only the total amount of the two commodities in bundles determines their preference rank-order.,Extreme Cases of Indifference Curves; Perfect Substitutes,x2,x1,8,8,15,15,Slopes are constant at - 1.,I2,I
12、1,Bundles in I2 all have a total of 15 units and are strictly preferred to all bundles in I1, which have a total of only 8 units in them.,Extreme Cases of Indifference Curves; Perfect Complements,If a consumer always consumes commodities 1 and 2 in fixed proportion (e.g. one-to-one), then the commod
13、ities are perfect complements and only the number of pairs of units of the two commodities determines the preference rank-order of bundles.,Extreme Cases of Indifference Curves; Perfect Complements,x2,x1,I1,45o,5,9,5,9,Each of (5,5), (5,9) and (9,5) contains 5 pairs so each is equally preferred.,Ext
14、reme Cases of Indifference Curves; Perfect Complements,x2,x1,I2,I1,45o,5,9,5,9,Since each of (5,5), (5,9) and (9,5) contains 5 pairs, each is less preferred than the bundle (9,9) which contains 9 pairs.,Preferences Exhibiting Satiation,A bundle strictly preferred to any other is a satiation point or
15、 a bliss point. What do indifference curves look like for preferences exhibiting satiation?,Indifference Curves Exhibiting Satiation,x2,x1,Satiation (bliss) point,Indifference Curves Exhibiting Satiation,x2,x1,Better,Better,Better,Satiation (bliss) point,Indifference Curves Exhibiting Satiation,x2,x1,Better,Better,Better,Satiation (bliss) point,Indifference Curves for Discrete Commodities,A commodity is infinitely divisible if it can be acquired in any quantity; e.g. water or cheese. A commodity is discrete if it comes in un
