# Problem Set I: Preferences, W.A.R.P., consumer choice

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1 Problem Set I: Preferences, W.A.R.P., consumer choice Paolo Crosetto Exercises solved in class on 18th January 2009 Recap:,, Definition 1. The strict preference relation is x y x y but not y x Definition 2. The indifference relation is x y x y and y x Recap: rationality assumptions is rational if it is Complete: x, y X, we have x y or y x or both; Transitive: x, y, z X, if x y and y z, then x z. Recap: and utility function u( ) Definition 3. A function u : X R is a utility function representing if x, y X : x y u(x) u(y) 1. MWG, Exercise 1.B B.2: properties of Prove that if is rational (complete and transitive), then 1. is both irreflexive (x x never holds) and transitive (if x y and y z, then x z); 2. is reflexive (x x, x), transitive (if x y and y z, then x z) and symmetric (if x y then y x); 3. if x y z then x z. Solution: property 3 first Proof. Property 3: if x y z then x z 1. By definition, x y means that x y but not y x; 2. then, x y z means x y z; 3. for transitivity (assumed), this means that x z. 4. Now, let s suppose that z x. Since y z, by transitivity we d have y x 5. but this is a contradiction, since we had in the beginning that x y. 6. So, we have x z but we cannot have z x: this means that x z 1

2 Solution: property 1 Proof. Property 1: is irreflexive and transitive 1. Irreflexivity. Use completeness: x y, x, y X: 2. hence, it must hold also for x x, x X; 3. this means that in no case there can be x x. 4. Transitivity. Suppose x y and y z: 5. this means that at least x y z. 6. But we have proved before that this means x z. Solution: property 2 Proof. Property 2: is reflexive, transitive and symmetric 1. Reflexivity. By completeness, x x, x X: 2. this implies also that x x, x X, by definition of. 3. Transitivity. Suppose x y and y z: 4. by the definition of, this means that all of these hold: 5. x y, y x, y z, z y. 6. By transitivity of, this implies both x z and z x: hence x z. 7. Symmetry. Suppose x y: by definition, then x y and y x. 8. But the latter is also the definition of y x, if you look it the other way around. 9. hence, x y implies y x. 2. MWG 1.B B.4.: and u( ) Show that if f : R R is a strictly increasing function and u : X R is a utility function representing the preference relation, then the function v : X R defined by v(x) = f (u(x)) is also a utility function representing ; Consider a preference relation and a function u : X R. Show that if u(x) = u(y) implies x y and if u(x) > u(y) implies x y then u( ) is a utility function representing. Solution: strictly increasing function, intuition We will prove that a utility function associated with is ordinal and not cardinal in nature. This is important: among other things, it implies that it is impossible to make interpersonal utility comparisons directly. Note the definition of strictly increasing function: Definition 4. A function f (x) is said to be strictly increasing over an interval I if f (b) > f (a) for all b > a, when a, b I. Example 5. Functions that are strictly increasing over their whole domain are among others all positive straight lines (y = ax, a > 0) and positive exponentials (y = a x, a > 0); other functions can be increasing over a part of their domain, as parabola (y = x 2, for x > 0). Tip: a strictly increasing function on interval I has its derivative positive on I. 2

3 Solution: strictly increasing functions, plots Solution: strictly increasing function, proof Proof. A strictly increasing transformation of a utility function is still a utility function 1. Let s take x, y X. Since u( ) represents, by definition: 2. if x y then u(x) u(y). 3. since f ( ) is strictly increasing, applying f ( ) to u( ) does not change order, but only magnitude; 4. hence, f (u(x)) f (u(y)), i.e. v(x) v(y) when x y: 5. hence, v( ) is a utility function representing. Solution: u( ), and. Proof. if x y, then u(x) u(y) 1. Suppose x y. 2. if at this we add y x, then x y and u(x) = u(y). 3. if instead we don t have y x, then x y and u(x) > u(y). 4. hence, if x y, then u(x) u(y) Proof. if u(x) u(y), then x y 1. Suppose u(x) u(y). 2. if at this we add u(x) = u(y), then x y. 3. if instead we add u(x) > u(y), then x y. 4. hence, if u(x) u(y), then x y. 3

4 3. MWG 1.B.5: and u( ), II Show that if X is finite and is a rational preference relation on X, then there is a utility function u : X R that represents. Solution: intuition Since X is finite, the set of pairwise combination of elements of X is finite too; Since is rational (hence complete and transitive): it defines a preference over all of the finite set of pairs; it excludes contradictory cycles of preferences. Hence, intuitively it is possible to rank all x, y X according to ; it must then be possible to build a utility function with such a complete ranking using. Solution: proving by induction A proof by induction is done by showing that something is true for n = 1 and then for n + 1; it then follows that it must be true for all n up to N. Proof by induction is used in the set of natural numbers N. More formally, for any proposition P(n) about positive integers: Prove that P(1) is true (base case); Prove that for each k 1, if P(k) is true, then P(k + 1) is true (inductive step). Example 6. Consider a set of domino tiles. If domino tile n falls, tile n + 1 will fall. If we prove that tile 1 has fallen, then we can conclude that all tiles will fall. Solution: proof, x y Proof. if X is finite, then there exists a u( ) representing : no indifference 1. Start considering that no two items are indifferent, i.e. x y, x, y X; 2. Let s prove by induction that in such a setting there exists a u( ) representing. 3. Base case: if N = 1 there is nothing to prove. 4. Inductive step: Let s suppose the claim is true for N 1, and let s prove it is still ture for N. 1. Let s take X = {x 1, x 2,..., x N 1, x N }. 2. By hypothesis, there exists a u( ) on defined up to x N Let s order the x: let s assume u(x 1 ) > u(x 2 ) > > u(x N 1 ). 4. Since we have assumed no indifference, the above ranking means exclusively: i < N, x N x i i < N, x i x N i < N and j < N s.t. x i x N x j 4

5 Solution: proof, x y continued Proof....continued In all the three cases above we can find a value of u( ) that is consistent: 1. In Case 1, we can take u(n) > u(x 1 ); 2. In Case 2, we can take u(n) < u(x N 1); 3. in Case 3: Define two intervals I = {i (1... N) : x i x N } and J = {j (1... N) : x N x j }; I and J are disjoint intervals on N by our hypotheses; then if i = max I, i + 1 = min J. We can then take u(x N ) to lie in the interval (u(i ), u(i + 1)). Hence, in all of three cases an utility function can be built. Recap: W.A.R.P., Definition 7 (WARP). A choice structure (B, C( )) satisfies the weak axiom if for some B B with x, y B we have x C(B), then for any B B with x, y B, if we have y C(B ) we must also have x C(B ) Which is indeed a minimal consistency requirement. Note that completeness and transitivity are not required. Definition 8 (Revealed preference relation ). Given a choice structure (B, C( )), the revealed preference relation is defined as: x y there is some B B such that x, y B and x C(B). Which is just attaching a preference relationship to choices Note again that nor completeness nor transitivity are implied. It is just descriptive. Recap: Rationalizability Rational preferences W.A.R.P. satisfied always W.A.R.P. satisfied Rational preferences not always Definition 9 (Rationalizability). Given a choice structure (B, C( )), the rational preference relation rationalizes C( ) relative to B if C(B) = C (B, ) for all B B. In other words, generates the choice structure (B, C( )). the W.A.R.P. is a necessary but not sufficient condition for rationalizability. if B includes all subsets of X of up to three elements, then it is also sufficient: intuitively, the three-members property implies transitivity Exercise on W.A.R.P. Consider a choice problem with choice set X = {x, y, z}. Consider the following choice structures: (B, C( )), in which B = {{x, y}, {y, z}, {x, z}, {x}, {y}, {z}} and C({x, y}) = {x}, C({y, z}) = {y}, C({x, z}) = {z}, C({x}) = {x}, C({y}) = {y}, C({z}) = {z}. (B, C( )), in which B = {{x, y, z}, {x, y}, {y, z}, {x, z}, {x}, {y}, {z}} and C({x, y, z}) = {x}, C({x, y}) = {x}, C({y, z}) = {z}, C({x, z}) = {z}, C({x}) = {x}, C({y}) = {y}, C({z}) = {z}. 5

6 (B, C( )), in which B = {{x, y, z}, {x, y}, {y, z}, {x, z}, {x}, {y}, {z}} and C({x, y, z}) = {x}, C({x, y}) = {x}, C({y, z}) = {y}, C({x, z}) = {x}, C({x}) = {x}, C({y}) = {y}, C({z}) = {z}. For every choice structure say if the WARP is satisfied and if it exists a rational preference relation that rationalizes C( ) relative to its B. If such a rationalization is possible, write it down. Comment on your results. Solution: (B, C( )) The choice structure can be summarised in these three relations: C({x, y}) = {x} reveals x y; C({y, z}) = {y} reveals y z C({x, z}) = {z} reveals z x 1. W.A.R.P. is trivially satisfied the same couple never appears more than once in different budgets; moreover, B does not include all budgets up to three elements. revealed preference relation is not necessarily transitive 2. B is NOT rationalizable: C({x, y}) = {x} is rationalised by x y; C({y, z}) = {y} is rationalised by y z; C({x, z}) = {z} is rationalised by z x. It is not transitive, hence (B, C( )) is not rationalisable. Solution: (B, C( )) The choice structure can be summarised in these relations: C({x, y, z}) = {x} reveals x y and x z C({x, y}) = {x} reveals x y; C({y, z}) = {y} reveals y z; C({x, z}) = {z} reveals z x. 1. W.A.R.P. is NOT satisfied x z and z x hold at the same time; in this case it exists x, z B : C(B) = {x}, but there is also......a x, z B : z C(B ) but not x C(B ) 2. B is NOT rationalisable: since in general if is rational satisfies W.A.R.P.; then, by using the contrapositive, if A B, it must be true that B A Hence (B, C( )) is not rationalisable 6

7 Solution: (B, C( )) The choice structure can be summarised in these relations: C({x, y, z}) = {x} reveals x y and x z C({x, y}) = {x} reveals x y; C({y, z}) = {y} reveals y z; C({x, z}) = {x} reveals x z. 1. W.A.R.P. is satisfied there are no violations of the type x y and y x; moreover, B includes all budgets up to three elements. 2. B is rationalizable: C({x, y, z}) = {x} reveals x y and x z C({x, y}) = {x} reveals x y; C({y, z}) = {y} reveals y z; C({x, z}) = {x} reveals x z. Hence x y z is complete and transitive and rationalises (B, C( )) 5. MWG 1.D.2: and W.A.R.P. Show that if X is finite, then any rational preference relation generates a nonempty choice rule; that is, C(B) = for any B X with B =. Solution Proof. X finite C(B) = 1. We proved earlier that if X is finite, then u( ) is a utility function representing a rational. (by induction. Remember??) 2. Since X is finite, for any B X with B = there exists x C(B) such that u(x) u(y) for all y B remember that finiteness implied that we could order all alternatives in X, and assign a value. 4. Then, it means that x C (B, ), i.e. x is chosen according to preference relation in B. 5. Hence, C (B, ) cannot be empty: C (B, ) = 7

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