# Prerequisites Almost essential Consumner Optimisation Frank Cowell Microeconomics

- Slides: 38

Prerequisites Almost essential Consumner: Optimisation Frank Cowell: Microeconomics Useful, but optional Firm: Optimisation September 2011 Household Demand Supply MICROECONOMICS Principles and Analysis Frank Cowell

Working out consumer responses Frank Cowell: Microeconomics n The analysis of consumer optimisation gives us some powerful tools: u u u n The primal problem of the consumer is what we are really interested in. Related dual problem can help us understand it. The analogy with the firm helps solve the dual. The work we have done can map out the consumer's responses u u to changes in prices to changes in income what we know about the primal

Overview. . . Frank Cowell: Microeconomics Household Demand & Supply Response functions The basics of the consumer demand system. Slutsky equation Supply of factors Examples

Solving the max-utility problem Frank Cowell: Microeconomics l The primal problem and its solution n max U(x) + m[ y – S pi xi ] Link to full discussion U 1 = mp 1 U 2(x*) = mp 2. . Un(x*) = mpn (x*) i=1 n ü ý þ The Lagrangean for the max U problem § The n+1 first-order conditions, assuming all goods purchased. § S pixi* = y i=1 l Solve this set of equations: x 1* = D 1(p, y) x 2* = D 2(p, y). . xn* = Dn(p, y) n S i=1 pi. Di(p, ü ý þ y) = y §Gives a set of demand functions, one for each good. Functions of prices and incomes. §A restriction on the n equations. Follows from the budget constraint

The response function Frank Cowell: Microeconomics response function for the primal problem is demand for good i: xi* = Di(p, y) h. The system of equations must have an “adding-up” property: §Should be treated as just one of a set of n equations. h. The n S pi Di(p, y) = y §Reason? This follows immediately from the budget constraint: left-hand side is total expenditure. i=1 h. Each equation in the system must be homogeneous of degree 0 in prices and income. For any t > 0: xi* = Di(p, y )= Di(tp, ty) Reason? Again follows immediately from the budget constraint. § To make more progress we need to exploit the relationship between primal and dual approaches again. . .

How you would use this in practice. . . Frank Cowell: Microeconomics n n n Consumer surveys give data on expenditure for each household over a number of categories… …and perhaps income, hours worked etc as well. Market data are available on prices. Given some assumptions about the structure of preferences… …we can estimate household demand functions for commodities. From this we can recover information about utility functions.

Overview. . . Frank Cowell: Microeconomics Household Demand & Supply Response functions A fundamental decomposition of the effects of a price change. Slutsky equation Supply of factors Examples

Consumer’s demand responses Frank Cowell: Microeconomics n n u u n Fixed income? Income endogenously determined? And on the type of budget change. u n Link to budget constraint What’s the effect of a budget change on demand? Depends on the type of budget constraint. Income alone? Price in primal type problem? Price in dual type problem? So let’s tackle the question in stages. Begin with a type 1 (exogenous income) budget constraint.

Effect of a change in income Frank Cowell: Microeconomics Take the basic equilibrium § Suppose income rises § x 2 §The effect of the income increase. Demand for each good does not fall if it is “normal” § x** x* But could the opposite happen? § x 1

An “inferior” good Frank Cowell: Microeconomics Take same original prices, but different preferences § Again suppose income rises § x 2 §The effect of the income increase. Demand for good 1 rises, but… § Demand for “inferior” good 2 falls a little § x* x** Can you think of any goods like this? § x 1 §How might it depend on the categorisation of goods?

A glimpse ahead. . . Frank Cowell: Microeconomics We can use the idea of an “income effect” in many applications. n Basic to an understanding of the effects of prices on the consumer. n Because a price cut makes a person better off, as would an income increase. . . n

Effect of a change in price Frank Cowell: Microeconomics Again take the basic equilibrium § x 2 § Allow price of good 1 to fall §The effect of the price fall. §The “journey” from x* to x** broken into two parts incomesubstitution effect ° x* x** x 1

And now let’s look at it in maths Frank Cowell: Microeconomics We want to take both primal and dual aspects of the problem. . . n. . . and work out the relationship between the response functions. . . n. . . using properties of the solution functions. n (Yes, it’s time for Shephard’s lemma again. . . ) n

A fundamental decomposition Frank Cowell: Microeconomics compensated demand h. Take ordinary demand the two methods of writing xi*: § Remember: they are two ways of representing the same thing Hi(p, u) = Di(p, y) h. Use cost function to substitute for y: §Gives us an implicit relation in prices and utility. Hi(p, u) = Di(p, C(p, u)) with respect to pj : Hji(p, u) = Dji(p, y) + Dyi(p, y)Cj(p, u) h. Differentiate § h. Simplify §Using : Hji(p, u) = Dji(p, y) + Dyi(p, y) Hj(p, u) = Dji(p, y) + Dyi(p, y) xj* h. And Uses function-of-a-function rule again. Remember y=C(p, u) cost function and Shephard’s Lemma again § From the comp. demand function § This is the Slutsky equation so we get: Dji(p, y) = Hji(p, u) – xj*Dyi(p, y)

The Slutsky equation Frank Cowell: Microeconomics Dji(p, y) = Hji(p, u) – xj*Dyi(p, y) Gives fundamental breakdown of effects of a price change § Income effect: “I'm better off if the price of jelly falls, so I buy more things, including icecream. I’m worse off if the price of jelly rises, so I buy less icecream” § l l x* x** “Substitution effect: When the price of jelly falls and I’m kept on the same utility level, I prefer to switch from icecream for dessert” §

Slutsky: Points to watch Frank Cowell: Microeconomics n Income effects for some goods may have “wrong” sign u u n For n > 2 the substitution effect for some pairs of goods could be positive… u u n net substitutes apples and bananas? … while that for others could be negative u u n for inferior goods… …get opposite effect to that on previous slide net complements gin and tonic? back to the maths Neat result is available if we look at special case where j = i

The Slutsky equation: own-price Frank Cowell: Microeconomics Link to firm’s input demand h. Set j = i to get the effect of the price of icecream on the demand for icecream Dii(p, y) = Hii(p, u) – xi*Dyi(p, y) h. Own-price substitution effect must be negative h– xi* income effect is nonpositive for normal goods h. So, Follows from the results on the firm § Price increase means less disposable income § if the demand for i does not decrease when y rises, then it must decrease when pi rises.

Price fall: normal good Frank Cowell: Microeconomics p 1 ordinary demand curve D 1(p, y) compensated (Hicksian) demand curve § The initial equilibrium § price fall: substitution effect § total effect: normal good § income effect: normal good H 1(p, u) price fall initial price level §For normal good income effect must be positive or zero Compensating Variation x*1 x** 1 x 1

Price fall: inferior good Frank Cowell: Microeconomics p 1 ordinary demand curve price fall initial price level § The initial equilibrium § price fall: substitution effect § total effect: inferior good § income effect: inferior good compensated demand curve §Note relative slopes of these curves in inferiorgood case. §For inferior good income effect must be negative Compensating Variation x*1 x** 1 x 1

Features of demand functions Frank Cowell: Microeconomics Homogeneous of degree zero n Satisfy the “adding-up” constraint n Symmetric substitution effects n Negative own-price substitution effects n Income effects could be positive or negative: n u in fact they are nearly always a pain.

Overview. . . Frank Cowell: Microeconomics Household Demand & Supply Response functions Extending the Slutsky analysis. Slutsky equation Supply of factors Examples

Frank Cowell: Microeconomics Link to budget constraint Consumer demand: alternative approach Now for an alternative way of modelling consumer responses. n Take a type-2 budget constraint (endogenous income). n Analyse the effect of price changes… n …allowing for the impact of price on the valuation of income n

Consumer equilibrium: another view Frank Cowell: Microeconomics so as to buy more good 2 x 2 Type 2 budget constraint: fixed resource endowment §Budget constraint with endogenous income § § Consumer's equilibrium §Its n n i=1 interpretation {x: S pi xi S pi. Ri } Equilibrium is familiar: same FOCs as before. § l x* consumer sells some of good 1. . l R x 1

Two useful concepts Frank Cowell: Microeconomics n 1. From the analysis of the endogenous-income case derive two other tools: The offer curve: u u 2. Path of equilibrium bundles mapped out by prices Depends on “pivot point” - the endowment vector R The household’s supply curve: u u The “mirror image” of household demand. Again the role of R is crucial.

The offer curve Frank Cowell: Microeconomics x 2 Take the consumer's equilibrium § Let the price of good 1 rise a bit more § l l § x*** Draw the locus of points x** This path is the offer curve. § l x* l R Amount of good 1 that household supplies to the market § x 1

Household supply Frank Cowell: Microeconomics Flip horizontally , to make supply clearer § Rescale the vertical axis to measure price of good 1. § § Plot p 1 against x 1. p 1 x 2 This path is the household’s supply curve of good 1. § l x*** l x** Note that the curve “bends back” on itself. § l l R x* supply of good 1 §Why?

Decomposition – another look Frank Cowell: Microeconomics h. Take ordinary demand for good i: xi* = Di(p, y) h. Substitute in for y : xi* = Di(p, Sj pj. Rj) direct effect of pj on demand h. Differentiate with respect §Income indirect effect of prices pj on demand via the impact on income to pj : dxi* dy — = Dji(p, y) + Dyi(p, y) — dpj = Dji(p, y) + Dyi(p, y) Rj h. Now recall the Slutsky relation: Dji(p, y) = Hji(p, u) – xj* Dyi(p, y) h. Use §Function of prices and income itself now depends on §The indirect effect uses function -of-a-function rule again §Just the same as on earlier slide this to substitute for Dji in the above: § dxi* — = Hji(p, u) + [Rj – xj*] Dyi(p, y) dpj This is the modified Slutsky equation

The modified Slutsky equation: Frank Cowell: Microeconomics dxi* ── = Hji(p, u) + [Rj – xj* ] Dyi(p, y) dpj h. Substitution effect has same interpretation as before h. Two terms to consider when interpreting the income effect h. This is just the same as before h. This term makes all the difference: o. Negative if the person is a net demander o. Positive if he is a net supplier some examples

Overview. . . Frank Cowell: Microeconomics Household Demand & Supply Response functions Labour supply, savings… Slutsky equation Supply of factors Examples

Some examples Frank Cowell: Microeconomics n Many important economic issues fit this type of model : u u u n It's important to identify the components of the model. u u u n Subsistence farming. Saving. Labour supply. How are the goods to be interpreted? How are prices to be interpreted? What fixes the resource endowment? To see how key questions can be addressed. u u How does the agent respond to a price change? Does this depend on the type of resource endowment?

Subsistence agriculture. . . Frank Cowell: Microeconomics x 2 Resource endowment includes a lot of rice § Slope of budget constraint increases with price of rice § § Consumer's equilibrium x 1, x 2 are “rice” and “other goods” § l x* supply. . l R Will the supply of rice to export rise with the world price. . . ? . § x 1

The savings problem. . . Frank Cowell: Microeconomics x 2 Resource endowment is non -interest income profile § Slope of budget constraint increases with interest rate, r § § Consumer's equilibrium § Its interpretation x 1, x 2 are consumption “today” and “tomorrow” § Determines time -profile of consumption § l x* saving. . l R What happens to saving when the interest rate changes. . . ? . § 1+r x 1

Labour supply. . . Frank Cowell: Microeconomics x 2 Endowment is total time available & non-labour income. § Slope of budget constraint is the wage rate § § Consumer's equilibrium x 1, x 2 are leisure and “consumption” § Determines labour supply § l labour supply. x* wage rate l R Will people work harder if their wage rate goes up? . § non-labour income. x 1

Modified Slutsky: labour supply Frank Cowell: Microeconomics h. Take the modified Slutsky: §The dxi* — = Hij(p, u) + [Rj – xj*] Diy(p, y) dpj h. Assume that supply of good i is the only source of income (so y= pi[Ri – xi]). Then, for the effect of pi on xi* we get: dxi* y — = Hii(p, u) + — Diy(p, y) dpi pi h. Rearranging : general form. We are going to make a further simplifying assumption pi dxi* pi y i (p, u) – ——* — = – —— H Diy(p, y) Total labour supply j * * Ri–xi dpelasticity: Rcould Ri–xi i i–xi bemust + orbe – h. Write positive (backward-bending) in elasticity form: negative if leisure is a normal good etotal = esubst + eincome §Suppose good i is labour time; then Ri – xi is the labour you sell in the market (I. e. leisure time not. consumed); pi is the wage rate §Divide by labour. supply; multiply by (-) wage rate §The Modified Slutsky equation in a simple form Estimate the whole demand system from family expenditure data. . .

Modified Slutsky: labour supply Frank Cowell: Microeconomics h. Take the modified Slutsky: dxi* — = Hii(p, u) + [Ri – xi*] Diy(p, y) dpi h. Assume that supply of good i is the only source of income (so y= pi[Ri – xi]). Then, for the effect of pi on xi* we get: dxi* y — = Hii(p, u) + — Diy(p, y) dpi pi h. Rearranging : §The general form. We are going to make a further simplifying assumption pi dxi* pi y i (p, u) – ——* — = – —— H Diy(p, y) j * * Ri–xi dpi Ri–xi h. Write in elasticity form: etotal = esubst + eincome §Suppose good i is labour time; then Ri – xi is the labour you sell in the market (I. e. leisure time not. consumed); pi is the wage rate §Divide by labour. supply; multiply by (-) wage rate §The Modified Slutsky equation in a simple form Estimate the whole demand system from family expenditure data. . .

Simple facts about labour supply Frank Cowell: Microeconomics § Source: Blundell and Walker (Economic Journal, 1982) The estimated elasticities. . . Men's labour supply is backward bending! § Leisure is a "normal good" for everyone § Children tie down women's substitution effect. . . § total Men: subst income – 0. 23 +0. 13 − 0. 36 No children +0. 43 +0. 65 − 0. 22 One child +0. 10 +0. 32 − 0. 22 Two children – 0. 19 +0. 03 − 0. 22 Women:

Summary Frank Cowell: Microeconomics Review n How it all fits together: Compensated (H) and ordinary (D) demand functions can be hooked together. n Slutsky equation breaks down effect of price i on demand for j. n Endogenous income introduces a new twist when prices change. n

What next? Frank Cowell: Microeconomics The welfare of the consumer n How to aggregate consumer behaviour in the market. n

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