Mankiw Chapter 11. Aggregate Demand I. Building the IS-LM Model

Transcription

1 Mankiw Chapter 11 Building the IS-LM Model 0

2 IN THIS CHAPTER, WE WILL COVER: the IS curve and its relation to: the Keynesian cross the LM curve and its relation to: the theory of liquidity preference how the IS-LM model determines income and the interest rate in the short run when P is fixed 1

3 Context Chapter 10 introduced the model of aggregate demand and aggregate supply. Long run: prices flexible output determined by factors of production & technology unemployment equals its natural rate Short run: prices fixed output determined by aggregate demand unemployment negatively related to output 2

4 Context This chapter develops the IS-LM model, the basis of the aggregate demand curve. We focus on the short run and assume the price level is fixed (so the SRAS curve is horizontal). We focus on the closed-economy case 3

5 The Keynesian cross A simple closed-economy model in which income is determined by expenditure. (due to J. M. Keynes) Notation: I = planned investment PE = C + I + G = planned expenditure Y = real GDP = actual expenditure Difference between actual & planned expenditure = unplanned inventory investment 4

6 Elements of the Keynesian cross consumption function: govt policy variables: C = C( Y T ) G = G, T = T for now, planned investment is exogenous: I = I planned expenditure: PE = C ( Y T ) + I + G equilibrium condition: actual expenditure = planned expenditure Y = PE 5

7 Graphing planned expenditure PE planned expenditure PE =C +I +G 1 MPC income, output, Y 6

8 Graphing the equilibrium condition PE planned PE =Y expenditure 45º income, output, Y 7

9 The equilibrium value of income PE planned expenditure PE =Y PE =C +I +G income, output, Y Equilibrium income 8

10 An increase in government purchases PE At Y 1, there is now an unplanned drop in inventory PE =C +I +G 2 PE =C +I +G 1 ΔG so firms increase output, and income rises toward a new equilibrium. PE 1 = Y 1 ΔY Y PE 2 = Y 2 9

11 Solving for ΔY Y = C + I + G Δ Y = Δ C + Δ I + ΔG = Δ C + ΔG = MPC Δ Y + ΔG Collect terms with ΔY on the left side of the equals sign: (1 MPC) Δ Y = ΔG equilibrium condition in changes because I exogenous because ΔC = MPC ΔY Solve for ΔY : 1 Δ Y = ΔG 1 MPC 10

12 The government purchases multiplier Definition: the increase in income resulting from a $1 increase in G. In this model, the govt purchases multiplier equals ΔY ΔG = 1 1 MPC Example: If MPC = 0.8, then ΔY 1 = = 5 ΔG An increase in G causes income to increase 5 times as much! 11

13 Why the multiplier is greater than 1 Initially, the increase in G causes an equal increase in Y: ΔY = ΔG. But Y C further Y further C further Y So the final impact on income is much bigger than the initial ΔG. 12

14 An increase in taxes Initially, the tax increase reduces consumption and therefore PE: PE PE =C 1 +I +G PE =C 2 +I +G ΔC = -MPC ΔT so firms reduce output, and income falls toward a new equilibrium PE 2 = Y 2 ΔY At Y 1, there is now an unplanned inventory buildup PE 1 = Y 1 Y 13

15 Solving for ΔY Δ Y = Δ C + Δ I + ΔG = ΔC eq m condition in changes I and G exogenous ( Y T ) = MPC Δ Δ Solving for ΔY : (1 MPC) Δ Y = MPC ΔT Final result: MPC Δ Y = ΔT 1 MPC 14

16 The tax multiplier def: the change in income resulting from a $1 increase in T : ΔY ΔT = MPC 1 MPC If MPC = 0.8, then the tax multiplier equals ΔY = = = ΔT

17 The tax multiplier is negative: A tax increase reduces C, which reduces income. is greater than one (in absolute value): A change in taxes has a multiplier effect on income. is smaller than the govt spending multiplier: Consumers save the fraction (1 MPC) of a tax cut, so the initial boost in spending from a tax cut is smaller than from an equal increase in G. 16

18 The IS curve def: a graph of all combinations of r and Y that result in goods market equilibrium i.e. actual expenditure (output) = planned expenditure The equation for the IS curve is: Y = C( Y T ) + I ( r) + G 17

19 Deriving the IS curve PE PE =Y PE =C +I (r 2 )+G r I PE =C +I (r 1 )+G PE ΔI Y r Y 1 Y 2 Y r 1 r 2 Y 1 Y 2 IS Y 18

20 Why the IS curve is negatively sloped A fall in the interest rate motivates firms to increase investment spending, which drives up total planned spending (PE ). To restore equilibrium in the goods market, output (a.k.a. actual expenditure, Y ) must increase. 19

21 Fiscal Policy and the IS curve We can use the IS-LM model to see how fiscal policy (G and T ) affects aggregate demand and output. Let s start by using the Keynesian cross to see how fiscal policy shifts the IS curve 20

22 Shifting the IS curve: ΔG At any value of r, G PE Y PE PE =Y PE =C +I (r 1 )+G 2 PE =C +I (r 1 )+G 1 so the IS curve shifts to the right. The horizontal distance of the IS shift equals ΔY r r 1 Y 1 Y 2 Y Y 1 ΔY Y 2 IS 1 IS 2 Y 21

23 EXAMPLE Shifting the IS curve: ΔT Use the diagram of the Keynesian cross to show how an increase in taxes shifts the IS curve. If you can, determine the size of the shift. 22

24 ANSWERS Shifting the IS curve: ΔT At any value of r, T C PE PE PE =Y PE =C 1 +I (r 1 )+G PE =C 2 +I (r 1 )+G so the IS curve shifts to the left. The horizontal distance of the IS shift equals MPC Δ Y = ΔT 1 MPC r r 1 Y 2 Y 1 ΔY Y 2 Y 1 IS 2 Y IS 1 Y 23

25 The theory of liquidity preference Due to John Maynard Keynes. A simple theory in which the interest rate is determined by money supply and money demand. 24

26 Money supply The supply of real money balances is fixed: ( MP) s = MP r interest rate ( MP) s M P M/P real money balances 25

27 Money demand Demand for real money balances: d ( MP) = Lr ( ) r interest rate ( MP) s L (r ) M P M/P real money balances 26

28 Equilibrium The interest rate adjusts to equate the supply and demand for money: M P r interest rate r 1 ( MP) s = L( r) L (r ) M P M/P real money balances 27

29 How the Fed raises the interest rate To increase r, Fed reduces M r interest rate r 2 r 1 M 2 P M 1 P L (r ) M/P real money balances 28

30 The LM curve We started with the money demand function L(r) but money demand also depends on income: ( MP) d The LM curve is a graph of all combinations of r and Y that equate the supply and demand for real money balances. The equation for the LM curve is: = LrY (, ) M P = L( r, Y ) 29

31 Deriving the LM curve (a) The market for real money balances r r (b) The LM curve LM r 2 r 2 L (r, Y 2 ) r 1 r 1 L (r, Y 1 ) M 1 P M/P Y 1 Y 2 Y 30

32 Why the LM curve is upward sloping An increase in income raises money demand. Since the supply of real balances is fixed, there is now excess demand in the money market at the initial interest rate. The interest rate must rise to restore equilibrium in the money market. 31

33 How ΔM shifts the LM curve (a) The market for real money balances r r (b) The LM curve LM 2 r 2 r 2 LM 1 r 1 L (r, Y 1 ) r 1 M 2 P M 1 P M/P Y 1 Y 32

34 EXAMPLE Shifting the LM curve Suppose a wave of credit card fraud causes consumers to use cash more frequently in transactions. Use the liquidity preference model to show how these events shift the LM curve. 33

35 ANSWERS Shifting the LM curve (a) The market for real money balances r r (b) The LM curve LM 2 r 2 r 1 L (r, Y 1 ) L (r, Y 1 ) r 2 LM 1 r 1 M 1 P M/P Y 1 Y 34

36 The short-run equilibrium The short-run equilibrium is the combination of r and Y that simultaneously satisfies the equilibrium conditions in the goods & money markets: r LM Y = C( Y T ) + I ( r) + G M P = L( r, Y ) Equilibrium interest rate IS Equilibrium level of income Y 35

37 The Big Picture Keynesian cross Theory of liquidity preference IS curve LM curve IS-LM model Explanation of short-run fluctuations Agg. demand curve Agg. supply curve Model of Agg. Demand and Agg. Supply 36

38 CHAPTER SUMMARY 1. Keynesian cross basic model of income determination takes fiscal policy & investment as exogenous fiscal policy has a multiplier effect on income 2. IS curve comes from Keynesian cross when planned investment depends negatively on interest rate shows all combinations of r and Y that equate planned expenditure with actual expenditure on goods & services 37

39 CHAPTER SUMMARY 3. Theory of liquidity preference basic model of interest rate determination takes money supply & price level as exogenous an increase in the money supply lowers the interest rate 4. LM curve comes from liquidity preference theory when money demand depends positively on income shows all combinations of r and Y that equate demand for real money balances with supply 38

40 5. IS-LM model CHAPTER SUMMARY Intersection of IS and LM curves shows the unique point (Y, r ) that satisfies equilibrium in both the goods and money markets. 39