# The Short – Run Macro Model

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The Short – Run Macro Model

The Short-Run Macro Model
In short-run, spending depends on income, and income depends on spending. The more income households have, the more they will spend. The more households spend, the more output firms will produce More income they will pay to their workers. Many ideas behind the model were originally developed by British economist John Maynard Keynes in 1930s. Short-run macro model focuses on spending in explaining economic fluctuations.

Review the Categories of Spending
Macroeconomists have found that the most useful approach is to divide those who purchase the GDP into four broad categories Households --- consumption spending (C) Business firms --- planned investment spending (IP) Government agencies --- government purchases (G) Foreigners --- net exports (NX) Nominal or real spending? Real terms

Consumption What factors affect households’ spending?
Disposable income: Yd (= Y – T) Wealth (= total assets – total liability) Price level Interest rate When interest rate falls, consumption rises Expectations about future

Figure: U.S. Consumption and Disposable Income, 1985-2002

Figure: The Consumption Function

Consumption and Disposable Income
Autonomous consumption spending Consumption spending when disposable income is zero Marginal propensity to consume, or MPC The slope of the consumption function MPC = Δ Consumption ÷ Δ Disposable Income MPC measures by how much consumption spending rises when disposable income rises by one dollar Logic and empirical evidence suggest that the MPC should be larger than zero, but less than 1 So, we assume that 0 < MPC < 1

Representing Consumption with an Equation
C = a + bYd Where C is consumption spending And a is the autonomous consumption spending And b is the marginal propensity to consume (MPC) Equation between consumption and total income Since Yd = Y – T, C = a + b(Y – T) So, C = (a- bT) + bY

Figure: The Consumption-Income Line

Shifts in the Consumption-Income Line
When a change in income causes consumption spending to change, we move along consumption-income line. When a change in anything else besides income causes consumption spending to change, the line will shift.

Figure: A Shift in the Consumption-Income Line

IP, G and NX For now, in the short-run macro model, planned investment spending, government purchases, and net exports are all treated as given or fixed values.

Summing Up: Aggregate Expenditure
Aggregate expenditure (AE) Sum of spending by households, businesses, government, and foreign sector on final goods and services produced in United States Aggregate expenditure = C + IP + G + NX AE plays a key role in explaining economic fluctuations Why? Because over several quarters or even a few years, business firms tend to respond to changes in aggregate expenditure by changing their level of output.

Finding Equilibrium GDP
When aggregate expenditure is less than GDP, inventories will increase and output will decline in future. When aggregate expenditure is greater than GDP, inventories will decrease and output will rise in future. In short-run, equilibrium GDP is level of output at which output and aggregate expenditure are equal.

Inventories and Equilibrium GDP
When firms produce more goods than they sell, what happens to unsold output? Added to their inventory stocks Find output level at which change in inventories is equal to zero. AE < GDP  ΔInventories > 0  GDP↓ in future periods AE > GDP  ΔInventories < 0  GDP↑ in future periods AE = GDP  ΔInventories = 0  No change in GDP Equilibrium output level is the one at which change in inventories equals zero.

Figure: Deriving the Aggregate Expenditure Line

Figure: Using a 45° to Translate Distances

Figure: Determining Equilibrium Real GDP

Equilibrium GDP and Employment
When economy operates at equilibrium, will it also be operating at full employment? Not necessarily For instance, insufficient spending causes business firms to decrease their demand for labor. Remember, in the long run (classical) macro model, it takes time for labor market to achieve full employment. In the short-run model, it would be quite a coincidence if our equilibrium GDP happened to be the full employment output level. In short-run macro model, output can be lower or higher than the full employment output level.

Figure: Short-Run Equilibrium GDP < Full Employment GDP

Figure: Short-Run Equilibrium GDP > Full-Employment GDP

A Change in Investment Spending
Suppose the initial equilibrium GDP in an economy is \$6,000 billion. Now, business firms increase their investment spending on plant and equipment by \$1,000 billion. Then, firms that sell investment goods receive \$1,000 billion as income, which is to be distributed as salary, rent, interest, and profit. What will households do with their \$1,000 billion in additional income? Spend the money ! How much to spend depends crucially on marginal propensity to consume (MPC): let’s assume MPC = 0.6

A Change in Investment Spending
When households spend an additional \$600 billion, firms that produce consumption goods and services will receive an additional \$600 billion in sales revenue. Which will become income for households that supply resources to these firms. At this point, total income has increased by \$1,000+\$600=\$1,600billion With an MPC of 0.6, consumption spending will further rise by 0.6 x \$600 billion = \$360 billion, creating still more sales revenue for firms, and so on and so on… At end of process, when economy has reached its new equilibrium. Total spending and total output are considerably higher.

Figure: The Effect of a Change in Investment Spending

The Expenditure Multiplier
Whatever the rise in investment spending, equilibrium GDP would increase by a factor of 2.5, so we can write ΔGDP = 2.5 x ΔIP Value of expenditure multiplier depends on value of MPC So, when the increase in planned investment spending is ΔIP, the increase in total income (GDP) is calculated as:

The Expenditure Multiplier
A sustained increase in investment spending will cause a sustained increase in GDP. Multiplier process works in both directions. Just as increases in investment spending cause equilibrium GDP to rise by a multiple of the change in spending. Decreases in investment spending cause equilibrium GDP to fall by a multiple of the change in spending.

Spending Shocks Shocks to economy can come from other sources besides investment spending. Government purchases (G) Net exports (NX) Autonomous consumption (a) Changes in planned investment, government purchases, net exports, or autonomous consumption lead to a multiplier effect on GDP.

Spending Shocks The effect of a change in spending on total income through expenditure multiplier

Figure: A Graphical View of the Multiplier

Application: The Recession of 2007-09
December 2007 recession began Aggregate expenditures declined Consumption spending declined Investment spending declined Recessionary expenditure gap

Application: The Recession of 2007-09
Federal government undertook Keynesian policies Tax rebate checks \$787 billion stimulus package

Say’s Law vs. Keynesian Theory
Classical economics Say’s Law Economy will automatically adjust Laissez-faire Keynesian economics Cyclical unemployment can occur Economy will not correct itself Government should actively manage macroeconomic instability

Automatic Stabilizers and the Multiplier
Automatic stabilizers reduce size of multiplier and therefore reduce impact of spending shocks. With milder fluctuations, economy is more stable. Some real-world automatic stabilizers we’ve ignored in the simple, short-run macro model of this chapter Taxes Transfer payments Interest rates Imports Forward-looking behavior

The Role of Saving In long-run, saving has positive effects on economy. But in short-run, automatic mechanisms of classical model do not keep economy operating at its potential. In long-run, an increase in desire to save leads to faster economic growth and rising living standards. In short-run, however, it can cause a recession that pushes output below its potential. Two sides to the “saving coin” Impact of increased saving is positive in long-run and potentially dangerous in short-run.

The Effect of Fiscal Policy
In classical model fiscal policy—changes in government spending or taxes designed to change equilibrium GDP—is completely ineffective – crowding out effect. In short-run, an increase in government purchases causes a multiplied increase in equilibrium GDP. Therefore, in short-run, fiscal policy can actually change equilibrium GDP. Observation suggests that fiscal policy could, in principle, play a role in altering path of economy. Indeed, in 1960s and early 1970s, this was the thinking of many economists. But very few economists believe this today.