The Aggregate Expenditure Model
Lesson 18


Introduction to Aggregate Expenditure Function

Aggregate consumption function - how aggregate consumption relates to GDP

  • Assume no income taxes; thus disposable income (DI) = GDP
    • Autonomous Spending (A) - the y-intercept
      • Spending is independent of income (i.e. GDP)
      • Consumers still need a level of spending with no income
        • Food
        • Shelter, etc.
    • MPC - the slope of the line
      • Give consumers $1 more dollar in income, MPC is the amount that goes to spending
    • Aggregate Expenditures (AE) - total spending by all institutions in an economy
      • The y-axis

The Consumption Function

A graph of the consumption is below:

The graph of the consumption function

  • Investment - assume investment is independent of GDP
    • Gross investment (Ig) - all investment in society

The gross investment function

Aggregate Expenditures (AE) - restrict ourselves to consumers and business investment

Aggregate Expenditures equation

  • Assume Equilibrium - Aggregate Expenditures = GDP, because AE is one definition of GDP
    • Equilibrium GDP is denoted as GDP*
    • This is the 450 line

The equilibrium condition: AE = GDP

The equilibrium GDP

How the multiplier enters the aggregate expenditure function
Substitute the consumption function and equilibrium condition into the AE and use algebra to solve for GDP*

Deriving the equilbrium GDP

Equilibrium is stable

The equilibrium GDP is stable

  • At GDP1, Aggregate Expenditures exceeds GDP
    • Businesses and consumers are spending more than the amount of goods and services produced in the economy
    • Inventories are falling
      • unplanned changes in inventories - businesses did not anticipate this
    • Businesses increase their production of goods and services
  • At GDP2, GDP exceeds Aggregate Expenditures
    • Businesses and consumers are spending less than the amount of businesses are producing
    • Inventories are increasing
      • unplanned changes in inventories
    • Businesses decrease their production of goods and services
  • At GDP* = AE*, economy is in equilibrium
    • Leakage - withdrawal from income / expenditure stream
      • Savings is a leakage
    • Injection - money is injected into income / expenditure stream
      • Investment is an injection
    • Consumer savings = Business Investment
      • S = Ig
    • No unexpected changes in business inventories


Expanding the Aggregate Expenditure Function

International Sector

  • Net exports (Xn) = Exports (X) - Imports (M)
    • If Xn > 0, country exports more than what it imports
      • Expands domestic production
      • Has multiplier effect
    • If Xn < 0, country imports more that what it exports
      • Expands production for foreign country
      • Negative multiplier effect
      • The foreign country gets the multiplier effect
  • Shifting level of net exports
    1. Prosperity abroad
      • Foreign countries have rising incomes
      • They buy more exports from U.S.
      • Boosts U.S. GDP
    2. Tariffs
      • Foreign countries impose tariffs
        • Decrease U.S. exports, because tariffs cause higher prices
        • Causes U.S. GDP to be smaller
      • U.S. imposes tariffs
        • Decreases imports, because tariffs cause higher prices here
        • Causes net exports to increase
        • Causes U.S. GDP to be higher
      • Tariffs could trigger a tariff war, as other countries retaliate to U.S. tariffs
    3. Exchange Rates
      • U.S. dollar appreciates
        • U.S. exports decrease
        • U.S. imports increase
        • Thus, net exports decrease, causing GDP to contract
      • U.S. dollar depreciates
        • U.S. exports increase
        • U.S. imports decrease
        • Thus, net imports increase, causing GDP to increase

Government Sector

  1. Government spends at level G
    • Independent of GDP
    • If government spending increases by $1, then AE shifts up by $1
    • GDP increases by $1 times the multiplier
  2. Government finances spending through taxes
    • Lump sum taxes - taxes are assessed as a fixed dollar amount
    • If government increases taxes by $1, then AE shifts down by $1*MPC
      • Households pay taxes through reduced consumption and from savings
        • Proportion of taxes from reduced consumption is MPC
        • Proportion of taxes from savings is MPS


  • Leakages - withdrawals from income / expenditure stream
    • Savings
    • Taxes
    • Imports
  • Injections - income is injected into income / expenditure stream
    • Investment
    • Government spending
    • Exports
  • Equilibrium
    • AE = GDP
    • AE = C + Ig + G +Xn
    • Leakages = Injections
    • S + T + M = Ig + G + X

The equilibrium GDP

Note - Similar substitute C = A + (MPC)(GDP) into AE = C + Ig + G + Xn

Blue arrow GDP* = multiplier [A + Ig + G +Xn]

Shifting the Aggregate Expenditure Function

  • Example 1 - President Bush gives every family $600
    • Increases autonomous spending
      • The $600 is independent from GDP income
    • A.E. function shifts upward by $600
      • GDP increases more than $600 from the multiplier effect

The Aggregate Expenditures shift upward

  • Example 2 - Businesses are pessimistic about the future
    • Businesses decrease gross investment
    • A.E. shifts downward by the decrease in gross investment
    • GDP decreases more than the drop from investment, because of the negative multiplier effect

The Aggregate Expenditures function shifts downward

  • Example 3 - Government increases government spending on the military, in order to invade a country
    • A.E. shifts upward by the increase in government spending
    • GDP increases more than the increase of government spending because of the multiplier effect

The Aggregate Expenditures function increases

  • Example 4 - Incomes for foreign countries fall from the 2008 financial crisis
    • Demand for U.S. exports fall, thus net exports fall
    • A.E. shifts downward by the decrease of net exports
    • GDP falls more than the decrease in net exports from the negative multiplier effect

The Aggregate Expenditures function decreases

  • Recession - GDP falls below the full-employment GDP
    • Decrease in consumer spending (A or MPC)
    • Decrease in government spending or increase in taxes
    • Decrease in business gross investment
    • Exports decrease or imports increase
  • Demand pull inflation - if A.E. increases where GDP grows faster than the full-employment GDP
    • Increase in consumer spending (A or MPC)
    • Increase in government spending or decrease in taxes
    • Increase in business gross investment
    • Exports increase or imports decrease
  • Model is simple
    • Many things in the economy are occurring
    • U.S. net exports have been negative for 40 years
    • Counter balanced from strong business investment, government spending, and consumer spending

Blue arrow
A.E. is only one graph of the economy. Macroeconomists use several graphs to explain how economy respond to changes in economic events.



  • aggregate consumption function
  • aggregate expenditures
  • autonomous spending
  • unplanned changes in inventories
  • leakage
  • injection
  • net exports
  • lump-sum tax
  • recession (in terms of the model)
  • demand pull inflation (in terms of the model)