AP Syllabus focus: ‘Supply of loanable funds rises with the real interest rate, and in a closed economy national saving equals private plus public saving.’
These notes explain why the supply of loanable funds is upward sloping in the real interest rate and how national saving is measured as the combined saving of households, businesses, and government in a closed economy.
Supply of Loanable Funds
What is “supply” in this market?
In the loanable funds market, the supply of loanable funds comes from income that is not used for consumption or government purchases and is instead made available for lending.
Supply of loanable funds: the quantity of financial capital that savers are willing and able to provide at each real interest rate.
The key price is the real interest rate, because it reflects the inflation-adjusted reward to saving and lending.
Why supply rises with the real interest rate
The supply curve is upward sloping because a higher real interest rate increases the incentive to save (and decreases the incentive to consume now).
Households are more willing to postpone consumption when the reward to saving is higher.
Firms with retained earnings may be more willing to place funds in interest-bearing assets rather than hold idle balances.
Financial institutions can attract more deposits when the return to savers rises, increasing funds available to lend.
This relationship is captured in the syllabus idea that “supply of loanable funds rises with the real interest rate.”
Movement along vs. shifts of supply
A change in the real interest rate causes a movement along the supply curve (more or less saving supplied).
A change in underlying saving behavior or fiscal conditions changes national saving, which shifts the supply curve.
National Saving in a Closed Economy
Core measurement relationship
In a closed economy (no international capital flows), the total pool of funds available to finance borrowing comes from national saving, split into private and public components.
National saving: total saving generated by the economy; in a closed economy it equals private saving + public saving.
Between these components, national saving connects household/business saving decisions and government budget outcomes to the overall supply of loanable funds.
Private saving and public saving
Private saving reflects saving by households and businesses, while public saving reflects the government’s budget position.
= National saving (real dollars per year)
= Private saving (real dollars per year)
= Public saving (real dollars per year)
= Income/output (real dollars per year)
= Net taxes (real dollars per year)
= Consumption (real dollars per year)
= Government purchases (real dollars per year)
These identities are used to track how changes in taxes, government purchases, or private consumption translate into changes in the economy’s total saving.
Interpreting public saving
If , then public saving is positive (a budget surplus), increasing national saving.
If , then public saving is negative (a budget deficit), reducing national saving.
When public saving falls (a larger deficit), national saving falls unless private saving rises enough to offset it, shrinking the funds available for lending in the closed economy framework.
Linking national saving to the supply curve
Because the supply of loanable funds is grounded in saving, the economy’s national saving helps determine the position of the supply curve:
Higher private saving and/or higher public saving → greater national saving → supply of loanable funds shifts right
Lower private saving and/or lower public saving → lower national saving → supply of loanable funds shifts left
This reinforces the syllabus statement that, in a closed economy, national saving is the combined total of private and public saving, and that the supply available to borrowers expands as the real interest rate rises.
FAQ
Public saving is based on net taxes and government purchases.
Transfers matter indirectly because they can reduce net taxes ($T$) for a given tax system, which can lower $S_g$.
Saving and lending decisions depend on purchasing power.
The real interest rate better reflects the true return to saving after inflation, so it is the relevant “price” in this market.
Yes.
If the government runs a large enough deficit (very negative $S_g$) and private saving ($S_p$) is insufficient to offset it, then $S = S_p + S_g$ can be negative.
It implies public saving fell by the same amount.
In symbols, if $S$ is unchanged and $S_p$ increases, then $S_g$ must decrease equivalently.
Not exactly.
Supply refers to the total flow of saving made available to borrowers through financial markets; bank lending is one channel, but funds can also be supplied via bonds and other credit instruments.
Practice Questions
(2 marks) Explain why the supply of loanable funds is positively related to the real interest rate.
1 mark: States that a higher real interest rate increases the incentive to save (reward for postponing consumption).
1 mark: Links higher saving to a greater quantity of loanable funds supplied (upward-sloping supply).
(6 marks) In a closed economy, define national saving and explain how private saving and public saving combine to determine it. Use the relationships among , , and in your explanation.
1 mark: Defines national saving as total saving in the economy.
1 mark: States .
2 marks: Correctly explains private saving and identifies it as saving by households/businesses (may reference ).
2 marks: Correctly explains public saving as the government budget balance (surplus/deficit) (may reference and that deficits imply negative public saving).
