Interest on reserves (IOR) is the interest rate a central bank pays commercial banks on the reserves they hold at the central bank. In an ample-reserves economy like the U.S., it is the primary administered rate used to steer the policy rate and conduct monetary policy (AP Macro Topic 4.6).
Interest on reserves is exactly what it sounds like. Banks keep reserve balances in accounts at the central bank, and the central bank pays them interest on those balances. That rate matters because it sets a floor on lending. No bank will lend money out at a rate lower than what it can earn risk-free by just parking funds at the Fed.
In the AP Macro CED (EK POL-1.D.2), interest on reserves is listed as one of the central bank's administered interest rates, alongside the discount rate. Here's the part the exam cares about most. The tool a central bank reaches for depends on whether the banking system has limited reserves or ample reserves. In a limited-reserves system, open market operations move the money supply and nudge interest rates. In an ample-reserves system, reserves are so plentiful that buying or selling a few bonds barely moves the needle, so the central bank steers rates directly by raising or lowering interest on reserves. The United States operates with ample reserves, which makes IOR the Fed's main lever today.
This term lives in Unit 4 (Financial Sector), Topic 4.6 (Monetary Policy) and supports learning objective 4.6.A, which asks you to define monetary policy and its tools. EK POL-1.D.2 is explicit that the tools differ between limited-reserves and ample-reserves economies, and that the U.S. banking system has ample reserves. That single sentence is the source of a whole family of exam questions. If you only know the old textbook story (Fed buys bonds, money supply rises, interest rates fall), you'll miss questions about how the Fed actually operates now. Interest on reserves is also your bridge to the transmission mechanism. Changing IOR changes the policy rate, which changes investment spending, which shifts aggregate demand. That chain is the backbone of monetary policy FRQs.
Keep studying AP® Macroeconomics Unit 4
Ample reserves framework (Unit 4)
Interest on reserves only works as the main tool because reserves are ample. When banks are already swimming in reserves, the demand-for-reserves curve is flat where it meets supply, so changing IOR moves the policy rate directly. IOR is the steering wheel of the ample-reserves framework.
Discount Rate (Unit 4)
Both are administered rates the central bank simply sets rather than letting markets determine. The discount rate is what banks pay to borrow FROM the Fed (a ceiling on rates), while interest on reserves is what banks earn by lending TO the Fed, effectively (a floor). Together they form a corridor that traps the policy rate.
Investment Spending (Units 3-4)
This is how IOR reaches the real economy. Lower IOR pulls market interest rates down, cheaper borrowing boosts investment spending, and aggregate demand shifts right. That chain is exactly what you trace when a question asks you to close a recessionary gap.
Inflationary Gap (Units 3-4)
When output is above full employment, the central bank raises interest on reserves. Banks earn more by holding reserves, lending tightens, interest rates rise, investment falls, and AD shifts left to cool the inflationary output gap.
Multiple-choice questions love the ample-versus-limited reserves distinction. Expect stems like "In an economy with ample reserves, which tool would be most effective to stimulate growth?" (answer: lower interest on reserves, not open market operations) or "What happens immediately when the Fed raises interest on reserves?" (banks hold more reserves and lend less, pushing market rates up). You may also see a question asking WHY the Fed shifted from open market operations to IOR as its primary tool. The answer hinges on reserves being ample, so OMOs lost their punch. On FRQs, the 2025 exam (Q2) gave two countries below full employment, one with limited reserves and one with ample reserves, and asked which tool each should use and in which direction. That's the move to practice. Name the right tool for the right framework, state the direction of the change, then trace the chain to interest rates, investment, AD, output, and the price level.
Both are administered rates set directly by the central bank, but they point in opposite directions. The discount rate is what commercial banks PAY when they borrow from the Fed; interest on reserves is what commercial banks EARN on balances they keep at the Fed. IOR acts as a floor on market rates (why lend below what the Fed pays you risk-free?), while the discount rate acts as a ceiling (why borrow elsewhere at a higher rate?). In the U.S. ample-reserves system, IOR is the primary tool; the discount rate is more of a backstop.
Interest on reserves is the rate the central bank pays commercial banks on reserve balances they hold at the central bank.
It is an administered rate, meaning the central bank sets it directly instead of letting supply and demand determine it.
In an ample-reserves economy like the United States, interest on reserves is the primary monetary policy tool because open market operations barely move rates when reserves are plentiful.
Raising IOR encourages banks to hold reserves rather than lend, which pushes market interest rates up and is contractionary; lowering IOR does the opposite and is expansionary.
IOR sets a floor under market interest rates because no bank will lend at a rate below what it can earn risk-free at the Fed.
On FRQs, always match the tool to the framework first (IOR for ample reserves, open market operations for limited reserves), then trace the effect through interest rates, investment, and aggregate demand.
It's the interest rate a central bank pays commercial banks on the reserves they hold at the central bank. Per EK POL-1.D.2, it's an administered rate and the main monetary policy tool in economies with ample reserves, like the United States.
No, not as its primary tool. Because the U.S. banking system has ample reserves, small bond purchases or sales barely move interest rates, so the Fed steers the policy rate by setting interest on reserves. The CED explicitly says U.S. monetary policy is implemented under an ample-reserves framework.
The discount rate is what banks pay to borrow from the Fed; interest on reserves is what banks earn on funds they keep at the Fed. Both are administered rates, but IOR works as a floor on market rates while the discount rate works as a ceiling.
Raising IOR is contractionary. Banks earn more by holding reserves, so they lend less, market interest rates rise, investment spending falls, and aggregate demand shifts left. Lowering IOR does the reverse to stimulate the economy.
Yes. It appears in Topic 4.6 under EK POL-1.D.2, shows up in multiple-choice questions about ample-reserves economies, and the 2025 FRQ Q2 asked which monetary policy tool fits an ample-reserves versus limited-reserves country.
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