Lesson 4.3: Interest Rates

Lesson Overview

Interest rates show up everywhere: credit cards, mortgages, student loans, business loans, and government bonds. But an interest rate is not just a random number or a “bank decision.” It is a price—specifically, the price of borrowing money (or the reward for lending it).

In this lesson you will learn two essential ideas:

• Loanable funds: how the supply of savings and the demand for borrowing determine interest rates.
• Transmission: how changes in interest rates ripple outward into spending, investment, inflation, and growth.

When you understand these, you can make sense of headlines like “rates are up,” “credit is tightening,” “bond yields are falling,” and “policy is restrictive.”

Learning Objectives

• Define an interest rate and explain why it exists.
• Distinguish between nominal interest rates and real interest rates.
• Use the loanable funds framework to explain how interest rates are determined.
• Identify factors that shift savings supply and borrowing demand.
• Explain the relationship between bond prices and yields.
• Describe major transmission channels from rates to the overall economy.

What Is an Interest Rate?

An interest rate is the percentage cost of borrowing money (or the percentage return for lending money) over a period of time.

Why does interest exist?

• Time preference: People generally prefer having resources now rather than later.
• Opportunity cost: Lending money means you cannot use it for something else today.
• Risk: The borrower might not repay.
• Inflation: Money may buy less in the future than it does today.

Put simply: interest is what makes borrowing and lending possible on a large scale.

Nominal vs. Real Interest Rates

Interest rates come in two flavors:

• Nominal interest rate: the rate you see on a contract (e.g., 6% on a loan).
• Real interest rate: the nominal rate adjusted for inflation (your “true” purchasing power cost).

Why the difference matters

If inflation is high, paying back a loan is “easier” in purchasing power terms because the dollars are worth less. If inflation is low (or negative), borrowing can be more expensive in real terms.

A simple rule of thumb (Fisher idea)

Real interest rate ≈ Nominal interest rate − Expected inflation

Example: If a loan is 7% and expected inflation is 3%, the real rate is roughly 4%.

Most macroeconomic decisions are driven by real rates, because people care about purchasing power, not just the number of dollars.

The Loanable Funds Market (The Big Picture)

The loanable funds framework treats interest rates like any other price: they are determined by supply and demand—specifically, the supply of funds available to lend and the demand to borrow.

Who supplies loanable funds?

• Households saving part of their income
• Businesses with retained earnings (profits kept inside the firm)
• Foreign investors buying domestic assets

Who demands loanable funds?

• Businesses borrowing to invest (factories, equipment, software, R&D)
• Households borrowing for s, education, and big purchases
• Governments borrowing to cover deficits

In this model, the interest rate is the price that balances how much people want to lend with how much others want to borrow.

Supply of Loanable Funds: Savings

The supply curve represents saving: the higher the interest rate, the more incentive people have to save (generally), because the reward for lending is higher.

What shifts the supply of loanable funds?

• Income growth: higher incomes can increase saving.
• Preferences: if households become more future-oriented, saving rises.
• Demographics: a larger working-age population can increase saving.
• Tax policy: taxes on interest income can reduce saving incentives.
• Foreign capital flows: more foreign investment increases available funds.

If saving increases, the supply curve shifts right, pushing interest rates down (all else equal).

Demand for Loanable Funds: Investment and Borrowing

The demand curve represents borrowing for investment and other uses. When interest rates are high, fewer projects are profitable (or affordable), so borrowing demand falls.

What shifts the demand for loanable funds?

• Expected profitability: if firms expect strong future demand, investment rises.
• Technology changes: new opportunities can increase investment demand.
• Business confidence: optimism increases borrowing; fear reduces it.
• Government deficits: more borrowing by government increases demand.
• Housing booms: higher mortgage demand can increase overall borrowing.

If borrowing demand increases, the demand curve shifts right, pushing interest rates up (all else equal).

Crowding Out: A Classic Application

When the government runs a deficit, it often borrows by issuing bonds. In the loanable funds framework, this increases the demand for loanable funds.

Result (in the basic model):

• Demand for loanable funds rises
• Interest rates rise
• Private investment falls relative to what it would have been

That reduction in private investment is called crowding out. It is “classic,” but not automatic in every situation—especially during recessions when private demand is weak. You’ll revisit this idea again in Lesson 4.4 (Fiscal Policy) and Lesson 4.5 (Government Debt).

Bond Prices and Yields: Why They Move Opposite

Many interest rates in the economy are connected to bond markets. A bond is essentially a loan contract: you lend money to a borrower (often a government or corporation) and receive payments over time.

The key relationship

When bond prices go up, yields (interest rates) go down.
When bond prices go down, yields (interest rates) go up.

Intuition

If a bond pays a fixed $50 per year and the bond price rises, that $50 is a smaller return relative to the price. If the bond price falls, the same $50 becomes a bigger return relative to the price.

This is why you’ll often hear: “yields fell as investors bought bonds.”

Risk and Term Premiums: Why Not All Rates Are the Same

There is no single interest rate in the economy. Different borrowers face different rates due to:

• Default risk: the chance the borrower won’t repay (risk premium).
• Time horizon: longer loans typically carry a term premium.
• Liquidity: some assets are easier to sell quickly without losing value.
• Tax treatment: certain interest income may be taxed differently.

That’s why a government can borrow at a lower rate than a new small business, and why a 30-year mortgage rate differs from an overnight rate.

Transmission Mechanisms: How Rates Affect the Economy

When a central bank changes short-term rates, it sets off a chain reaction through multiple channels. The details differ by country, but the big channels are consistent.

1) Borrowing and Spending Channel

Lower rates reduce the cost of borrowing for households and firms, encouraging spending on houses, cars, equipment, and expansion. Higher rates do the opposite.

2) Investment and Business Expansion Channel

Many business projects are justified only if the expected return exceeds the cost of financing. When rates rise, fewer investments “clear the hurdle.”

3) Asset Price Channel

When rates fall, the present value of future cash flows tends to rise, which can increase prices of assets like stocks and real estate. Rising asset values can increase spending through wealth effects and collateral value.

4) Exchange Rate Channel

Higher rates can attract foreign capital, increasing demand for the currency and making it stronger. A stronger currency makes imports cheaper and exports relatively more expensive, influencing inflation and growth.

5) Expectations Channel

If people believe inflation will fall (or rise) in the future, they adjust wage demands, pricing, and spending today. Central bank credibility matters because expectations can amplify or weaken policy effects.

Putting It Together: A Rate Hike Story

Suppose inflation is rising and the central bank raises its policy rate.

1. Short-term borrowing costs rise.
2. Credit becomes more expensive; some loans don’t happen.
3. Household and business spending cools over time.
4. Asset prices may soften; exchange rates may strengthen.
5. Demand pressure eases, and inflation tends to slow with a lag.

Notice the theme: interest rates are a coordinator. They guide how much society saves, borrows, invests, and spends.

Practice: Check Your Understanding

1. In your own words, what is an interest rate?
2. Calculate the approximate real interest rate: nominal rate 8%, expected inflation 5%.
3. Name two factors that shift the supply of loanable funds and two that shift demand.
4. What does “crowding out” mean in the loanable funds model?
5. Why do bond prices and yields move in opposite directions?
6. Choose one transmission channel and explain it with a real-life example.

Reflection Prompt

Think about a major purchase you might make in the next few years: a car, , education, or a business investment.

• How sensitive is that decision to interest rates?
• If rates rise by 2 percentage points, what changes about affordability or profitability?
• What matters more for you: the nominal rate, or the real rate after inflation?

What’s Next?

In Lesson 4.4: Fiscal Policy, we’ll shift from central bank policy to government budget choices: spending, taxation, and how fiscal decisions can stabilize (or destabilize) the economy.