(This post is a part of the series on Basics of Finance and Investing.)

An interest rate is the monthly rate you pay as a borrower, or receive as a creditor/lender. If you save money in bank, or invest in a money market, you are indirectly lending money to a borrowing corporation (or the government). If the interest rate goes up, the borrower must pay you more, which makes them unhappy but you happy (your bank balance soars). The mood swings the other way when interest rate goes down.

Here we are talking about a real interest rate, which is the rate after adjusting for inflation. So, for the purpose of this topic, just assume inflation does not exist (hard, I know, given the reality, but it simplifies our discussion).

Demand and supply influence the real interest rate

There are almost as many different types of interest rates as the investing and borrowing choices available to us (your bank pays you one rate, and you pay another rate to your credit card company). But they all respond to three fundamental market forces: supply of funds, demand for funds, and occasional government interventions.

Households supply funds via their invested assets, whereas corporations and the government borrow these funds to finance their needs (see this post on the interaction between investors and borrowers). This dynamics of supply and demand determine how all these different rates arise. It is easy to think in terms of a single abstract rate, as the picture below illustrates, which plots real interest rate against available fund.

The supply curve (solid blue graph) goes up from left to right, because with increasing interest rate the households save and invest more. By contrast, the demand curve (solid red graph) falls as more fund is available. The intersection of these two curves, point A, determines the equilibrium real interest rate.

Now suppose there is an increase in the budget deficit, which will raise the government’s borrowing demand. This pushes the demand curve to the right (dashed red graph), and lifts the interest rate to point B, which in turn may discourage businesses from further borrowing and slow the economy down. The government may then intervene by releasing more fund from the central bank (Federal Reserve). This action pushes the supply curve to right (dashed blue graph), and brings the interest rate down (to point C).

So, again, the fundamental market forces that set the interest rate, and run the investing world, are the demand and supply of funds.

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