Interest rates have their clearest effect on one particular form of investment spending: spending on residential construction—that is, on the construction of homes. The reason is straightforward: home builders only build houses they think they can sell, and houses are more affordable—and so more likely to sell—when the interest rate is low.

Consider a potential home-buying family that needs to borrow $150,000 to buy a house. At an interest rate of 7.5%, a 30-year home mortgage will mean payments of $1,048 per month. At an interest rate of 5.5%, those payments would be only $851 per month, making houses significantly more affordable. As described in the upcoming Economics in Action, interest rates actually did drop from roughly 7.5% to 5.5% between the late 1990s and 2003, helping set off the great housing boom described in this chapter’s opening story.

Interest rates also affect other forms of investment spending. Firms with investment spending projects will only go ahead with a project if they expect a rate of return higher than the cost of the funds they would have to borrow to finance that project. As we saw in Chapter 10, if the interest rate rises, fewer projects will pass that test, and as a result investment spending will be lower.

You might think that the trade-off a firm faces is different if it can fund its investment project with its past profits rather than through borrowing. Past profits used to finance investment spending are called retained earnings. But even if a firm pays for investment spending out of retained earnings, the trade-off it must make in deciding whether or not to fund a project remains the same because it must take into account the opportunity cost of its funds. For example, instead of purchasing new equipment, the firm could lend out the funds and earn interest. The forgone interest earned is the opportunity cost of using retained earnings to fund an investment project.

So the trade-off the firm faces when comparing a project’s rate of return to the market interest rate has not changed when it uses retained earnings rather than borrowed funds, which means that regardless of whether a firm funds investment spending through borrowing or retained earnings, a rise in the market interest rate makes any given investment project less profitable. Conversely, a fall in the interest rate makes some investment projects that were unprofitable before profitable at the now lower interest rate. So some projects that had been unfunded before will be funded now.

So planned investment spending—spending on investment projects that firms voluntarily decide whether or not to undertake—is negatively related to the interest rate. Other things equal, a higher interest rate leads to a lower level of planned investment spending.