How Interest-Rate Monetary Policy Works

Idea

Interest-rate monetary policy works through two channels. The primary channel is the cost of borrowing, which acts on domestic consumption and investment. The secondary channel runs through short-term capital flows and the exchange rate, which act on net exports. Both move aggregate demand in the same direction, so the two reinforce one another.

Mechanism

Expansionary monetary policy. The central bank lowers the official interest rate, and commercial savings and borrowing rates follow.

Primary effect — borrowing is cheaper, so households and firms borrow more for consumption ( $C$ ) and investment ( $I$ ), both of which rise.
Secondary effect — the lower domestic rate prompts an outflow of short-term capital (“hot money”) chasing higher returns abroad. Supply of the domestic currency rises and demand for it falls in the foreign-exchange market, so the currency depreciates. Exports become relatively cheaper to foreigners and imports relatively dearer at home, so export revenue ( $X$ ) rises, import expenditure ( $M$ ) falls, and net exports ( $X-M$ ) rise.

With $C$ , $I$ and $(X-M)$ all higher, aggregate demand shifts right and national income rises by a multiple via the multiplier.

Contractionary monetary policy. The central bank raises the interest rate. Borrowing is dearer, so $C$ and $I$ fall; the higher rate draws an inflow of hot money, the currency appreciates, $(X-M)$ falls. Aggregate demand shifts left, curbing demand-pull inflation.

Diagram

Expansionary monetary policy: lower interest rates raise C, I and net exports, shifting AD right from AD₀ to AD₁; the general price level rises from P₀ to P₁ and real output from Y₀ towards full employment Yf.

Contractionary monetary policy: higher interest rates lower C, I and net exports, shifting AD left from AD₀ to AD₁ at full employment Yf; the general price level falls from P₀ to P₁ while output stays at Yf, easing demand-pull inflation.

Limitations

Interest (in)sensitivity. In a downturn, poor confidence can leave $C$ and $I$ unresponsive even to large rate cuts; in a boom, high confidence can leave them unresponsive to rate rises.
Inability or unwillingness to lend. Rising loan defaults in a downturn leave banks with bad debt and less willing or able to lend, blunting an expansion.
Zero lower bound. Nominal rates cannot fall much below zero, so expansionary policy runs out of room once rates are near zero.
Size of the multiplier. As with fiscal policy, a small multiplier means a small eventual change in national income.
Asset bubbles. Cheap credit can be borrowed for speculation rather than $C$ and $I$ , inflating asset prices; raising rates abruptly to deflate a bubble risks bursting it and triggering a hard landing.
Inflation and unemployment trade-offs. Over-loose policy (especially alongside money-supply expansion) risks excessive demand-pull inflation; over-tight policy risks demand-deficient unemployment.

Context

Singapore does not operate an independent interest-rate policy: under the exchange-rate-centred framework it accepts that domestic interest rates are largely set by world rates and capital flows, the price of having chosen to manage the exchange rate instead. The interest-rate mechanism above is therefore the generic central-bank model, exemplified by economies such as the United States, against which Singapore’s choice is contrasted.