The Control Stack Behind the Exchange Rate

A pegging central bank does not have one instrument called “the peg.” It has a set of operating tools that influence rates, change the quantity of reserves, absorb forecast errors, and provide liquidity when markets cannot redistribute funds. This chapter explains the distinct job of each layer. The practical value is diagnostic: once a shock has been traced through the peg’s balance sheet and markets, the reader can ask which tool addresses that shock and which tool would create a new incentive or risk.

Two levers, different jobs

The first distinction is between the policy rate and liquidity operations. They can move together, but they are not interchangeable.

A policy rate is a price signal. Under a peg and an open capital account, the central bank may set it with reference to the anchor-country rate and an assessment of country or liquidity risk. If capital flows respond strongly to the local rate, changing that rate can influence the demand for foreign currency and the pressure on the peg. A higher rate can make local assets more attractive and help offset an outflow. A lower rate can make local assets less attractive and encourage an outflow when the banking system is carrying a persistent reserve surplus.

This use of the policy rate is not the same as setting rates to close an inflation or output gap in a fully flexible exchange-rate regime. The IMF framework describes the rate under a peg as potentially serving external stability first. A central bank may need to move in step with the anchor country when the FX market is calm, then adjust the differential when capital flows put pressure on the fixed rate. The rate is doing work through investors’ and banks’ incentives.

Open-market operations are different. They change the quantity or composition of liquidity available to banks. A central bank can lend against collateral, absorb reserves through securities or deposits, or conduct other operations that offset a short-term movement in autonomous factors. The purpose is not necessarily to change the desired external rate relationship. It is to keep a temporary reserve shock from making the overnight market rate more volatile than the operating framework requires.

The distinction can be stated with two questions. If the problem is that capital is leaving because the return on local assets is inconsistent with the peg, is the policy rate the relevant signal? If the problem is that a government payment or an FX settlement has unexpectedly changed the banking system’s reserve balance, is a liquidity operation the more direct response? The two situations can coexist, but treating every reserve movement as a reason to change the policy rate would make the rate carry work that an operation could perform more precisely.

The forecast problem is unusually important under a peg. The central bank fixes the price at which it will buy or sell foreign currency but does not fully control the volume of those transactions. Capital flows and terms-of-trade shocks can therefore change the quantity of reserves in ways that are difficult to predict.