Sterilized Intervention
What is Sterilized Intervention?
Sterilized intervention is a central bank foreign currency trade offset by an opposite open market operation, leaving the domestic monetary base unchanged.
A central bank's monetary base equals its net foreign assets plus its net domestic assets, so buying foreign currency injects domestic reserves unless something drains them back out. Sterilization is that drain: the bank sells domestic government securities in the same amount, holding the monetary base and short-term interest rates where domestic policy needs them while still shifting its foreign exchange position. What changes is the composition of assets held by the bank and by the private sector, not the quantity of money, which is why the exchange rate effect is disputed. If domestic and foreign bonds are close substitutes and capital moves freely, swapping one for the other should barely move the exchange rate, and estimated effects in deep advanced-economy markets are typically small and short-lived. Effects are larger where assets are poor substitutes, where capital controls bind, or where the operation signals a coming change in interest rates, and sterilization is not free: the bank often pays more on the securities it sells than it earns on the reserves it buys.
Sterilized Intervention: a worked example
A central bank wants to slow an appreciation, so it buys $2 billion of foreign currency at 10 pesos per dollar, paying with 20 billion pesos of newly created reserves. Net foreign assets rise by 20 billion pesos. It then sterilizes by selling 20 billion pesos of domestic treasury bills out of its portfolio, cutting net domestic assets by 20 billion pesos, so the change in the monetary base is plus 20 minus 20 = 0. Left unsterilized, with a money multiplier of 4, that injection would have expanded broad money by 4 times 20 billion = 80 billion pesos. Holding the position is costly: if the bills pay 7 percent while the reserves earn 2 percent, the annual carry cost is 20 billion times (0.07 minus 0.02) = 1 billion pesos.
The mistake students make with sterilized intervention
The offsetting operation gets pointed the wrong way. Buying foreign currency adds reserves to the banking system, so sterilizing it means selling domestic securities, not buying them; selling foreign currency drains reserves, so sterilizing that means buying domestic securities. A second slip is concluding that because the monetary base is unchanged, nothing on the balance sheet moved. The size of the balance sheet is held constant while its composition shifts toward foreign assets and away from domestic ones, and that shift is the whole point of the operation.
Sterilized Intervention questions
Why not just let the intervention change the money supply?
Because the central bank usually has a domestic target as well as an exchange rate concern. Buying foreign currency without sterilizing expands the monetary base, pushes short-term interest rates down and can add to inflation, which conflicts with an inflation target. Sterilizing lets the bank take a position on the exchange rate while keeping domestic monetary conditions where its inflation or output objective requires.
Does sterilized intervention actually move the exchange rate?
Estimated effects for large advanced economies with open capital markets are usually small and fade within days, because domestic and foreign bonds are close substitutes and swapping one for the other barely changes the returns investors demand. Effects show up more clearly in emerging markets, where assets are poorer substitutes and capital controls limit arbitrage. Interventions also tend to bite harder when several central banks act together, or when the trade is read as a signal about future interest rate decisions rather than a one-off portfolio shift.
What does sterilization cost a central bank?
It usually runs a negative carry. The bank holds low-yielding foreign reserves and funds them by selling or issuing higher-yielding domestic securities, so the interest it pays exceeds the interest it earns. On 20 billion of reserves with a 5 percentage point gap, that is 1 billion a year. Those losses land on the central bank's own accounts and, indirectly, on the government that owns it, which is one reason prolonged sterilization is hard to sustain.
Formula / Example
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