A Central Bank Can Intervene in Exchange Markets Without Loosening Policy
Intervening in currency markets injects domestic money into the economy. Sterilisation is the offsetting operation that removes it again, letting a central bank pursue two goals that would otherwise conflict.
The Side Effect of Intervention
Suppose a central bank wants to stop its currency appreciating. It buys foreign currency and pays with newly created domestic currency.
That purchase does what it was meant to do, and it also increases the domestic money supply. More money in the banking system pushes interest rates down and is expansionary, which the central bank may not want at all if it is trying to contain inflation.
The Offsetting Operation
Sterilisation removes the domestic money that intervention created. The central bank sells government securities into the domestic market, taking currency back out of circulation in an amount matching what the intervention put in.
The net result is a change in the composition of the central bank balance sheet, more foreign assets and fewer domestic ones, with no change in the total money supply.
Sterilised intervention is an attempt to influence the exchange rate without influencing domestic monetary conditions, which are normally two sides of the same action.
Why This Is Attempted
The motivation is the constraint sometimes called the impossible trinity: a country cannot simultaneously have a fixed exchange rate, free capital movement, and independent monetary policy. It must give up one.
Sterilisation is an attempt to evade that constraint, managing the exchange rate while keeping domestic policy independent, with capital still moving freely. It works partially and temporarily rather than fully.
| Goal | Unsterilised intervention | Sterilised intervention |
|---|---|---|
| Move exchange rate | Effective | Weaker effect |
| Preserve money supply | No | Yes |
| Sustainable long term | Depends | Costly to maintain |
Why It Weakens Over Time
Two problems accumulate.
The first is cost. The central bank holds foreign assets, typically low yielding reserves, and has issued domestic securities paying the domestic interest rate. Where domestic rates exceed foreign yields, the operation loses money continuously, and the losses grow with the size of the position.
The second is that the effect is limited. Unsterilised intervention changes monetary conditions and therefore has a strong effect on the exchange rate. Sterilised intervention leaves monetary conditions unchanged, so its effect works through weaker channels: shifting the relative supply of assets, and signalling what the central bank intends. Evidence suggests the signalling channel does most of the work, which means it depends on credibility rather than on the transaction.
Where It Is Used
It is most common in economies receiving large capital inflows they consider temporary or destabilising. A country experiencing a commodity boom, or sudden portfolio inflows, may want to prevent an appreciation that would damage its other export industries.
Countries running persistent export surpluses have accumulated very large reserves this way, and the scale of those holdings is itself evidence of how long the operation can be sustained when the fundamental pressure does not reverse.
The Limits Worth Knowing
Against sustained market pressure driven by fundamentals, intervention generally fails. A currency under pressure because inflation is high or because the external position is unsustainable will move eventually, and the reserves spent defending it are simply transferred to the traders on the other side.
Intervention works best against disorderly short term movements, where it provides liquidity and signals a view, and worst when used to defend a level that the underlying economics do not support.
The Bottom Line
Sterilisation lets a central bank buy or sell foreign currency without changing domestic monetary conditions, which is the only way to pursue an exchange rate objective and an inflation objective at once. It is expensive to maintain, works mainly through signalling rather than mechanics, and cannot hold a level that fundamentals contradict.