What it means
When a central bank sells dollars to prop up its currency, the cash leaves the banking system, and what happens to that cash next defines whether the intervention is sterilised or not. In unsterilised intervention, the answer is nothing: the monetary base simply shrinks or grows with the operation, so the exchange rate move and domestic monetary policy pull in the same direction.
The New York Fed's research materials on foreign exchange intervention draw the line precisely: unsterilised operations involve domestic monetary base changes, while sterilisation offsets them. Sterilisation is the alternative, in which the central bank reverses the base effect with an open-market operation, buying or selling domestic bonds so the money supply returns to where it was.
The difference decides the intervention's power, because unsterilised operations are monetary policy with an exchange-rate accent, while sterilised ones attempt to move the currency without touching domestic rates. Theory predicts unsterilised intervention should work better, since it changes the interest differential driving the exchange rate, and the empirical literature largely agrees that sterilised effects are weaker and shorter-lived.
In practice most major central banks sterilise by default, because their money supply is set by an inflation target and letting currency operations leak into it would surrender the policy framework. For a non-finance reader, the distinction is whether the fire department drains the neighbourhood pool to fight the fire.
Unsterilised intervention accepts the drained pool, while sterilised intervention refills it from the tanker while the hoses run. Emerging-market central banks face the choice more often than reserve-rich ones.
With thin reserves and open capital accounts, their interventions are routinely unsterilised in effect, because offsetting operations would require credibility and instruments they lack, so their currency defences and monetary policy are permanently entangled whether they like it or not.
In practice
Real-world examples.
Example
A central bank sells reserves by day and buys domestic bonds by night, sterilising out of habit. The monetary base holds steady, rates do not move and the currency slides anyway. Traders read the operation as a defence the bank is unwilling to pay for.
Example
A research memo reviews past currency defences and finds that every successful one paired intervention with real monetary consequences. The bank's own history shows that the sterilised-only attempts eventually folded. The memo becomes the argument for changing the playbook.
Example
A central bank stops sterilising, so the monetary base contracts and overnight rates climb by 200 basis points. The higher rate differential draws capital back and the currency finds a floor within days. The cost is slower growth, which the board accepts with its eyes open.
Formula
Calculation
Change in monetary base from intervention = domestic currency paid for foreign currency bought, minus domestic currency received for foreign currency sold, plus or minus any offsetting open-market operations. An intervention is unsterilised when the offsetting term is zero.
Worked example. A central bank sells $2,000,000,000 of reserves to defend its currency, and the buyers pay in domestic currency worth $2,000,000,000 at the prevailing rate. Unsterilised, the monetary base falls by $2,000,000,000, which tightens money markets and lifts short-term rates. Sterilised, the bank also buys $2,000,000,000 of domestic bonds, so the net base change is -$2,000,000,000 + $2,000,000,000 = $0 and rates are untouched. Daily liquidity management can unintentionally mimic or mask sterilisation.Case study
Seen in the real world.
This case study is fictional and illustrative. A made-up emerging-market central bank fights a run on its currency with reserves, and the governor's morning meeting divides over one question: sterilise or not. The intervention chief wants the operation to bite, while the monetary policy chief wants the inflation target untouched. The first week runs sterilised by habit: reserves sell by day, domestic bonds buy by night, the base holds steady, and the currency slides anyway, because the market reads a central bank defending a level while refusing to pay for it with tighter money. The memo that changes policy comes from the bank's own research unit: every successful defence in their sample paired intervention with monetary consequences, and every sterilised-only defence in the sample eventually folded.
The governor orders the shift: intervention continues, sterilisation stops, the monetary base contracts, overnight rates climb 200 basis points, and the currency finds a floor within days, at the price of a growth forecast the board accepts with its eyes open. The governor's summary to parliament is one sentence: we stopped telling the currency market we disagreed with it and started charging it interest for the disagreement. The research unit's dataset grows with their own episode added, and its summary statistic hardens: interventions with monetary backing hold more than twice as often as sterilised-only defences. The bank's intervention playbook now opens with the sterilisation decision instead of burying it in annexes.
Watch out
Common mistakes.
- Assuming sterilised intervention is fake; it can work through signalling and portfolio channels, just with weaker and shorter effects.
- Forgetting the framework conflict; under an inflation target, unsterilised operations are a policy change, not a free option.
- Reading all reserve use as intervention; valuation effects, interest income and other operations move reserves without any currency defence occurring.
Questions
People also ask.
What is unsterilised intervention?
Central bank foreign exchange intervention allowed to change the domestic monetary base, so the operation tightens or eases monetary conditions.
How does sterilised intervention differ?
The base effect is offset with open-market operations, attempting to move the exchange rate without changing domestic monetary policy.
Which works better?
Theory and evidence favour unsterilised intervention, since it shifts the interest differential; sterilised effects are generally weaker and briefer.
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