The Impossible Trinity: What a Central Bank Pays for a Fixed Exchange Rate

The impossible trinity is the proposition that, out of the set of goals "fixed exchange rate", "free movement of capital" and "independent interest rate", a state can hold any combination except the complete set. Something from the set has to be given up as payment: either the exchange rate (it starts to float), or the openness of the capital account (restrictions appear on moving money out and in), or the country's own monetary policy (the interest rate stops being a tool for domestic objectives and follows someone else's). This is not an ideological choice and not a question of the quality of governance — it is a consequence of arbitrage, which is carried out with the money of private holders, not by a decision of the regulator.

Why this is an identity, not a doctrine

The mechanism is simpler than the wording. Imagine that the exchange rate is rigidly pegged, the borders are open to capital, and the domestic interest rate has turned out to be higher than the foreign rate. Then any holder of currency borrows more cheaply abroad, converts at the announced rate, places the money at a higher return at home — and takes on no currency risk, because the exchange rate has been promised as fixed. Money keeps flowing in until the rates converge, that is, until the domestic rate ceases to be independent. If the regulator wants to hold both the exchange rate and the interest rate, all that remains is to switch off the channel itself — to impose restrictions on the movement of capital. If it does not want restrictions and does want its own rate, it has to let the exchange rate go: the difference in rates is then absorbed not by arbitrage but by currency risk, and the equilibrium rests on expectations, not on a promise.

Hence the practical meaning of the construct for the reader: it predicts not a magnitude but which variable will start to move when the pressure builds. If the exchange rate is declared firm and the borders are open, the strain will inevitably come out through reserves and the interest rate. If the exchange rate floats, the strain comes out through the exchange rate — and that is exactly why, in floating regimes, the currency reacts to rate decisions faster and more sharply than exports or imports manage to adjust.

Which configurations practice has chosen

The Bretton Woods system (built in 1944, dismantled by 1971) held exchange rates and policy independence — at the cost of capital restrictions, which were considered the norm at the time. The single European currency is the opposite choice: the exchange rate inside the zone is locked as tightly as it can be, capital is free, and an individual country has no interest rate of its own at all — it has been delegated. The classic free float with an open account is the choice of most advanced economies after the collapse of Bretton Woods: the rate is their own, capital is free, and the exchange rate takes the blow. Economies with a managed exchange rate and retained control over capital flows are the third configuration, and it requires an administrative machine to maintain that control.

The Russian regime went through a change of configuration before the eyes of today's market participants: the move to a floating exchange rate in 2014 meant abandoning the promise on the exchange rate in favour of an independent interest rate with an open account, while the measures of 2022 temporarily added restrictions on the movement of capital — that is, they shifted the point at which the strain comes out. Understanding this matters more than remembering specific values: the configuration determines which asset class is repriced first.

What this means for a portfolio

For a holder of securities, the trinity works as a filter. With a floating exchange rate and an independent interest rate, monetary policy decisions hit long-dated debt above all: yields on OFZ move together with rate expectations, and the long end of the curve reacts more strongly than the short end. The corporate segment of the bond market adds to this a credit spread, which in regime shifts widens separately from the rate. On the equity market, the configuration sends exporters and domestic-demand companies in different directions: a weaker exchange rate improves revenue for some, and for others raises the cost of purchases and of servicing debt. Restrictions on the movement of capital separately change the mechanics of funds — access to foreign assets and the ability to move money between jurisdictional perimeters cease to be a technical matter.

There is a calendar dimension too: rate-setting meetings and macroeconomic releases are the dates on which the regime shows itself publicly, and they are worth following in the events calendar, while commentary on the decisions is in the news. Definitions of the terms that come up around this topic — capital account, yield curve, carry trade — are collected in the glossary.

Where the construct stops working

There is a serious objection to the trinity: with a fully open account, global capital movements are so large that a floating exchange rate does not insulate domestic policy but merely transmits the impulse by another route. On this reading, the choice narrows to "open account or independent policy", and the exchange rate regime becomes secondary. This does not cancel the logic of arbitrage — it remains valid — but it changes the assessment of how much freedom a float actually buys.

A separate, honest caveat: this article explains the mechanism and contains no values for the exchange rate, the interest rate, reserves or capital flows — no such data were supplied in the brief, and inventing them here would be worse than sending the reader to the relevant sections of the platform. If you need the factual picture as it stands today, see the instrument sections and the news feed, links to which are given above.