The Impossible Trinity: Why Hong Kong's Peg and China's Capital Controls Are Two Sides of the Same Trade-Off

In early May 2025, Hong Kong's monetary authority sold a total of HK$129.4 billion of its own currency to defend the strong side of the exchange rate across four separate interventions in a single week, then watched short-term interest rates fall by hundreds of basis points within weeks as the resulting flood of currency sat in the banking system. The central bank did not choose that outcome. It was the mechanical result of a fixed exchange rate meeting free capital flows. Thousands of miles away, China runs one of the tightest capital-control regimes among major economies for the opposite reason: to keep the interest-rate freedom that Hong Kong just gave up. Both are following the same rule. They have simply picked different corners of it.

The Three Goals a Government Can Only Ever Buy Two Of

International economists call this rule the impossible trinity, or the trilemma: a country can combine, at most, two of three policy goals at the same time. A fixed exchange rate, free movement of capital across its borders, and an independent monetary policy set by its own central bank cannot all hold at once. Economists Robert Mundell and John Marcus Fleming worked out the underlying mechanics independently in the early 1960s, and the logic is arithmetic rather than ideological. If a country fixes its exchange rate and keeps its capital account open, then cuts interest rates to stimulate its own economy, investors can borrow at the new, lower rate and move that money abroad for a better return. The resulting capital outflow puts downward pressure on the currency, forcing the central bank to either raise rates back up to defend the peg or abandon the peg altogether. The independent rate cut and the fixed exchange rate cannot coexist once capital is free to move.

The diagram below lays out the three corners as a triangle. Each edge names the two goals that combination keeps, the one goal it sacrifices, and a real economy currently sitting on that edge.

Hong Kong and China sit on two different edges of that triangle. What each gave up to get there is visible in the data both have generated over the past two years.

Hong Kong's Peg Buys Stability by Giving Up Its Own Interest Rate

Hong Kong has run a currency board since 17 October 1983, according to the Hong Kong Monetary Authority, and since 2005 has kept the Hong Kong dollar inside a narrow band of 7.75 to 7.85 per US dollar, with every Hong Kong dollar in circulation backed by foreign-currency reserves. Hong Kong also keeps its capital account fully open. Under the trilemma, that combination leaves exactly one thing off the table: an interest rate set for Hong Kong's own economy.

The trade-off stopped being theoretical in the spring of 2025. Mainland capital inflows and a weakening US dollar pushed the Hong Kong dollar to the strong side of its trading band, and in defending the 7.75 limit the HKMA triggered the strong-side Convertibility Undertaking four times in a single week. The side effect was a flood of Hong Kong dollar liquidity into the banking system: one-month HIBOR fell by 270 basis points to 0.70% over the following two months, according to the HKMA's own currency board operations report. Hong Kong's financial account surplus reached HKD 160.4 billion in the first quarter of 2025, up from HKD 138.4 billion the previous quarter and the highest level on record since the statistic was first tracked in 1999. None of that was a Hong Kong policy choice about Hong Kong's economy; it was the currency board absorbing capital flows that had nothing to do with local conditions.

The same mechanism runs the other direction, too. Hong Kong's base rate has tracked the US Federal Reserve's target range almost exactly: the HKMA held its rate at 4.75% alongside the Fed's 5.25%-5.50% target in the middle of 2025, and later held it at 4.00% when the Fed's target range had fallen to 3.00%-3.75%. Whatever Hong Kong's own inflation or growth numbers said in between, the peg required the rate to move with Washington's decision, not Hong Kong's.

China Keeps Its Own Interest Rate by Closing the Capital Account Instead

China chose the opposite two corners. The People's Bank of China runs a managed float, letting the yuan trade in a band roughly 2% either side of a daily reference rate it sets itself, and it retains real monetary autonomy: interest-rate decisions inside China do not have to match the Federal Reserve's. What China gives up is the free movement of capital. The PBOC leans on tools such as adjustable risk-reserve requirements on foreign-exchange forwards and macroprudential limits on cross-border financing to slow money moving in or out, according to a Reserve Bank of Australia analysis of China's monetary framework.

The cost of that choice shows up in how far the yuan has, and has not, spread internationally. According to SWIFT's own RMB Tracker data, the yuan's share of global payments by value climbed from around 2% in 2023 toward a peak near 4.7% in late 2024, then slid back to about 2.93% in August and 3.17% in September of the following year, even as the Bank of China's own internationalization white paper counted more than 43 trillion yuan (roughly $6.04 trillion) in cross-border yuan settlement in 2024. That gap between rising settlement volume and a shrinking SWIFT share is what capital controls look like in currency data: transactions are increasingly routed through Beijing-administered channels such as the Cross-Border Interbank Payment System rather than the open market that a fully convertible currency would use. Consistent with that priority, PBOC governor Pan Gongsheng announced in January 2025 that China would raise the share of its foreign-exchange reserves and assets allocated in Hong Kong, reinforcing the managed channel rather than opening the capital account further.

Placing Hong Kong and China's choices side by side, using figures each institution has separately disclosed, makes the shape of the trade-off concrete rather than abstract:

Where the Triangle Doesn't Fully Explain a Currency Union

The trilemma is a clean way to think about a single sovereign currency, but two extensions to the framework show where a plain triangle stops being enough. First, economists Joshua Aizenman, Menzie Chinn, and Hiro Ito built indexes that score more than 170 countries from 1970 onward on each of the three goals as a continuum from zero to one rather than a binary choice, and their data shows most countries sitting somewhere in the middle of all three rather than fully on one edge. Second, Aizenman has argued the three-way framework understates the picture, proposing that financial stability deserves treatment as a fourth constraint after the deleveraging costs of crises such as 2008 showed that exchange-rate and capital-account choices alone do not capture systemic risk.

A currency union raises a separate version of the same problem. Members of a monetary union such as the eurozone give up their own exchange rate entirely against each other, which should resolve the classic trilemma inside the bloc. Research on the eurozone's sovereign-debt period found that doing so exposes a second, related conflict: a single monetary policy, open capital markets between member states, and each government's own fiscal sovereignty cannot all be sustained together once a member's debt position diverges sharply from the rest of the bloc. The original triangle explains why Hong Kong's rates track Washington's and why the yuan trades where it does. It does not, on its own, explain what happens when the thing being fixed is not an exchange rate between two currencies but the shared currency of a group of governments that still borrow separately.