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Lesson M20.L04: Capital Mobility and the Impossible Trilemma

Module: Open Economy Macroeconomics Part I Level: intermediate Duration: 30 minutes Learning Objective: Apply the impossible trilemma to evaluate Australia's macroeconomic policy framework. Data as of: 2024 Provenance: RBA Exchange Rate Research | IMF World Economic Outlook

Explanation

The impossible trilemma (also called the monetary trilemma or Mundell-Fleming trilemma) states that a country can achieve at most two of the following three macroeconomic objectives simultaneously:

  1. Fixed exchange rate โ€” stable, predictable currency value for trade and investment
  2. Free capital mobility โ€” unrestricted cross-border investment flows
  3. Independent monetary policy โ€” ability to set domestic interest rates to manage inflation and output

The trilemma arises directly from the Mundell-Fleming model. If a country has both (1) and (2) โ€” a fixed rate and free capital โ€” then domestic interest rates must equal the world rate i* at all times. Any deviation triggers capital flows that either appreciate or depreciate the currency, requiring intervention that offsets the monetary policy action. So (3) is impossible.

Three policy regimes โ€” one corner per choice:

Corner Chosen Abandoned Example
Gold Standard / Currency Union Fixed rate + Free capital Independent monetary policy Eurozone, Bretton Woods
Capital Controls Fixed rate + Independent monetary policy Free capital mobility China (managed), Bretton Woods with controls
Floating Rate Free capital + Independent monetary policy Fixed exchange rate Australia (post-1983), USA, UK

Australia's choice. In December 1983, the Hawke-Keating government floated the AUD and progressively removed capital controls. Australia chose the floating rate corner: free capital mobility + independent monetary policy. The RBA can set the cash rate to target inflation (2โ€“3% band) without being constrained by exchange rate defence. The exchange rate adjusts freely, acting as an automatic stabiliser.

The AUD as a shock absorber. When commodity prices rise (iron ore, coal boom), export revenues increase โ†’ AUD appreciates โ†’ dampens domestic inflationary pressure and external competitiveness. When commodity prices crash (2014โ€“15), AUD depreciates โ†’ export competitiveness improves โ†’ partially offsets the income shock. This buffering function would be lost under a fixed exchange rate.

China's choice. China long operated near the "fixed rate + independent monetary policy" corner, maintaining capital controls to reconcile the two. As China liberalises its capital account, it faces the trilemma directly: it must either allow more exchange rate flexibility or accept reduced monetary independence.

Eurozone's choice. Eurozone members chose "fixed rate + free capital" (the single currency is the ultimate fixed rate) but surrendered monetary policy to the ECB. Greece, Spain, and Ireland could not devalue or set independent interest rates during the 2010โ€“12 sovereign debt crisis.

Notation: - i = domestic interest rate; i* = world interest rate - e = nominal exchange rate; ฤ“ = fixed target - Capital account openness: measure of restrictions on cross-border capital flows

Worked Example

Question: Illustrate the impossible trilemma quantitatively. Suppose the world interest rate is i* = 3%. An economy has a fixed exchange rate and free capital mobility (Eurozone-style). The ECB raises i to 5% to fight inflation. Trace what happens.

Step 1 โ€” Initial conditions.

  • Domestic i = i* = 3% (parity holds because of fixed rate + free capital)
  • Exchange rate: ฤ“ = 1.0 (fixed)

Step 2 โ€” ECB attempts to raise i to 5%.

ECB open market operations: sell bonds โ†’ M falls โ†’ i rises toward 5%.

Step 3 โ€” Interest rate differential triggers capital flows.

Investors observe: domestic i = 5% > i* = 3%. Differential = +2pp.

Capital inflows: funds flood into domestic assets seeking higher returns.

Step 4 โ€” Exchange rate pressure.

Capital inflows โ†’ demand for domestic currency โ†’ appreciation pressure. Under the fixed rate, the central bank must intervene: sell domestic currency (buy foreign reserves) โ†’ M expands.

Step 5 โ€” Money supply returns to equilibrium.

The monetary expansion from reserve purchases offsets the original contractionary open market operations. The net change in M is zero. i returns to i* = 3%.

Step 6 โ€” Calculate net effect.

\[\Delta i_{actual} = 0 \quad (\text{monetary policy fully offset})$$ $$\Delta Y = 0 \quad (\text{no domestic demand effect})\]

The attempted tightening is completely neutralised. This is exactly what ECB-peripheral country tensions looked like: the ECB sets rates for Germany, but those rates may be too high or too low for Ireland, Greece, or Spain โ€” and the fixed rate (euro) prevents adjustment.

Contrast โ€” Australia (floating rate + free capital).

RBA raises cash rate from 4.10% to 4.35% (November 2023 move).

  • i > i* โ†’ capital inflows โ†’ AUD appreciates.
  • AUD appreciation โ†’ import prices fall โ†’ inflation dampened (exchange rate channel).
  • AUD appreciation โ†’ NX falls โ†’ aggregate demand reduced.
  • Monetary policy IS effective because the exchange rate reinforces the tightening (rather than reversing it via reserve intervention).

Result: ฮ”i = +0.25pp transmits to real economy via both the interest rate channel AND the exchange rate channel.

Common Misconception

Misconception: "A country can avoid the trilemma by 'leaning against the wind' โ€” intervening in the FX market occasionally while keeping capital mostly free."

Correction: Occasional intervention ("managed float") does not escape the trilemma โ€” it merely positions a country between corners. The trilemma is an equilibrium constraint: any persistent attempt to hold all three simultaneously leads to a crisis (reserve exhaustion, currency attack, or interest rate surrender). "Dirty floats" โ€” like Australia's occasional RBA intervention โ€” are feasible only when the central bank accepts that monetary independence is the primary goal and exchange rate stability is secondary. The moment FX intervention becomes so large that it dominates monetary operations, the country has effectively moved toward the fixed-rate corner and loses monetary independence. The IMF classifies exchange rate regimes along a spectrum from free float to hard peg precisely because countries sit at different points along the trilemma triangle.

Practice Prompts

  1. Conceptual: The Eurozone chose "fixed exchange rate + free capital mobility." What did member countries surrender, and how did this constrain policy during the 2010โ€“12 sovereign debt crisis?

โ†’ Answer: Eurozone members surrendered independent monetary policy. The ECB sets one interest rate for all 19 (now 20) members. During 2010โ€“12, Greece, Ireland, Portugal, and Spain needed looser monetary policy (low rates, currency depreciation) to restore competitiveness and stimulate growth, but the ECB rate was calibrated partly for Germany. Without the ability to depreciate (no independent exchange rate) or set lower interest rates (no independent monetary policy), peripheral countries had to rely solely on fiscal austerity and internal devaluation (wage cuts) โ€” a painful and slow adjustment. Had they had floating currencies, depreciation would have quickly restored export competitiveness.

  1. Numerical: An economy has free capital mobility. The world interest rate is i* = 2.5%. The central bank tries to set i = 4.0% under a fixed exchange rate. How many basis points of capital-flow-driven money supply expansion would be required to return i to i* = 2.5% if each 100bp rise in M/P reduces i by 0.5pp? Show the algebra.

โ†’ Answer: - Required reduction in i: 4.0% โˆ’ 2.5% = 1.5pp = 150 basis points - Each 100bp rise in M/P reduces i by 0.5pp - Required rise in M/P = 150 bp รท (0.5pp per 100bp) = 150/0.5 ร— 100 = 300 basis points of M/P expansion - Equivalently: M/P must rise by 3 units (per the specified relationship) - This 3-unit expansion in M/P exactly reverses the initial monetary tightening โ€” confirming monetary policy impotence under fixed rates.

  1. Application: Australia floated the AUD in December 1983. How does the floating rate benefit RBA monetary policy during terms-of-trade shocks, and what would be the consequences of re-pegging the AUD to the USD?

โ†’ Answer: Under the float: a terms-of-trade boom (e.g., 2003โ€“2011 iron ore boom) โ†’ AUD appreciates naturally โ†’ dampens domestic inflation and investment overheating โ†’ RBA can focus on domestic conditions. A terms-of-trade crash (e.g., 2014โ€“15 iron ore collapse) โ†’ AUD depreciates โ†’ export revenue in AUD remains partly buffered โ†’ adjustment is smoother. Re-pegging to USD would eliminate this shock absorber: the RBA would have to match Fed rate decisions regardless of Australia's domestic conditions. During the 2008โ€“09 GFC, the RBA cut rates to 3% while the Fed cut to 0% โ€” under a USD peg, Australia would have had to follow the Fed much more closely, potentially undershooting optimal domestic policy. The float allows the RBA to respond to Australia-specific shocks independently.

Further Resources