Central Banks, Pegs & Currency Crises

So far this course has treated exchange rates like the weather: something that happens to you, which you hedge, arbitrage, or carry-trade against. But the weather has a meddler. Behind almost every major currency sits a central bank — an official-sector player with a printing press, a vault of reserves, and a mandate to care about where the rate sits. Sometimes it lets the wind blow freely. Sometimes it pins the rate in place with a clothespin and dares the market to rip it off. And every so often the market does rip it off, in a spectacle called a currency crisis that mints billionaires and bankrupts nations in the same afternoon. This lesson is about that meddler: what tools it has, what it can and can’t do, and the iron law — the impossible trinity — that no central bank has ever escaped.

What moves a currency

Before the central bank can meddle, we need the thing it’s meddling with: the price of a currency, set — like any price — by supply and demand. The twist is that a currency’s price is quoted in another currency (the exchange rate), and “demand for the dollar” means people wanting to hold or buy dollars.

Analogy. Think of a currency as concert tickets for a country’s economy. Three crowds bid for them:

• Trade flows. A German carmaker sells $2 billion of cars in the US, gets paid in dollars, and converts them to euros to pay German workers — that’s demand for euros, supply of dollars. Exporters earn foreign currency and sell it; importers do the reverse.
• Capital flows. A US pension fund decides Japanese stocks look cheap; it must buy yen to buy them — demand for yen. When foreign investors pour into a country’s assets, they bid its currency up; when they flee (capital flight), they dump it.
• Interest-rate expectations. Money chases yield (remember the carry trade from lesson 4). If the market expects a central bank to hike rates, the currency becomes more attractive to hold today, lifting demand before a single rate has moved.

Definition. A currency’s exchange rate is the market-clearing price where the quantity supplied (people wanting to sell it) equals the quantity demanded (people wanting to buy it). A surge in demand for a currency — foreigners buying in — pushes its exchange rate up (it appreciates); a surge in supply — holders dumping it — pushes the rate down (it depreciates).

Drag the sliders below. Shift demand up (foreign capital flooding in) and watch the rate climb; shift supply up (capital flight, locals fleeing to dollars) and watch it sink:

The single most important modern fact here: capital flows now dwarf trade flows. Global goods trade is roughly $25 trillion a year, but the FX market turns over around $7.5 trillion every single day — the vast majority of it financial, not commercial. The old textbook story (exchange rates settle trade balances) is a rounding error next to a hedge fund repricing a country’s bonds. That’s why a central bank can be overwhelmed: it’s not fighting carmakers, it’s fighting the entire global capital pool.

Exchange-rate regimes

A central bank’s first decision is philosophical: how much do we control the rate at all? The answer lives on a spectrum, from hands-off to handcuffed.

Analogy. Think of a thermostat. A free float is opening the windows and letting the room be whatever temperature the weather dictates. A hard peg is a thermostat bolted to exactly 21°C with the dial superglued. In between are every degree of “we’ll nudge it but not fix it.”

From loosest to tightest:

As you slide right, the bank trades flexibility for credibility. A free float can absorb shocks by letting the rate move, but offers no certainty to traders and savers. A currency board offers rock-solid certainty — Hong Kong’s peg has survived 40+ years — but the bank surrenders almost all room to fight a recession. Dollarization is the nuclear option: you import another country’s monetary policy wholesale and can never devalue, because you have nothing to devalue.

Intervention

When a central bank decides to push the rate, the act is called intervention: it steps into the FX market and trades its own currency, using its hoard of foreign-exchange reserves (dollars, euros, gold, US Treasuries).

Analogy. Intervention is a central bank acting as the world’s biggest dam operator. To lower the river (weaken its currency), it releases its own currency into the market — and it has an infinite reservoir, because it can print. To raise the river (strengthen its currency), it must pump water back up using reserves from a finite tank. One direction is gravity; the other is a pump with a fuel gauge.

Definition. To weaken its currency, a central bank sells its own currency and buys foreign assets — increasing supply of its currency. To strengthen (defend) it, the bank buys its own currency and sells foreign reserves — increasing demand. The asymmetry from the pretest lives here: selling your own currency is unlimited (print more), but buying it back requires reserves you can run dry.

Sterilized vs unsterilized — the money-supply side effect

Intervention has a sneaky side effect on the domestic money supply, and how the bank handles it has a name.

• Unsterilized intervention: the bank lets the money-supply effect ride. If it buys foreign reserves by printing local currency, the local money supply expands — which is itself loosening monetary policy. The FX move and the monetary effect both happen.
• Sterilized intervention: the bank offsets the money-supply effect with a second, opposite transaction (typically selling government bonds to soak the new money back up). The exchange-rate nudge happens, but the domestic money supply is left unchanged.

Why bother sterilizing? Because otherwise FX intervention secretly hijacks your interest-rate policy. A bank might want to weaken its currency without flooding the economy with cash and stoking inflation — sterilization lets it separate the two. The catch: sterilized intervention is widely considered weaker and more temporary, because it doesn’t change the fundamental rate differential driving the flows. You’ve fought the symptom, not the disease.

Worked example — the reserve burn

Suppose a central bank is defending a peg and the market is relentlessly selling its currency. To hold the line, the bank must buy its own currency, paying out reserves. Say it starts with $80 billion in reserves and the daily selling pressure forces it to absorb $10 billion per day:

Notice the outflow accelerates. As reserves visibly drain, speculators smell blood and pile in harder — the very act of defending advertises how close the bank is to the cliff. This reflexive feedback (defending shrinks reserves → shrinking reserves invites attack → bigger attack drains reserves faster) is the engine of a currency crisis. The math is brutally simple: reserves ÷ daily outflow = days until the peg breaks, and that denominator grows as the market scents the kill.

The impossible trinity (the trilemma)

Here is the iron law of the whole lesson — the constraint no central bank has ever wriggled out of. It’s called the impossible trinity or the policy trilemma.

Definition. A country can have at most two of the following three things at once:

1. A fixed exchange rate (a peg).
2. Free cross-border capital flows (money can move in and out freely).
3. Independent monetary policy (the bank sets its own interest rates for domestic goals).

Pick any two; the third becomes impossible. You cannot have all three.

Analogy. It’s a “pick two” menu, like the old “fast, cheap, good — choose two” joke for contractors. Want fast and cheap? It won’t be good. In FX: want a fixed rate and open borders? Then your interest rates are no longer yours — they’re chained to whatever keeps the peg alive.

Walk the three corners, each with a real country:

The intuition: if money can move freely and you peg the rate, then any gap between your interest rate and the world’s invites unlimited arbitrage — capital floods in or out until your domestic rate is dragged to the level that sustains the peg. So with open capital + a peg, the peg dictates your rates. The only way to reclaim independent rates while pegging is to slam the door on capital (China’s old model). And the only way to have both open capital and independent rates is to let the currency float (the US, the euro area, Japan).

Defending a peg — and how it breaks

Now we put intervention and the trilemma together and watch a peg die in real time. The villain (or hero, depending on your portfolio) is the speculative attack.

Analogy. Defending an overvalued peg is like a shopkeeper promising to buy back his own loyalty coupons at $1 each, even though everyone knows they’re really worth 70 cents. As long as nobody calls his bluff, fine. But the moment a big player starts dumping coupons by the truckload demanding $1 apiece, the shopkeeper either pays out until his till is empty — or admits the coupons are worth 70 cents (devalues). A speculative attack is the truckload. And here’s the kicker: betting against a peg is a one-way bet. If the peg holds, the speculator loses only their transaction costs. If it breaks, they make a fortune. Heads I win big, tails I lose lunch money — which is exactly why pegs attract attacks like blood attracts sharks.

Definition. A speculative attack is a coordinated, large-scale sale of a pegged currency by traders betting the central bank lacks the reserves (or the will) to defend it. Defending an overvalued peg (a rate the market thinks is too strong) forces the bank to (a) burn reserves buying its own currency, and (b) jack domestic interest rates punishingly high to make holding the currency attractive enough to stem the outflow — which throttles the domestic economy. At some point the pain of defending exceeds the pain of surrender, and the bank lets go. The peg breaks, the currency gaps to its market level, and everyone who shorted it gets paid.

Black Wednesday, 1992 — Soros vs the Bank of England

Britain had joined the European Exchange Rate Mechanism (ERM), pinning the pound to the Deutsche Mark within a band. Problem: the UK’s economy needed low rates (recession), but Germany, post-reunification, was running high rates to fight inflation. With open capital and a quasi-peg, the trilemma came collecting — Britain couldn’t have its own low rates and hold the DM peg.

George Soros’s Quantum Fund (and others) saw the pound was overvalued and unsustainable, and shorted it massively — reportedly a $10 billion position. On 16 September 1992, the Bank of England fought back with both defensive tools and lost both fights:

By evening Britain crashed out of the ERM and let the pound float (and fall). Soros’s fund reportedly made around $1 billion in a day, earning him the title “the man who broke the Bank of England.” The lesson: with open capital, a peg that fights the country’s own economic needs is a sitting duck.

The 1997 Asian crisis — the Thai baht

Thailand pegged the baht near 25 per USD while running a current-account deficit and a pile of short-term, dollar-denominated debt (more on why that’s lethal in the next section). Foreign capital had flooded in chasing high local yields — until confidence cracked in 1997 and it all tried to leave at once.

The collapse spread — Indonesia, South Korea, Malaysia, the Philippines all got hammered as investors fled the whole region (contagion). And here’s the spaced-recall link to lesson 4: the Asian crisis was, in part, a giant carry-trade unwind. Investors had borrowed cheap dollars to buy high-yielding Asian assets; when the pegs broke, the currencies they were long collapsed, the dollars they owed got more expensive, and the rush to unwind the carry accelerated the crash. The carry trade and the currency crisis are the same machine running in reverse.

Anatomy of a currency crisis

Economists sort currency crises into three “generations” of models — not three eras, but three mechanisms, often tangled together in any real blow-up.

Definition — the three generations:

• First-generation (bad fundamentals). The classic Krugman story: a government runs unsustainable policies — typically a fiscal deficit financed by printing money — while pegging the rate. The fundamentals and the peg are flatly incompatible. Reserves bleed predictably, and once they hit a threshold, a rational speculative attack finishes the peg in one stroke. The crisis is deserved: bad policy, inevitable break.
• Second-generation (self-fulfilling attacks). Here the fundamentals are ambiguous — the peg could survive. But defending is costly (high rates, recession), so if enough speculators believe the bank will give up rather than endure the pain, they attack, the cost of defending spikes, and the bank rationally capitulates. The attack causes the outcome it predicted. Multiple equilibria: the same country could have been fine or crashed, depending purely on market psychology. The 1992 ERM exit has strong second-generation flavor.
• Third-generation (balance sheets / foreign-currency debt). The 1997 Asian model. Banks and firms borrowed heavily in foreign currency (dollars) but earned in local currency. When the currency falls, their dollar debts explode in local-currency terms, wrecking balance sheets, triggering defaults — which scares capital, which sinks the currency further, which deepens the debt. A vicious doom loop where the devaluation and the financial collapse feed each other.

The warning signs a savvy analyst watches for — the smoke before the fire:

• Overvaluation — the real exchange rate looks too strong versus fundamentals.
• Thin reserves — low reserves relative to short-term external debt (the “months of import cover” or reserves-to-short-term-debt ratio).
• FX-denominated debt — lots of borrowing in dollars/euros by entities earning in local currency (the third-generation time bomb).
• Current-account deficits — the country relies on continuous foreign capital inflows, which can reverse violently.

Argentina, 2001–02 — when the currency board cracked

Argentina ran a currency board through the 1990s, legally pinning the peso 1-to-1 to the US dollar. It worked — until it didn’t. A strong dollar made Argentine exports uncompetitive, the country piled up dollar debt, and a deep recession plus capital flight drained the system. The hard peg that was supposed to be unbreakable became a straitjacket: Argentina couldn’t devalue to regain competitiveness or print to fight the slump (that’s the trilemma’s sacrifice, cashed in at the worst moment).

In late 2001 the board collapsed: deposits were frozen, the peg abandoned, the peso devalued, and Argentina defaulted on its sovereign debt. It’s the textbook reminder that even a legally bolted peg is only as durable as the economy underneath it — and that the trilemma’s price (no independent policy, no devaluation valve) comes due exactly when you can least afford it.

Putting it together

Behind every exchange rate sits a central bank with three tools and one inescapable constraint. It can choose a regime anywhere from free float to dollarization. It can intervene — unlimited when weakening its currency, countdown-limited when defending it. But it can never beat the impossible trinity: pick at most two of a fixed rate, open capital, and independent monetary policy. Every great currency crisis — Britain in ‘92, Thailand and Asia in ‘97, Argentina in ‘01 — is a country that tried to hold all three and got forced by the market to drop one, almost always the peg. Here’s the whole machine on one card:

One mixed recap before the takeaways:

Key Takeaways