Case study: the carry trade and its unwind
Near-zero Japanese rates against everyone else's made the yen the world's funding currency: borrow it cheaply, invest the proceeds elsewhere, earn steadily, then lose catastrophically when everyone unwinds at once.
A currency is two economies in one price, and no pair teaches the machinery better than the yen. For thirty years Japan ran the developed world's lowest interest rates, which turned its currency into the planet's favorite thing to borrow, and turned dollar-yen into a live seminar on rate differentials, the carry trade, intervention, and the violent negative skew that defines currency risk.
Why the yen is the teaching currency
Three features make dollar-yen the ideal case. First, a persistent, enormous rate differential: Japan pinned rates at or below zero for a generation while the rest of the developed world paid more, so the single cleanest driver in FX was on maximal display. Second, deep liquidity and free convertibility: the trade could be put on at scale, so its collective behavior (crowding, then cascading) is visible. Third, an active finance ministry that intervenes, giving a live study of what official firepower can and cannot do against the tide. Every FX concept from the analysis guide appears here in high contrast.
The event history
| Period | Event | The FX lesson |
|---|---|---|
| 1985 | Plaza Accord: coordinated dollar devaluation; yen roughly doubles vs USD over two years (~240 to ~120) | Coordinated official action CAN move a major currency: rare, and it needs consensus |
| 1990s | Asset bubble bursts; rates cut to near zero; deflation sets in | The rate differential that would define the yen for 30 years is born |
| 1998-2007 | The classic carry era: borrow yen, buy higher-yield everything; USDJPY drifts up on carry | Carry earns quietly for years while the funding currency slowly weakens: as theory says it should not, but does |
| 2008 | Risk-off: the carry trade unwinds violently; yen surges ~20%+ as borrowers buy it back | Negative skew realized: years of carry given back in weeks |
| 2013-2016 | Abenomics: massive QE weakens the yen deliberately; ~80 to ~120 | A determined central bank can debase its own currency on purpose |
| 2016-2022 | Yield-curve control: the BOJ pins the 10-year JGB near zero | Pinning the CURVE (not just the overnight rate) is the extreme version, and it built enormous pressure |
| 2022-2024 | The Fed hikes hard while the BOJ holds YCC: the differential explodes; yen collapses past 150 then toward 160, a 34-year low; Japan intervenes repeatedly | The differential is the master variable: when it gapped, nothing else mattered |
| Aug 2024 | BOJ hikes as the Fed signals cuts; the differential narrows; the carry trade unwinds in days: a global risk-asset air pocket | The unwind is a market-wide event, not a yen event: funding currencies are wired into everything |
The carry trade, embodied
borrow yen at ~0% -> convert to USD -> buy US assets at ~5% gross carry ≈ +5%/yr, BEFORE any move in USDJPY uncovered interest parity SAYS the yen should appreciate ~5% to erase it. for years it did the OPPOSITE (weakened on flow), paying carry twice: the yield AND the currency. this is the forward-premium puzzle, live. then, periodically: risk-off -> forced unwinds -> yen spikes 10-20% in weeks -> years of carry erased. steady premium, sudden ruin.
The yen carry trade is the canonical negatively-skewed strategy: it wins in the great majority of months and loses catastrophically in the rare ones, exactly like selling insurance. Its Sharpe ratio looks superb in any sample without a crisis, and lethal in any sample with one, which is why the risk-and-return guide warns that a high Sharpe with negative skew is a premium collected before the claim arrives. Position sizing that respects the AUGUST-2024-shaped tail, not the placid average, is the entire discipline: the trade that looks best on a spreadsheet is the one built to blow up.
How professionals analyze the yen
- The two-year yield differential (JP vs US, and vs the trade-weighted basket) is the workhorse: over months, USDJPY tracks it tightly, so the currency view reduces to a view on the two central banks' PATHS, not their current levels: what is the market pricing for the Fed and BOJ, and which side of it surprises?
- Balance of payments. Japan's traditional current-account surplus (an underlying yen bid) shifted as energy imports and outbound investment grew; the structural picture matters for the multi-year trend beneath the rate-driven swings.
- Positioning and crowding. CFTC data and options skew (risk reversals) show when the funding-currency short is crowded: the condition for a squeeze. In 2024 the record short was the fuel the August unwind lit.
- Real effective exchange rate. By 2024 the yen was among the most undervalued major currencies on a purchasing-power basis in decades: the PPP guide's slow anchor, saying "cheap" for years while the rate differential said "cheaper still." Valuation and catalyst, disagreeing, exactly as expected.
Intervention and its limits
Japan's Ministry of Finance intervened repeatedly in 2022-24, spending tens of billions of dollars of reserves to buy yen. The case teaches the general rule precisely: intervention can slow and punctuate a move (inflicting losses on crowded shorts, buying time) but cannot reverse a trend the fundamentals are driving. As long as the rate differential stayed enormous, each intervention bounce faded, because it fought the carry math without changing it. Intervention works durably only when it accompanies a policy turn (as August 2024's hike did) or a coordinated agreement (Plaza). Alone, it is a speed bump the tide flows over: reserves are finite, the differential is not.
The unwinds: 2008 and 2024, one pattern
Both great yen surges share a mechanism worth memorizing. Funding-currency shorts accumulate quietly for years; a shock (2008's credit freeze, 2024's sudden differential narrowing) forces deleveraging; unwinding the trade means BUYING yen; the buying is one-directional and self-reinforcing; and because the same funding currency underwrote positions all over the world, the unwind transmits into global equities as a synchronized risk-off. The yen did not just move; it moved everything. This is why macro desks watch the funding-currency crosses as a systemic gauge, not merely an FX trade: the carry unwind is one of the few reliably contagious events in markets.
The transferable lessons for any currency
- Find the master variable. For most major pairs in most eras it is the expected rate-differential PATH; trade the surprise to what is priced, not the level.
- Carry is selling insurance. Steady until it is not; size for the tail, and never confuse a crisis-free sample's Sharpe with skill.
- Positioning is the fuel, the catalyst is the spark. Crowded funding shorts plus any narrowing of the differential is the setup the biggest FX moves come from.
- Valuation (PPP) and catalyst (rates) routinely disagree for years. The yen was "cheap" long before it stopped falling: value tells you the direction of the eventual mean-reversion, not its date.
- Intervention is a speed bump, not a dam. It punctuates trends the fundamentals own; durable reversals need a policy turn.
- Funding currencies are wired into everything. Their unwinds are systemic, which is why an FX trade can become the whole market's story overnight.
- Rate differential
- The gap between two economies' interest rates, the fast anchor of exchange rates. The two-year government yield spread tracks major pairs remarkably well over months.
- Carry trade
- Borrow the low-rate currency, deposit in the high-rate one, harvest the differential. Pays steadily until a risk shock unwinds everyone at once; the yen unwinds of 2008 and 2024 are the case studies.
- Uncovered interest parity
- The theory that rate differentials should be erased by currency depreciation. Empirically it fails for years at a time (the forward premium puzzle), which is why carry exists as a strategy.
- Negative skew
- A return profile of many small gains and rare large losses, the shape of selling insurance. Carry trades and option selling share it; averages flatter it, and sizing must respect the tail rather than the average.
- Purchasing power parity
- The long-run tendency of exchange rates toward equalizing what money buys across borders. Useless for timing, excellent for knowing which side of expensive a currency starts from.
- Current account
- A country's net trade and income with the world. Persistent deficits must be financed by daily capital imports, which is fine until the financing mood changes: the anatomy of most currency crises.
- Risk reversal (FX)
- The implied-volatility gap between out-of-the-money calls and puts on a currency: the options market's directional fear gauge, and a crowding check before entering.
- Sharpe ratio
- Excess return over the risk-free rate, per unit of volatility. Around 1 is genuinely good long-run; above 2 deserves an audit. Gameable by strategies whose defining loss has not happened inside the sample yet.
- Crowding
- Too much capital in one trade. Crowded strategies unwind together, briefly correlating at the worst moment; measuring how much of a story is already positioned is half of risk management.
- Dollar smile
- The dollar strengthens when the US booms AND in global crisis (dollar debts must be serviced; dollars get hoarded), softening only in the mild middle. Every portfolio has a dollar exposure whether chosen or not.
- Currency hedging
- Removing FX risk from an international position, usually with forwards. The professional default: hedge where the currency is not the thesis, size it separately where it is.