The yen carry trade and the $790 billion morning it unwound

On the morning of August 5, 2024, Japan's Nikkei 225 fell 12.4% in a single session — its worst day since the 1987 Black Monday crash. About $790 billion was wiped from the broad Tokyo market that day, and the shock rolled west: the VIX volatility index spiked above 60, and the S&P 500 lost roughly 6% over three days. The trigger was not a war or a bankruptcy. It was a quarter-point interest-rate hike, and the quiet, enormous trade it broke.

That trade was the yen carry trade, and it is one of the cleanest real-world lessons in currency risk on the whole curriculum.

How a carry trade works

Borrow money where it is cheap, and invest it where it pays more. For years Japan held interest rates near zero while the rest of the world paid far more. So traders borrowed yen at almost no cost, converted it to dollars, and bought higher-yielding assets — US Treasuries, tech stocks, anything with a better return. The gap between the two interest rates was, in effect, free money.

Two forces decide whether the trade wins:

• The rate differential — the gap between what you pay to borrow and what you earn. The wider the gap, the richer the carry.
• The exchange rate — because you borrowed in yen, you must eventually buy yen back to repay the loan. If the yen weakens, you repay with cheaper currency and keep the difference. If the yen strengthens, repaying gets more expensive, and that cost can swamp the entire interest-rate gain.

That second force is where the risk lives.

Why it broke

On July 31, 2024, the Bank of Japan raised its policy rate from near zero to about 0.25% — tiny in absolute terms, but it narrowed the gap the whole trade depended on. Days later, a weak US jobs report stoked bets that the Federal Reserve would soon cut rates, squeezing the gap from the other side. The yen, long the cheap funding currency, suddenly surged, roughly 6% in a week.

Now run the trade in reverse. A stronger yen meant every carry position was losing money on the currency leg. Leverage magnified the losses; losses triggered margin calls; meeting the calls meant selling the assets and buying back yen, which pushed the yen up further and the assets down further. A crowded trade unwound all at once.

The exam version of the trade

Underneath the headlines, the carry trade is built from a few ideas the exam tests directly. Interest-rate differentials drive currency flows. Uncovered interest rate parity holds that, in theory, the higher-yielding currency should depreciate just enough to erase the carry. In practice it rarely does, which is why the trade can pay for years at a stretch. The exam tests that tension: in theory the parity neutralizes the carry, but the market routinely ignores it until a shock forces the whole adjustment at once. And leverage turns a sensible position into a fragile one, because it makes you sell at the worst possible moment.

A carry trade looks like free money right up until the currency moves against you. That is the core lesson in currency risk: the interest you earn is visible and steady, but the exchange-rate loss that can erase it stays hidden until it suddenly appears.