For most of the 2010s and early 2020s, one of the most consistently profitable trades in global currency markets was also one of the simplest: borrow in Japanese yen (near-zero interest rate), convert to US dollars or Australian dollars (higher interest rate), invest in higher-yielding assets, and pocket the interest rate differential. This is the carry trade — and its July 2024 unwind was the proximate cause of the sharpest global equity sell-off in years, demonstrating how a currency strategy can become a macro risk factor affecting every asset class.
Uncovered interest rate parity — the theory the carry trade violates
Standard economic theory predicts that carry trades should not work. Uncovered interest rate parity (UIP) states that a high-interest-rate currency should depreciate by exactly the amount of its interest rate advantage: if Japan's rate is 0% and the US rate is 5%, the yen should appreciate 5% per year against the dollar — perfectly offsetting the carry profit. In practice, this does not happen. The "forward premium puzzle" (or UIP puzzle) is one of the most robust empirical findings in international finance: high-interest-rate currencies tend to depreciate less than UIP predicts and sometimes even appreciate. The carry trade systematically earns positive returns over long periods — but with significant crash risk.
The crash risk: why carry trades unwind violently
The carry trade is sometimes described as "picking up nickels in front of a steamroller." Returns accumulate slowly and consistently — until they do not. Carry trades are systemically correlated: when they work, everybody does them, building up large positions. When they stop working — typically in a risk-off episode when the funding currency (yen, Swiss franc) strengthens sharply — everyone unwinds simultaneously. This creates a self-reinforcing feedback loop: carry unwind strengthens the funding currency, which creates more losses for carry positions, which forces more unwind.
Carry as a systematic factor
Academic research has documented carry as a persistent risk factor in currency markets: currencies with high interest rates earn excess returns over currencies with low interest rates, on average, over long time horizons. This premium exists not because carry traders are irrational but because they are being compensated for bearing crash risk — the tail event of the funding currency appreciating sharply in a crisis. The return to carry is essentially a liquidity risk premium: carry positions tend to lose value precisely when investors most need liquidity and are most risk-averse. This explains why carry returns are highly correlated with VIX, credit spreads, and other risk sentiment indicators — and why carry trades are typically unwound in exactly the moments that are most costly.