FX spot moves because global capital reallocates between safer and riskier currencies as volatility, commodity prices, policy expectations, and institutional balance sheets shift.

The dominant driver—30–50% of DM FX variation and 40–70% of EM FX variation—is the global volatility cycle. When volatility is low, investors borrow in low-yield funding currencies (JPY, CHF, USD in stress periods) and take leveraged long positions in higher-yield or higher-beta currencies (AUD, NZD, NOK, CAD, MXN, BRL, ZAR, KRW) by purchasing local assets or entering FX forwards. This is economically equivalent to synthetic lending to riskier economies because the position replicates borrowing in a safe country and lending in a risky one. The capital inflows—whether through direct purchases of local assets or FX forwards that dealers must hedge—strengthen the exchange. When volatility rises, this synthetic lending is withdrawn simultaneously across leveraged institutions. Positions must be cut: investors sell high-beta currencies and buy back JPY/CHF/USD. These synchronized outflows generate the characteristic risk-off pattern and explain the majority of short-horizon drawdowns and reversals in spot FX.

Commodity and terms-of-trade shocks contribute 20–40% of variation for commodity-linked currencies. Exporters of oil and metals (CAD, NOK, RUB, AUD, CLP, ZAR) appreciate when export prices rise because external balances improve; major importers (JPY, KRW, INR) weaken when energy costs rise. The mechanism is direct: higher commodity revenues increase foreign-currency inflows to exporters, raising demand for the local currency, while importers face larger outflows that weaken theirs. These shocks produce persistent, medium-horizon spot adjustments independent of the volatility cycle.

Monetary-policy surprises explain 10–25% of FX variation, mainly around central-bank events. Spot currencies respond to changes in expected future interest rates—hawkish repricing strengthens a currency, dovish repricing weakens it. The static rate differential itself explains little of spot because it is already priced into the forward rate under covered interest parity. The forward reflects today’s rate gap; spot moves only when the expected future gap changes.

Large, one-sided institutional flows add episodic but sometimes dominant variation. Japanese life insurers adjusting currency hedges on foreign bond portfolios require massive forward purchases that mechanically drive USD/JPY spot higher. Reserve-manager rebalancing shifts entire EM baskets. Equity and bond flows materially influence KRW, TWD, SEK, NOK, AUD, and MXN. These flows can override risk and policy forces in the short run because they are large relative to market depth and because the institutions executing them face balance-sheet or regulatory constraints that make them price-insensitive.

Momentum amplifies all of these forces at multi-month horizons, adding 5–15% to total variation. Because macro information diffuses slowly and position adjustments are staggered, FX trends persist, extending both risk-on rallies and risk-off selloffs beyond what fundamentals alone would generate. This slow adjustment means past returns predict future returns at horizons of three to twelve months, creating the most reliable cross-asset anomaly in systematic investing.