Correlation and diversification: the risk you don't see
You think you're diversified because you have three trades open in different pairs. But if those pairs move together, you actually have one bet at triple the size. Understanding correlation is understanding the risk hiding in plain sight.
What correlation is
Correlation measures how much two assets move at the same time. It runs from +1 (they move identically) to −1 (they mirror each other), through 0 (independent). In forex it's key because many pairs share a currency: EUR/USD and GBP/USD rise and fall almost in unison (high positive correlation), while EUR/USD and USD/CHF tend to move in opposite directions (negative correlation).
Notice: the two lines rise and fall almost the same. Buying both pairs doesn't spread the risk — it doubles it on the same idea (here, "the dollar falls").
The illusion of diversification
The most expensive mistake: opening EUR/USD, GBP/USD and AUD/USD "to diversify". Since all three are "something against the dollar", if the dollar strengthens suddenly, all three lose at once — your real risk is 3× what you thought. Real diversification means taking positions that don't depend on the same driver. And beware: in crises, correlations that looked low snap to +1 ("everything falls together"), exactly when it hurts most.
Typical correlations (indicative)
| Pair | Usual relationship | Why |
|---|---|---|
| EUR/USD ↔ GBP/USD | high positive | Both "euro/pound against dollar"; they share the USD side. |
| EUR/USD ↔ USD/CHF | high negative | USD is on opposite sides; the franc tracks the euro. |
| AUD/USD ↔ Gold | positive | Australia exports commodities; the "aussie" follows risk/commodities. |
| USD/CAD ↔ Oil | negative | Canada exports crude; higher oil tends to strengthen the CAD (USD/CAD falls). |
These are tendencies, not laws: correlations change over time and market regime. Check them, don't assume them.
What to do about it
- Count your risk by "theme", not by trade. Three correlated pairs = one large position. Add them up when working out how much you really risk with your position size.
- Avoid stacking the same bet. If you're already long EUR/USD, opening GBP/USD in the same direction doesn't diversify: it concentrates.
- Beware "free" hedges. Buying EUR/USD and USD/CHF at once (negatively correlated) nearly cancels out — you pay two spreads to risk almost nothing.