Position sizing controls how much one trade can hurt you. Correlation risk is the loophole: five perfectly-sized trades that are secretly the same trade, presenting one five-sized loss on the night the market moves. It is the most common way disciplined-looking accounts take undisciplined-sized hits.
How the stack builds
Correlated exposure accumulates innocently. Each position passes the per-trade risk check; nobody checks what they sum to. The classic forex book: long EUR/USD, long GBP/USD, short USD/JPY, long gold — four tickets, four "1% risks," and one theme: short the US dollar. A hawkish Fed surprise moves all four against you in the same minute, and the account takes a ~4% hit from what the journal records as a single unlucky evening.
The stack has three common sources. Shared legs — pairs containing the same currency move together by arithmetic. Shared drivers — AUD, NZD, CAD and copper all lean on global growth; oil and CAD; risk appetite and every yen cross. And shared strategy — five breakout trades on five unrelated pairs still share one factor: whether markets are trending or chopping this week. Strategy correlation is the sneakiest, because no correlation matrix shows it.
The crisis multiplier
The property that makes this risk dangerous rather than merely untidy: correlations are highest exactly when it matters most. In calm markets, pairs wander semi-independently and diversification looks real. In a stress event, one factor — fear — takes over every screen, correlations lurch toward one, and positions that spent months uncorrelated fall in unison. Diversification measured in good times is partly an illusion; the honest question is never "are these correlated on average?" but "will these be correlated on the worst day?" For anything sharing a funding currency, a commodity, or a risk-appetite link, the worst-day answer is usually yes.
Auditing your book
Three checks, none requiring software. The theme test: describe every open position in factor terms — short dollar, long risk, long oil — and sum risk per theme; if one theme carries more than your single-trade limit ×2, you are concentrated, whatever the ticket count says. The shock test: ask what the account does tonight if the dollar gaps 1%, equities drop 3%, or oil moves 5%; if one scenario hits everything, that scenario is your real position. The funding test: list how many positions share each currency — four positions with a yen leg is a yen position with extra steps.
Managing it
Cap theme risk, not just trade risk. A common structure: 1% per trade, but no more than 2–3% across all positions sharing a driver, counted honestly (gold counts toward short-dollar; yen crosses count toward risk-on).
Pick the best expression instead of all of them. If EUR/USD, GBP/USD and AUD/USD all signal the same dollar view, take the cleanest chart at full theme size rather than three at "diversified" size that isn't.
Beware offsetting positions that aren't hedges. Long EUR/USD plus short GBP/USD nets out to long EUR/GBP — a new trade, not a reduction. Netting only reduces risk when you actually computed the residual.
Re-check after regime changes. A correlation audit from a calm quarter is stale the week volatility returns. The five minutes after a central-bank surprise is precisely when yesterday's matrix is wrong.
The discipline compresses to one sentence: your position is what moves together, not what is listed separately. Count it that way and correlation risk becomes a sizing input like any other; ignore it and the market will eventually do the summation for you, on its choice of night.