Correlation risk is the danger that positions you think are separate actually move together — so what looks like a diversified book is really one concentrated bet in disguise. It's one of the most underestimated risks in trading, because it hides until the moment everything moves at once.
The illusion of diversification
Imagine you're long five different tech stocks. It feels like five positions, but they're highly correlated — when the sector sells off, they all fall together. You don't have five independent bets; you have one big sector bet, sized five times larger than you realized. The diversification was cosmetic. On index-correlated instruments, this is especially acute — most stocks move with the market.
Why it wrecks accounts
Correlated positions destroy the math of position sizing. You size each trade to risk a small amount, assuming they're independent — but if they all move together, a single market event hits every position simultaneously, and your total risk is far larger than any single position implied. Correlations also spike toward 1 in a crisis: "everything goes down together" exactly when you needed diversification most.
In a crisis, correlations converge to one. The diversification you counted on vanishes precisely when you need it.
Seeing your true exposure
Managing correlation risk means measuring your net exposure to a common factor — a sector, the broad market, a single theme — not just counting positions. A book of correlated longs is one directional bet and must be sized as such. This is why a disciplined process thinks in terms of total, factor-aware risk and ruin avoidance, not the comforting but false sense of safety from simply holding "many" positions.