Updated October 2026 · Mike Shell, Chief Investment Officer

Correlation measures how closely two investments move together, from +1 (in step) to −1 (opposite). It describes direction, not size, and it changes: in a selloff, correlations tend to rise, just when diversification is needed most.

Correlation is measured in calm markets and tested in crashes.

Why it matters. Every diversified portfolio rests on correlation estimates, usually taken from years when markets were calm. The estimates don’t hold still. When investors sell in a panic, they sell what they can, and markets that moved independently start falling together. Something can look different on a statement and still depend on the same thing as everything else.

The math. Correlation is the covariance of two return series divided by their two standard deviations multiplied together. Two points get missed. A correlation of 0.9 says two assets moved in the same direction, not by the same amount: one can fall 10% while the other falls 50%. And a correlation near zero doesn’t mean one gains when the other loses. From October 9, 2007, to March 9, 2009, with dividends reinvested, U.S. REITs fell 69%, emerging-market stocks 63%, international stocks 60%, the S&P 500 55%, high-yield bonds 33% and commodities 27%. Only high-grade bonds rose, about 7%.

How it’s applied in ASYMMETRY® Managed Portfolios. Different rules can still depend on the same market, so the combination is judged, especially when everything is under stress at once. Correlation is checked in the worst periods, not only on average. And the predefined risk across every position is added up, because holdings that look different can fall together.

Common misconceptions. Low correlation isn’t a hedge. A hedge is built to gain when the position falls; an uncorrelated asset can fall at the same time. A ten-year average hides the months that matter. And private assets priced by appraisal look less correlated with stocks than they are, because their prices move slowly.

Related terms. Diversification: what correlation is supposed to deliver. Asymmetric correlation: why correlations rise in declines. When diversification fails, tail risk hedging and systemic risk follow in the lexicon. Part of the ASYMMETRY® guide to asymmetric investing.

Mike Shell is the Founder and Chief Investment Officer of Shell Capital Management, LLC, and portfolio manager of ASYMMETRY® Managed Portfolios, with nearly three decades of live-market experience across multiple full market cycles.