Correlation

Correlation measures the degree to which two assets' returns move together over time, expressed as a number between -1 and +1. A correlation of +1 means two assets move perfectly in the same direction at the same time; a correlation of -1 means they move perfectly in opposite directions; a correlation near 0 means their movements are essentially unrelated.

Correlation is the mathematical backbone of diversification and of asset allocation itself. Combining two assets that are highly correlated with each other — like SPY and VOO, which both track the S&P 500 — provides very little diversification benefit, since they rise and fall together. Combining assets with low or negative correlation, such as stocks and gold during certain periods, can meaningfully reduce a portfolio's overall volatility, because when one asset is falling the other is more likely to be flat or rising, smoothing the combined ride.

A crucial caveat: correlations are not fixed. They are calculated over a historical window and can shift, sometimes sharply, especially during systemic market stress when many normally uncorrelated assets fall together — as stocks and bonds did in 2022. Relationships that look reliably diversifying in calm markets can temporarily break down in a crisis, which is exactly when investors need them most.

The idea of building a portfolio from assets chosen for their low mutual correlation is taken to its logical conclusion in risk parity strategies like the All Weather Portfolio. This site's correlation matrix tool shows historical correlations across major asset classes so you can see which combinations have provided the most genuine diversification over time.