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. 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 tend to rise and fall together. Combining two 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 out the combined ride.

A crucial caveat: correlations are not fixed. They are calculated over a historical period and can shift, sometimes sharply, especially during systemic market stress when many assets that are normally uncorrelated start falling together (as happened with stocks and bonds in 2022). 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.