Glossary term
Correlation
Correlation is a measure of how closely two investments move together, helping investors judge how much diversification a combination may provide.
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Written by: Editorial Team
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What Is Correlation?
Correlation is a measure of how closely two investments or return series move together. In portfolio management, it helps investors judge whether two holdings tend to rise and fall in similar ways, in opposite ways, or with little consistent relationship at all. That makes correlation one of the most useful concepts behind diversification, because the benefit of combining assets depends partly on how similarly those assets behave.
Owning different labels is not the same thing as owning truly different exposures. Two funds can have different names and still move almost in lockstep. On the other hand, two assets that respond differently to economic conditions may help reduce portfolio volatility when held together. Correlation helps investors think beyond labels and focus on behavior.
Key Takeaways
- Correlation measures how closely two investments move together.
- It is usually expressed on a scale from -1 to 1.
- Lower correlation can improve diversification because portfolio components are less likely to move in the same way at the same time.
- Correlation helps inform asset allocation and portfolio construction.
- Correlation changes over time, especially during stressed markets.
How Correlation Works
At a high level, correlation describes the direction and closeness of co-movement between two return streams. If two assets usually rise and fall together, their correlation is positive. If they tend to move in opposite directions, correlation is negative. If there is no strong consistent relationship, correlation is near zero.
In finance, the idea is usually summarized with the correlation coefficient, which ranges from -1 to 1. Investors do not need to calculate the statistic by hand to benefit from the concept, but it helps to understand what the number is trying to describe.
Correlation level | What it generally suggests |
|---|---|
1 | Two return series move together almost perfectly |
Between 0 and 1 | They usually move in the same direction, but not perfectly |
0 | There is little consistent linear relationship |
Between -1 and 0 | They often move in opposite directions |
-1 | They move in opposite directions almost perfectly |
How Correlation Shapes Portfolio Diversification
Correlation shapes portfolio diversification because portfolio risk depends on how the pieces interact, not just on how risky each piece is alone. Two volatile assets can still provide diversification benefit if they do not move together all the time. Two calm-looking assets may provide less diversification than expected if they respond to the same economic driver.
Correlation sits near the center of portfolio design. It helps explain why mixing multiple asset classes can reduce overall volatility, and why a portfolio that appears diverse on paper can still carry hidden overlap if its major holdings are highly correlated.
Correlation Versus Diversification
Correlation and diversification are closely linked, but they are not the same term. Correlation is the measurement idea. Diversification is the portfolio result investors are trying to achieve. Lower correlation can improve diversification, but it does not guarantee a perfectly diversified portfolio by itself.
For example, two holdings can have moderate correlation and still create concentration risk if they are both tied to the same sector or style. Correlation is therefore one tool for evaluating a portfolio, not the only one.
How Low Correlation Supports Diversification
When portfolio components do not all move together, a loss in one area may be partly offset by stability or gains elsewhere. That can smooth the overall portfolio path and make the investor less dependent on a single economic outcome. This is one reason portfolios often combine stocks, bonds, and cash rather than relying entirely on one category.
That said, low correlation is not the same thing as better return. Sometimes a lower-correlation asset lowers expected returns as well as volatility. The goal is usually not to maximize difference for its own sake, but to create a more balanced relationship between risk and return.
How Correlation Limits Show Up in Real Markets
Correlation is useful, but it has important limits. It is based on historical relationships, and those relationships can change. Two assets that behaved differently for years may move much more closely together during a crisis. That is one reason diversification can feel weaker precisely when investors need it most.
Correlation also does not imply causation. If two assets move together, that does not mean one is causing the other's performance. It may simply mean both are reacting to the same broader force, such as growth expectations, interest rates, or risk appetite.
How Investors Use Correlation in Practice
Investors use correlation when evaluating whether a new holding improves the portfolio or merely adds more of the same exposure. It can influence fund selection, rebalancing decisions, and how aggressively a portfolio should lean into one theme or asset class. Professional portfolio managers often track correlations in greater detail, but the practical lesson for households is simpler: different-looking investments are not always different enough to improve the portfolio.
This is also why broad asset allocation decisions matter so much. A portfolio built from multiple asset classes usually has a better chance of mixing different return patterns than a portfolio built from one narrow area of the market.
The Bottom Line
Correlation is a measure of how closely two investments move together. Lower correlation can improve diversification and help investors build portfolios that are less dependent on one source of risk, even though correlation is not fixed and should never be treated as a permanent guarantee.