By Harry Prabandham · Updated September 28, 2026
Investing & Markets
Correlation and Diversification: How Portfolios Spread Risk
Correlation measures how two assets move together, from +1 to -1. Combining assets that aren't perfectly correlated can lower portfolio volatility.
Correlation and Diversification: How Portfolios Spread Risk
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What is correlation, and why does it matter for diversification?
- Correlation runs from +1 (moving together) to -1 (moving opposite); zero means no consistent linear relationship.
- Combining assets with correlation below +1 makes portfolio volatility lower than the weighted average of the individual volatilities.
- Diversification reduces company-specific risk, but not the market-wide risk that affects nearly all stocks.
- It lowers the chance that one holding sinks the portfolio; it does not prevent losses.
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Full Guide
What is correlation, and why does it matter for diversification?
- Correlation runs from +1 (moving together) to -1 (moving opposite); zero means no consistent linear relationship.
- Combining assets with correlation below +1 makes portfolio volatility lower than the weighted average of the individual volatilities.
- Diversification reduces company-specific risk, but not the market-wide risk that affects nearly all stocks.
- It lowers the chance that one holding sinks the portfolio; it does not prevent losses.
How does modern portfolio theory use correlation?
- Harry Markowitz's 1952 work showed that portfolio risk depends on how holdings move together, not just on each holding's volatility.
- The efficient frontier is the set of portfolios with the highest expected return for each level of risk, given the assumed inputs.
- Results depend on estimates of returns, volatilities and correlations, which are uncertain and change over time.
- Rubric's portfolio construction uses quantitative models grounded in modern portfolio theory, with judgment applied to their inputs.
Why do correlations rise in a crisis?
- Correlations among risky assets tend to rise in sharp sell-offs, as in 2008, when many asset classes fell together.
- In 2022, stocks and high-quality bonds fell together as inflation and interest rates rose, showing the stock-bond relationship isn't fixed.
- Diversification measured in calm periods can overstate how much it helps in stress.
- High-quality government bonds and cash have tended to hold up better than riskier assets in many, but not all, sell-offs.
How does a concentrated stock position affect diversification?
- A single stock carries company-specific risk that diversification is meant to reduce, and it typically swings more than a broad index.
- Holding employer stock alongside salary and unvested equity ties both income and savings to one company.
- Adding stocks from the same sector, or with the same economic drivers, diversifies less than the number of holdings suggests.
- Reducing a concentrated position can have a tax cost, so pacing and tax planning are part of the decision.
Measuring and monitoring correlation
- Rolling windows, such as 36 months of returns, show how relationships shift over time.
- Stress tests against historical episodes can reveal common exposures that averages hide.
- Looking at underlying drivers, such as growth, inflation and interest rates, is more informative than asset class labels.
- Periodic rebalancing restores target weights as returns diverge.
Key takeaways
- Correlation below +1 is what allows diversification to lower portfolio volatility.
- Correlations shift, and they often rise when diversification is needed most.
- Concentration in one company is the most common gap in diversification for tech employees.
- Diversification lowers single-holding risk but cannot remove market risk or prevent losses.
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