A summary of John Edwards' article on market correlation, portfolio diversification, and volatility.
What is Correlation?
Correlation is a statistical measure—ranging from -1 (perfect negative) to +1 (perfect positive)—that dictates how assets or broader markets move in relation to one another. Perfect correlations are rare, but understanding these relationships is a fundamental piece of risk management.
Market and Portfolio Implications
Correlation gives investors perspective on the broader market dynamics:
High Correlation Environments: When assets are highly correlated (e.g., the S&P 500 sectors exhibiting a 95% correlation in 2011), stocks move in lockstep. This makes stock-picking difficult and diminishes the benefits of holding a variety of stocks. In these environments, simple index funds often suffice.
Modern Portfolio Theory (MPT): In portfolio management, MPT uses correlation to determine the efficient frontier, optimizing expected returns against risk. Combining assets with low correlation structurally reduces overall portfolio risk.
The Catch: Correlation is not stable; assets that were previously uncorrelated can suddenly move together, exposing a key shortcoming of MPT which assumes stable historical correlations.
The Impact of Volatility
During periods of market stress and heightened volatility (such as the 2008 financial crisis or the 2016 oil price drop), correlations have a dangerous tendency to converge toward 1.
Cross-Sector Contagion: Assets and international markets that are usually uncorrelated often become highly correlated during crises.
Diminished Hedges: This implies that diversification often fails when investors need it the most, highlighting the importance of seeking truly uncorrelated assets to manage downside risk.
Actionable Takeaways for Diversification
To build a resilient portfolio, investors must deliberately seek assets with low correlation to traditional equities:
Bonds: The classic diversifier, as they historically maintain a lower correlation with stocks.
Commodities: Precious metals like gold and silver often serve as a hedge against equity market declines.
Frontier Markets: Investing in frontier markets—economies less developed than emerging markets—can provide a strong counterbalance to U.S. equities. For instance, frontier market ETFs have historically demonstrated low correlation (e.g., ~0.54) with the S&P 500.