Diversification: What It Does, What It Does Not Do, and When It Can Break Down

The most oversold promise in investing

"Do not put all your eggs in one basket." It is the oldest advice in investing. And it is correct, as far as it goes. The problem is that most investors believe diversification protects them far more than it actually does.

During the 2008 financial crisis, the S&P 500 fell 37%. International developed stocks (MSCI EAFE) fell 43%. Emerging market stocks fell 53%. REITs fell 37%. High-yield bonds fell 26%. Almost everything fell at the same time. The investor who thought they were diversified across stocks, international markets, real estate, and corporate bonds lost money in every category simultaneously [Morningstar, 2008 Annual Returns by Asset Class].

Diversification is valuable. But understanding what it can and cannot do is essential, because overestimating its protection can lead to nasty surprises at the worst possible time.

What Diversification Actually Does

Diversification reduces the impact of any single investment failing. If you hold 500 stocks and one goes bankrupt, you lose 0.2% of your portfolio. If you hold 5 stocks and one goes bankrupt, you lose 20%.

This is the core benefit: spreading exposure so that no single bad outcome can devastate your total wealth. Owning a broad index fund like the S&P 500 instead of a handful of individual stocks is the simplest and most effective form of diversification.

Within an asset class

Holding 500 stocks instead of 5 dramatically reduces company-specific risk (also called unsystematic risk or idiosyncratic risk). This type of risk can be almost entirely eliminated through broad diversification. Research shows that most of the diversification benefit within equities is captured with approximately 30-50 stocks, and virtually all of it with a broad index [Statman, "How Many Stocks Make a Diversified Portfolio?" Journal of Financial and Quantitative Analysis, 1987].

Across asset classes

Holding stocks AND bonds provides a different kind of diversification. Because stocks and bonds often (though not always) move in different directions, combining them reduces overall portfolio volatility. This is the basis of the classic 60/40 portfolio.

Across geographies

U.S., international developed, and emerging market stocks do not move in perfect lockstep. Adding international exposure can reduce volatility that is specific to any single country's economy or market.

What Diversification Does Not Do

It does not protect against systemic risk

Systemic risk is the risk that the entire financial system declines simultaneously. During a major crisis, nearly all risky assets fall together. The 2008 financial crisis and the 2020 COVID crash both demonstrated this: stocks, international stocks, real estate, and corporate bonds all declined in the same periods. (Notably, in 2022, broad commodity exposure was one of the few asset classes that held up while stocks and bonds both fell, illustrating that systemic selloffs do not always hit every category equally.)

Diversification across risky assets provides protection against idiosyncratic events (one company failing, one sector struggling). It provides limited protection against broad market declines where fear drives selling across the board.

It does not eliminate losses

A diversified portfolio will still lose money during market downturns. A 60/40 stock/bond portfolio lost approximately 26% during the 2008 crisis. The loss was smaller than a 100% stock portfolio (-37%), but it was still a significant decline. Diversification reduces the magnitude of losses. It does not prevent them.

It does not guarantee higher returns

Diversification is a risk management tool, not a return enhancement tool. In years when U.S. large-cap stocks dominate (as they have for much of 2010-2023), a diversified portfolio that includes international stocks and bonds will underperform a concentrated U.S. stock portfolio. The diversified investor accepts this underperformance in good times as the price of protection in bad times.

Correlation: The Number That Explains Everything

The effectiveness of diversification depends entirely on correlation: how much two investments move together.

Correlation of 1.0: Two investments move in perfect lockstep. Owning both provides zero diversification benefit. They rise and fall by the same amount at the same time.

Correlation of 0.0: Two investments are completely unrelated. Owning both provides substantial diversification benefit, as gains in one can offset losses in the other.

Correlation of -1.0: Two investments move in exactly opposite directions. This is perfect diversification, though it rarely exists in practice outside of hedging instruments.

Here is the critical problem: correlations are not stable. They change over time, and they tend to increase during exactly the periods when you need diversification most.

Correlation during normal markets vs. crisis markets

Consider U.S. stocks and international developed stocks:

• During normal markets (2012-2019): correlation approximately 0.7
• During the 2008 crisis: correlation spiked above 0.9
• During the 2020 COVID crash: correlation spiked above 0.9

[Based on MSCI index return data; exact figures vary by calculation window and methodology]

During calm periods, the correlation of 0.7 provides meaningful diversification. During crises, the correlation spikes toward 1.0, meaning both asset classes fall together, and diversification provides far less protection than the historical average suggested.

This is sometimes called "correlation breakdown" or "diversification failure," though it is more accurate to say that correlations increase during stress. The diversification is not broken. It is working as it always does in crises: less effectively than in normal times.

The Diversification Myth: What the Data Shows

The industry narrative around diversification often oversells it. "Own a diversified portfolio and you will be protected" is an incomplete statement. Here is a more accurate one:

Diversification reliably protects against company-specific and sector-specific risks. It provides moderate protection against broad market declines. It provides limited protection against systemic crises where correlations spike.

The most effective diversification comes from combining asset classes with genuinely different risk drivers:

• Stocks + high-quality bonds (bonds often rise when stocks fall during recessions)
• Stocks + Treasury bills (cash does not decline)
• U.S. stocks + commodities (different economic drivers)

The least effective diversification comes from combining assets that appear different but share the same underlying risk:

• U.S. large-cap stocks + U.S. small-cap stocks (both are U.S. equities)
• U.S. stocks + international stocks during a global crisis (both are equities)
• Corporate bonds + stocks during a credit crisis (both are exposed to corporate default risk)

How to Think About Diversification Honestly

Layer 1: Within equities (high value, low cost)

Owning a broad index fund instead of a handful of individual stocks eliminates company-specific risk almost entirely. This is the single most effective diversification decision, and it costs nothing (index funds are typically the cheapest option).

Layer 2: Across asset classes (moderate value, requires tradeoffs)

Adding bonds to an all-stock portfolio reduces volatility, especially during equity bear markets. The cost is lower expected returns in good years. The benefit is a smoother ride and better sleep during bad years.

Layer 3: Across geographies (some value, long-term)

International diversification has merit over very long periods but has underperformed U.S.-only portfolios for the past decade. The value is insurance against a scenario where U.S. markets do not continue to outperform the rest of the world, which has happened before and will happen again.

Layer 4: Across time (dollar-cost averaging)

Investing a fixed amount regularly, regardless of market conditions, is a form of diversification across time. It ensures you are not making a single large bet at a market peak.

Common Mistakes

Mistake 1: Believing diversification eliminates risk

It does not. It reduces specific types of risk and moderates the magnitude of losses. A well-diversified portfolio can still drop 20-30% in a severe bear market. If that level of decline would cause you to panic and sell, diversification alone is not enough. Your allocation and your expectations need to match your actual behavior.

Mistake 2: Over-diversifying within the same asset class

Owning six U.S. large-cap mutual funds is not diversification. It is redundancy. The holdings overlap significantly, the fees multiply, and the portfolio complexity increases without meaningful risk reduction. One broad index fund provides more diversification at lower cost.

Mistake 3: Assuming historical correlations will hold in a crisis

If your diversification strategy is based on backtested correlations during normal markets, it will underperform your expectations during the next crisis. Always stress-test your portfolio against crisis-period correlations, not average correlations.

Mistake 4: Diversifying away from concentrated positions too slowly

Employees with large positions in their employer's stock face enormous concentration risk. Enron employees who held company stock in their 401(k) lost both their jobs and their retirement savings simultaneously. Diversifying out of concentrated positions is one of the most important risk management actions an investor can take, even if it means paying taxes on the gains.

What This Means for You

Diversification is a valuable tool. It is not a magic shield. It will protect you from the risk of any single investment blowing up, it will moderate your losses during broad market declines, and it will reduce the overall volatility of your portfolio.

It will not protect you from systemic market crashes. It will not prevent losses. And it can become less effective precisely when you need it most, because correlations increase during periods of market stress.

The honest approach is to diversify broadly (eliminating needless concentrated risk), understand the limitations (you will still lose money in downturns), and plan accordingly (maintain enough bonds or cash to avoid being forced to sell stocks at the bottom).

Key Takeaways

• Diversification reliably eliminates company-specific risk. Owning a broad index fund instead of a handful of stocks is the single most effective and cheapest diversification decision.
• Diversification across asset classes (stocks, bonds, cash) reduces overall portfolio volatility but does not eliminate losses during broad market declines.
• Correlations between assets increase during market crises. This means diversification provides less protection during the periods when you need it most.
• The industry often oversells diversification. It is a risk reduction tool, not risk elimination. Understanding its limitations is as important as understanding its benefits.
• Stress-test your portfolio against crisis-period correlations, not average correlations, to get a realistic picture of how it might behave in a downturn.

Try the Correlation Calculator to see how this applies to your situation.

MyAvere provides tools and education, not investment advice. Always consult a qualified financial professional for personalized guidance.

References

1. Statman, Meir. "How Many Stocks Make a Diversified Portfolio?" Journal of Financial and Quantitative Analysis, September 1987. https://www.jstor.org/stable/2330969
2. Morningstar. Annual Returns by Asset Class, 2008 and historical data. https://www.morningstar.com/
3. MSCI. Global equity index data and methodology. https://www.msci.com/indexes
4. Federal Reserve Bank of St. Louis. FRED data on asset class returns during crisis periods. https://fred.stlouisfed.org/
5. Vanguard Group. "Global Equity Investing: The Benefits of Diversification and Sizing Your Allocation." Research publication, 2023. https://corporate.vanguard.com/content/corporatesite/us/en/corp/articles/global-equity-investing-benefits-diversification.html