Diversification is supposed to reduce risk.
But during severe market selloffs, something uncomfortable can happen:
assets that normally move differently can suddenly start falling together.
This is known as correlation convergence.
It helps explain why a portfolio that looks diversified in normal markets can experience much larger losses during a crisis.
Educational research only. This article is not investment advice.
What Is Correlation?
Correlation measures how two assets tend to move relative to each other.
It ranges from -1 to +1.
+1 correlation
The assets tend to move in the same direction.
0 correlation
Their movements have little consistent relationship.
-1 correlation
They tend to move in opposite directions.
Diversification works best when a portfolio contains assets that do not all respond to the same forces in the same way.
Why Correlations Can Rise During Crashes
During normal conditions, investors distinguish between:
- sectors;
- countries;
- industries;
- growth prospects;
- individual companies.
During a crisis, one factor can suddenly dominate everything:
the need to reduce risk.
Investors may sell many assets at once.
That can cause previously different investments to start moving together.
Forced Selling Makes It Worse
Leverage can amplify this effect.
If prices fall sharply, leveraged investors may face margin calls.
They may then sell whatever assets they can—not necessarily the assets they want to sell.
The chain can become:
Prices fall
↓
Losses increase
↓
Margin calls appear
↓
Investors sell more assets
↓
Correlations rise
This can spread stress across markets.
Liquidity Can Disappear
Diversification also depends on markets functioning normally.
During severe stress, buyers can become harder to find.
Investors seeking cash may sell their most liquid holdings first.
As selling spreads, different assets can decline together.
This means the problem is not always that the investments suddenly became economically identical.
Sometimes they are simply reacting to the same liquidity shock.
Why Diversification Can Disappoint
Imagine a portfolio containing:
- technology stocks;
- banks;
- small caps;
- international equities;
- crypto.
On paper, that may look diversified.
But all of those positions still carry significant exposure to risk appetite.
During a major market shock, they may all fall simultaneously.
The portfolio contained many assets.
But it may not have contained many different sources of risk.
That distinction matters.
Asset Diversification vs Risk Diversification
Owning more investments does not automatically create better diversification.
For example:
10 technology stocks
may still represent one broad technology risk.
Stocks across several countries
may still share global equity risk.
Stocks plus crypto
may still be heavily exposed to liquidity and investor risk appetite.
True diversification asks:
What makes these assets behave differently?
Not simply:
How many assets do I own?
Why Historical Correlation Can Mislead
Correlation is not fixed.
Two assets may show low correlation during normal periods but become strongly correlated during stress.
That means historical averages can hide important behavior.
Suppose two assets usually have low correlation.
During a crisis, both suddenly fall 20%.
Their long-run correlation may still look reasonable.
But the period when diversification mattered most was exactly when the relationship changed.
This is why risk analysis should consider stress conditions, not only averages.
How Risk Simulation Helps
TradingSimuLab’s Risk Simulation looks beyond one expected outcome.
Important outputs include:
VaR
Where does severe downside begin?
CVaR
How large are losses once the portfolio moves beyond that severe-loss threshold?
Max Drawdown
How far could the investment fall from peak to trough?
Probability of Gain
How often do simulated paths finish above their starting level?
Terminal Price Range
How wide is the range of possible ending outcomes?
These measures help reveal something that simple diversification claims can miss:
the path matters.
A portfolio may have an attractive average outcome while still carrying significant downside during stressful scenarios.
Does Diversification Stop Working?
Not necessarily.
Diversification can still reduce risk.
But its protection can become weaker precisely when markets are under the most pressure.
The solution is not to abandon diversification.
It is to understand that:
correlations change,
liquidity changes,
and:
risk regimes change.
Different assets should therefore be evaluated by how they behave under stress—not only during calm markets.
A Simple Diversification Checklist
Ask:
Do my assets depend on the same economic conditions?
Would they all suffer if interest rates rise?
Would they all fall if liquidity disappears?
How did their relationships change during previous selloffs?
What happens to portfolio drawdown if correlations increase?
Those questions reveal more than simply counting how many investments are in a portfolio.
Final Takeaway
Diversification works because different assets behave differently.
But during market crashes, fear, leverage and liquidity pressure can push correlations higher.
The risk chain is:
Market shock → forced selling → liquidity pressure → rising correlations → weaker diversification
That does not make diversification useless.
It means diversification should be tested under stress, not judged only during normal markets.
The better question is not:
“How many assets do I own?”
It is:
“How differently will those assets behave when markets are under pressure?”
For more market risk tools, Monte Carlo simulations and downside analysis, sign up to TradingSimuLab and explore the platform.