A common misconception
It's a widely repeated idea that diversification simply means owning many different stocks. But owning 50 different companies that all belong to the same industry, region, or economic cycle isn't necessarily more diversified than owning five companies spread across unrelated sectors. The number of holdings matters less than how those holdings behave relative to one another.
The concept of correlation
Correlation describes how closely two investments move in relation to each other. When two assets are highly correlated, they tend to rise and fall together. When they're uncorrelated or negatively correlated, one may hold steady or rise while the other falls. Diversification works best when a portfolio combines assets that don't all react the same way to the same event.
For example, a portfolio holding only technology company stocks may look diversified because it includes dozens of companies, but if a single industry-wide event affects technology spending broadly, many of those holdings could decline together.
Layers of diversification
Diversification can be thought of across several layers, not just one.
1. Across asset classes
Stocks, bonds, real estate, and cash each tend to respond differently to the same economic conditions. Bonds, for instance, have historically behaved differently from stocks during certain periods of market stress, though this relationship isn't fixed or guaranteed.
2. Across sectors and industries
Within stocks alone, sectors like energy, health care, financials, and technology don't always move in tandem. A portfolio concentrated in one sector carries risk tied to that sector's specific challenges, regardless of how many companies are held within it.
3. Across geographies
Domestic and international markets don't move in lockstep. Economic policy, currency movements, and regional events can affect one market more than another at a given time.
4. Across company size
Large, established companies and smaller, growth-stage companies often behave differently across market cycles, with smaller companies historically showing more volatility in both directions.
5. Across time
Investing a lump sum at a single point in time carries different risk than spreading contributions across months or years — a concept distinct from asset diversification but related to overall portfolio risk management.
A simplified illustration
The table below is a simplified, illustrative example of how three hypothetical portfolios might be constructed — not a recommendation, and not representative of any actual fund or performance figures.
| Portfolio | Holdings count | Diversification across asset classes/sectors |
|---|---|---|
| A | 60 stocks, one sector | Low — concentrated in a single industry |
| B | 15 stocks, one country | Moderate — spread across several sectors |
| C | Broad stock fund + bond fund + international fund | Higher — spread across asset classes and geographies |
Portfolio A has the most individual holdings but arguably the least diversification in the broader sense, since a downturn affecting its one sector could affect most of its holdings simultaneously.
What diversification does and doesn't do
Diversification is generally understood as a way to reduce the impact of any single investment, sector, or event on an overall portfolio. It does not eliminate risk altogether — a broad, diversified portfolio can still lose value during a widespread market downturn, since some level of correlation across nearly all asset classes tends to increase during periods of severe financial stress.
Over-diversification is also possible
There's also a point at which adding more holdings does little to reduce risk further, while potentially making a portfolio harder to track and adding complexity. A handful of broad, low-cost funds covering different asset classes can sometimes achieve similar diversification benefits to holding hundreds of individual securities.
Rebalancing keeps diversification intact
Because different assets grow at different rates, a portfolio's original mix can drift over time. A portfolio that started as 60% stocks and 40% bonds might, after a strong period for stocks, become 75% stocks and 25% bonds — a materially different risk profile than originally intended. Periodically rebalancing back toward a target mix is one way investors maintain the diversification they originally set out to have.
Key takeaways
- Owning many individual stocks doesn't guarantee diversification if those holdings are highly correlated.
- Diversification spans multiple layers: asset class, sector, geography, company size, and time.
- A diversified portfolio can still lose value in a broad market downturn; diversification reduces certain risks, not all of them.
- Adding holdings beyond a certain point may add complexity without meaningfully reducing risk.
- Rebalancing helps a portfolio maintain its intended level of diversification as markets move.
