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Risk of an Individual Stock vs. an Index: Why Diversification Is Not Just Advice — It Is Mathematics
Key takeaways
- The risk of an individual stock consists of unsystematic (company-specific) and systematic (market-wide) components.
- Unsystematic risk disappears through diversification — 20–30 stocks from different sectors suffice.
- Systematic risk (a decline of the whole market) cannot be eliminated by diversification.
- An index automatically eliminates unsystematic risk by holding hundreds or thousands of stocks.
- Adding one more stock helps very little — adding a hundred changes everything.
Every individual stock carries two layers of risk: unsystematic (specific to that company) and systematic (the movement of the whole market) — and only unsystematic risk can be eliminated by diversification.
Two layers of risk
Unsystematic risk arises from company-specific events: bad earnings, a lawsuit, a CEO departure, a product failure. This risk is independent of the rest of the market — it can strike one company while the index rises. Systematic risk is the movement of the whole market — recession, an inflation shock, geopolitics. It hits all stocks simultaneously and cannot be removed by diversification.
The mathematics of diversification
Research (Statman, 1987, and others) shows that a portfolio of 20–30 stocks from different sectors eliminates roughly 90% of unsystematic risk. Beyond 30–40 holdings, the marginal benefits are minimal. An index with 500 or 1,600 constituents carries virtually zero unsystematic risk.
- 1 stock: full unsystematic risk, full systematic risk.
- 10 stocks: unsystematic risk reduced by ~60%.
- 30 stocks from different sectors: unsystematic risk almost fully eliminated.
- Index ETF (500+ holdings): unsystematic risk practically nil.
Practical impact for the individual investor
If you own five technology stocks and consider yourself diversified, you probably are not. You have core exposure to a single sector. Concentration risk rises with the correlation among holdings. A global All-World ETF, by contrast, holds thousands of stocks from dozens of countries and sectors and automatically delivers the maximum spread of unsystematic risk.
When individual stocks do make sense
If you have deep knowledge of a specific company, read its annual reports, and understand its business better than the market does — then adding 1–2 individual positions as a "satellite" alongside a core index holding can make sense. Never as the primary strategy.
FAQ
What is the difference between systematic and unsystematic risk?
Unsystematic risk is specific to a given company and can be eliminated by diversification. Systematic risk is the movement of the whole market and cannot be removed by diversification — it hits all stocks simultaneously.
How many stocks do I need for proper diversification?
Research shows that 20–30 stocks from different sectors eliminate roughly 90% of unsystematic risk. Beyond 30 holdings, marginal benefits are small. An index ETF with 500+ stocks is practically fully diversified.
Is an ETF safer than an individual stock?
An ETF eliminates unsystematic risk by holding hundreds of stocks. Systematic market risk remains. So an ETF is more resilient to the bankruptcy of a single company, but it still falls when the whole market falls.