APT 2: Systematic vs. Idiosyncratic Risk
Systematic risk hits the whole market, like recessions or interest rate changes. Idiosyncratic (unsystematic) risk belongs to one company, like a failed product or a lawsuit. Diversification can wash out idiosyncratic risk, so models like CAPM and APT only reward systematic risk.
Systematic vs idiosyncratic risk explained simply
Systematic risk hits the whole market (recessions, interest rates); idiosyncratic risk belongs to one company (a failed product, a lawsuit). Diversification can wash out idiosyncratic risk, so CAPM and APT only reward systematic risk. In the example, about 47% of the stock's variance is systematic and 53% idiosyncratic.
What this systematic risk lesson covers
- What systematic (market) risk is, with real-world examples
- What idiosyncratic (firm-specific) risk is, with examples
- Why diversification removes one but not the other
- How to split total variance into systematic and idiosyncratic parts
- Why investors are only paid for bearing systematic risk
Risk split formula, explained
- σ_total²
- total variance of the stock's returns
- β² × σ_m²
- systematic variance
- σ_e²
- idiosyncratic (firm-specific) variance
How to split risk step by step (worked example)
A stock has beta 1.2 and a total standard deviation of 35%. The market's standard deviation is 20%. How much of its risk is systematic?
- Total variance = 0.35² = 0.1225
- Systematic variance = 1.2² × 0.20² = 0.0576
- Idiosyncratic variance = 0.1225 − 0.0576 = 0.0649
- Systematic share = 0.0576 ÷ 0.1225 ≈ 47%
Answer: About 47% of the stock's variance is systematic and about 53% is idiosyncratic (rounded).
Systematic vs idiosyncratic risk
| Systematic (market) risk | Idiosyncratic (company) risk | |
|---|---|---|
| Examples | Recession, interest rates, inflation | Product failure, lawsuit, a CEO leaving |
| Removed by diversification? | No | Mostly, yes |
| Rewarded with higher expected return? | Yes | No (in CAPM and APT) |
| Measured by | Beta | The leftover variance |
Which risks should you never take? David's take
Diversification works because most company-specific surprises are small and they cancel out. That fits a soundbite of mine: the road to success has infinite tiny mistakes, some big mistakes and zero vital mistakes. Spread across many holdings, one company's disaster is a tiny mistake. Put everything in that one company and the same event becomes a vital mistake. Not all mistakes are the same, so size your bets so that none can wipe you out.
From David's concept: Choose Your Mistakes. This is David's opinion, alongside the standard textbook answer above.
Harder systematic risk questions
In a well-diversified portfolio, how much risk would this stock add?
Only its systematic part: β × σ_market = 1.2 × 20% = 24% standard deviation, instead of the 35% it has on its own. The idiosyncratic part largely disappears.
Why aren't investors paid for idiosyncratic risk?
Because anyone can remove it cheaply by diversifying. The market only pays a premium for risk that can't be diversified away.
Common systematic risk questions
What is the difference between systematic and unsystematic risk?
Systematic risk affects the whole market, such as recessions, inflation and interest rate changes, and cannot be diversified away. Unsystematic risk is specific to one company or industry, such as a product recall or a strike, and can be reduced by holding many stocks.
What is idiosyncratic risk?
Idiosyncratic risk is another name for unsystematic or firm-specific risk. It comes from events that affect one company, such as a CEO leaving, a lawsuit or a failed product launch. In a well-diversified portfolio these surprises largely cancel each other out.
Can diversification eliminate systematic risk?
No. Diversification removes idiosyncratic risk because company-specific surprises offset each other. Systematic risk hits nearly all stocks at once, so adding more stocks does not remove it. Hedging or holding safer assets is needed to reduce it.
Why are investors not compensated for unsystematic risk?
Because investors can remove it cheaply by diversifying. Markets will not pay a premium for risk you could have avoided for free, so required returns in CAPM and APT depend only on exposure to systematic risk.