Black Swan
A black swan is a rare, hard-to-predict event with extreme market consequences that people often rationalise only after it has happened.
A black swan is a rare, extreme event that almost nobody expected, with huge consequences. The idea was made famous by Nassim Taleb. After it happens, people often explain it as if it had been predictable. Since it can't be forecast, traders focus on surviving one rather than predicting it.
- Category
- Risk Management
- Reading time
- 3 min
- Also called
- Black swan event, Tail event
- Updated
Black swans in plain words
Nassim Taleb popularised the term for events that look almost impossible beforehand yet keep happening.1
In markets it usually means a rare, damaging shock that nobody saw coming because nothing like it had happened before.2
Risk models learn from the past, so they struggle with events that have no past to learn from.3 Explanations after the fact can make them look more predictable than they were.
How to prepare for one
Accept you can't predict it
Accept that your models will probably not flag the next black swan.3
Don't trust history alone
Remember why: models are built on history, and true black swans have no history.3
Expect correlations to jump
Plan for diversification to fail in a crisis, when unrelated assets can suddenly fall together.3
Stress-test
Stress-test how easily you could sell your holdings in a panic.3
Weigh hedging costs
Weigh the cost of protection: hedges and insurance cost money every year and drag on returns.3
Size for survival
Since such events happen with regularity, keep position sizes you could survive in an extreme move.1
Worked examples
Example 1 — Leverage meets a shock
−100%A hypothetical account of 10 uses 5× leverage; the market gaps down 20% overnight.
| Exposure | 50 |
| Loss | 50 × 20% = 10 |
Nothing in the trader's history looked like this event, so the model gave no warning and the account was wiped out.3
Example 2 — Small size survives
−6%A hypothetical account of 100 holds 30 in the same market with no leverage.
| Loss | 30 × 20% = 6 |
Different assets may suddenly become highly correlated, but the small size kept the loss to 6%.3
Example 3 — Cost of insurance
−5A hypothetical investor pays 1 a year for protection that pays nothing for five years.
| Cost | 5 × 1 = 5 |
The protection worked in the crash, but its yearly cost had been trimming returns all along.3
Black swan vs normal volatility
| Aspect | Black swan | Normal volatility |
|---|---|---|
| Frequency | Rare | Constant |
| Predictable | No | Partly |
| Size | Extreme | Ordinary |
| In models | Missed | Captured |
Where it matters
Leverage
Limiting Leverage that a shock could wipe out.
Sizing
Using conservative Position Sizing.
Hedging
Weighing the cost of Hedging.
Ruin risk
Thinking about Risk of Ruin.
Advantages and disadvantages
Advantages
- Reminds you models have limits.
- Encourages stress-testing.
- Promotes survivable sizing.
Disadvantages
- Can't be predicted or timed.
- Hedging costs add up.
- Hindsight makes it look obvious.
- Can lead to too much fear.
Going deeper
Read as far as you need. Each level builds on the one before it.
Common mistakes
Thinking you can predict it
Plan to survive instead.
Over-leveraging
Keep leverage modest.
Trusting past data fully
History lacks the event.
Calling it obvious later
That's hindsight.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- A black swan is rare, extreme and unexpected.
- It is often explained after the fact.
- Models built on history miss it.
- Correlations can jump in a shock.
- Size positions to survive one.
Frequently asked questions
Who coined the term?
Can I predict one?
How do I prepare?
Further learning
- Risk of Ruin
Surviving extremes.
- Value at Risk
A model with limits.
- Hedging
Paying for protection.
- Diversification
Why it can fail.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- CFA Institute. Nassim Taleb and Daniel Kahneman: Black Swan Shows Fragility under Heavy Weight of Anchoring (opens in a new tab) Checked .
- Morningstar. What Prior Market Crashes Taught Us in 2020 (opens in a new tab) Checked .
- Fidelity International. How to better protect against black swans (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.