What is a black swan event, and which crash counted as one?
The essentials in 30 seconds
- A black swan event is a rare shock that carries extreme impact and gets explained only after it happens.
- A 3-question test sorts any shock into black swan, grey swan, or known risk.
- Black Monday on 19 October 1987 saw the Dow fall 22.6% and no official body calls it a black swan.
- Experts split on the 2020 shock, and Nassim Nicholas Taleb says it was expected.
- Investors in Singapore can reuse the test on every new headline.
What is a black swan event, and which stock market crash counted as one? A black swan event is a rare shock with extreme impact that people explain only after it happens. As of October 2026, this page gives the plain definition, a 3-question test, and a reading of two famous falls. It sits under the guide to black swan events, and it works as a reference you can reuse.
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What is a black swan event in plain words?
A black swan event is a surprise big enough to change the picture. The word swan comes from an old belief that every swan is white, and one sighting of a black swan ended the belief. In finance the sighting is a market move that models called almost impossible, followed by the realisation that the models were too narrow. The lesson for a stock market crash is practical: a model built only on past calm underestimates the range of outcomes.
The term became popular through Nassim Nicholas Taleb and his book The Black Swan, first published in 2007. He asks readers to separate what they have seen from what can happen. A long record of calm days tells you about the days you saw, and says little about the days that remain.
Which 3 questions decide whether a shock is a black swan event?
Question 1 asks whether the event lies outside regular expectations, so that credible forecasts gave it almost no weight. Question 2 asks whether the impact was extreme, with large and lasting effects on prices or institutions. Question 3 asks whether people explained it afterwards as if it had been predictable. A yes to all 3 earns the label.
A partial score is useful too. An event that scores yes on impact and hindsight, and no on surprise, belongs to the group of known risks that were ignored. Sorting events this way trains attention on the first question, because surprise is the attribute that preparation can reduce the most.
Did the 1987 stock market crash count as a black swan event?
On 19 October 1987 the Dow Jones Industrial Average fell 508 points, or 22.6%, the largest one-day decline in history according to Federal Reserve History. Sources describe the fall as largely unexpected, which fits question 1, and the size fits question 2. Analysts then listed causes such as portfolio insurance, program trading, and overvaluation, which is the hindsight of question 3.
No official body attaches the label to Black Monday, so the case stays debated. The Federal Reserve History essay adds a fact that shapes the answer: the losses were not followed by an economic recession or a banking crisis. The impact on prices was extreme, and the impact on the wider economy stayed contained.
Was the 2020 COVID-19 shock a black swan event for markets?
Authors disagree. Some call the COVID-19 shock a black swan, others call it a grey swan, which means a rare but foreseeable event. Taleb himself says it was not a black swan, as it was expected with great certainty, because epidemics were a known risk. The disagreement shows that the label depends on whose expectations count.
The economic effect was sharp and short. The National Bureau of Economic Research dates the 2020 contraction from February 2020 to April 2020, a span of 2 months. Compare it with the 43 months of the 1929 to 1933 contraction, and you see why size of impact and length of impact need separate questions.
How does a grey swan differ from a black swan event?
A grey swan is a shock that experts can describe in advance, even when its timing and size remain unclear. A pandemic, a bank failure, or a sudden policy change all qualify. The risk has a name, a mechanism, and often a history, so a prepared institution can price it and hold a buffer.
A black swan sits beyond that line. Nobody has a name or a mechanism ready, and the first response happens in real time. The distinction brings good news: a grey swan can be planned for, and planning reduces the damage.
How can you use the black swan event test on a new headline?
Write the 3 questions on a card and answer them in 5 minutes whenever a headline calls a shock unprecedented. Look for earlier warnings in central bank reports, regulator notes, and research. If you find them, the shock is a grey swan or a known risk, and your plan can reference the earlier work. Keep the card answers in a notebook, so that your sorting improves with each event.
Then size your exposure. Ask which position, contract, or deadline would fail first if prices fell 20% in a week. The EQOS Asia simulator turns that question into a scenario you can run, and the EQOS Asia method page explains how the score is built. This page is general information, not investment advice.
What comes after the black swan event definition?
The sister page on tail risk moves from the definition to the measure. It shows how the far end of a return distribution is counted, why simple models understate it, and how investors price protection against the rare fall that matters most. Read it next if you want a number to attach to the definition.
For a decision in Singapore, ASEAN, or Hong Kong that depends on how a stock market crash would reach your balance sheet, a 30-minute pre-diagnostic with EQOS Asia applies the same scoring to your case and returns a ranked set of options.
See also in this guide
Frequently asked questions
What is a black swan event in simple terms?
A black swan event is a surprise shock with a very large effect, which people explain after the fact as if it had been predictable. The term comes from the old belief that all swans are white. Taleb popularised it in 2007, and investors now use it for rare market falls.
Which stock market crash counted as a black swan event?
No official body names one. Black Monday in 1987 was largely unexpected and the Dow fell 22.6%, yet opinions stay divided. For the 2020 shock, some authors say black swan, others say grey swan, and Taleb says it was expected. Use the 3-question test to form your own view.
Is a black swan event always bad for markets?
The label describes surprise and size, so it can describe a positive shock as well. In practice the term appears mostly after falls, because losses draw attention. The test works in both directions: ask whether the event was an outlier, whether it was extreme, and whether people explained it afterwards.
How can an investor prepare for a black swan event?
Preparation targets exposure, because the shock itself is unknowable. Hold liquid reserves sized to your obligations, spread counterparties, and stage large commitments. Review your plan each quarter. A 5-minute use of the 3-question test on each new headline keeps your attention on the right risks.
Test your knowledge
1. What did one sighting of a black swan end?
- The belief that all swans are white
- A stock exchange
- A tax rule
Show the answer
The belief that all swans are white.
2. How many questions make up the test on this page?
- 2
- 5
- 3
Show the answer
3.
3. How far did the Dow fall on 19 October 1987?
- 22.6%
- 2.6%
- 12%
Show the answer
22.6%.
4. How long was the 2020 contraction according to the NBER?
- 43 months
- 2 months
- 18 months
Show the answer
2 months.
5. What is a grey swan?
- A rare event nobody can name
- A positive surprise only
- A rare but foreseeable event
Show the answer
A rare but foreseeable event.
About the author
Stéphane Jambu
President, EQOS Forecast Asia
Stéphane Jambu leads EQOS Forecast Asia and works as a consultant in e-commerce, search visibility, and AI. LinkedIn profile
Sources: EQOS Asia simulator (eqos-asia.com/simulator.html), method page (eqos-asia.com/method.html) and contact page (eqos-asia.com/contact.html), Federal Reserve History, Stock Market Crash of 1987 (federalreservehistory.org/essays/stock-market-crash-of-1987), NBER US business cycle expansions and contractions (nber.org/research/data/us-business-cycle-expansions-and-contractions), Black swan theory (en.wikipedia.org/wiki/Black_swan_theory), consulted on 22 October 2026. Page updated on 22 October 2026.